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In chemistry , the oxidation state , or oxidation number , is the hypothetical charge of an atom if all of its bonds to other atoms are fully ionic . It describes the degree of oxidation (loss of electrons ) of an atom in a chemical compound . Conceptually, the oxidation state may be positive, negative or zero. Beside nearly-pure ionic bonding , many covalent bonds exhibit a strong ionicity, making oxidation state a useful predictor of charge.
The oxidation state of an atom does not represent the "real" charge on that atom, or any other actual atomic property. This is particularly true of high oxidation states, where the ionization energy required to produce a multiply positive ion is far greater than the energies available in chemical reactions. Additionally, the oxidation states of atoms in a given compound may vary depending on the choice of electronegativity scale used in their calculation. Thus, the oxidation state of an atom in a compound is purely a formalism. It is nevertheless important in understanding the nomenclature conventions of inorganic compounds . Also, several observations regarding chemical reactions may be explained at a basic level in terms of oxidation states.
Oxidation states are typically represented by integers which may be positive, zero, or negative. In some cases, the average oxidation state of an element is a fraction, such as 8 / 3 for iron in magnetite Fe 3 O 4 ( see below ). The highest known oxidation state is reported to be +9, displayed by iridium in the tetroxoiridium(IX) cation ( IrO + 4 ). [ 1 ] It is predicted that even a +10 oxidation state may be achieved by platinum in tetroxoplatinum(X), PtO 2+ 4 . [ 2 ] The lowest oxidation state is −5, as for boron in Al 3 BC [ 3 ] and gallium in pentamagnesium digallide ( Mg 5 Ga 2 ).
In Stock nomenclature , which is commonly used for inorganic compounds, the oxidation state is represented by a Roman numeral placed after the element name inside parentheses or as a superscript after the element symbol, e.g. Iron(III) oxide . The term oxidation was first used by Antoine Lavoisier to signify the reaction of a substance with oxygen . Much later, it was realized that the substance, upon being oxidized, loses electrons, and the meaning was extended to include other reactions in which electrons are lost, regardless of whether oxygen was involved.
The increase in the oxidation state of an atom, through a chemical reaction, is known as oxidation; a decrease in oxidation state is known as a reduction . Such reactions involve the formal transfer of electrons: a net gain in electrons being a reduction, and a net loss of electrons being oxidation. For pure elements, the oxidation state is zero.
Oxidation numbers are assigned to elements in a molecule such that the overall sum is zero in a neutral molecule. The number indicates the degree of oxidation of each element caused by molecular bonding. In ionic compounds, the oxidation numbers are the same as the element's ionic charge. Thus for KCl, potassium is assigned +1 and chlorine is assigned −1. [ 4 ] The complete set of rules for assigning oxidation numbers are discussed in the following sections.
Oxidation numbers are fundamental to the chemical nomenclature of ionic compounds. For example, Cu compounds with Cu oxidation state +2 are called cupric and those with state +1 are cuprous . [ 4 ] : 172 The oxidation numbers of elements allow predictions of chemical formula and reactions, especially oxidation-reduction reactions .
The oxidation numbers of the most stable chemical compounds follow trends in the periodic table. [ 5 ] : 140
International Union of Pure and Applied Chemistry (IUPAC) has published a "Comprehensive definition of oxidation state (IUPAC Recommendations 2016)". [ 6 ] It is a distillation of an IUPAC technical report: "Toward a comprehensive definition of oxidation state". [ 7 ] According to the IUPAC Gold Book : "The oxidation state of an atom is the charge of this atom after ionic approximation of its heteronuclear bonds." [ 8 ] The term oxidation number is nearly synonymous. [ 9 ]
The ionic approximation means extrapolating bonds to ionic. Several criteria [ 10 ] were considered for the ionic approximation:
In a bond between two different elements, the bond's electrons are assigned to its main atomic contributor typically of higher electronegativity; in a bond between two atoms of the same element, the electrons are divided equally. Most electronegativity scales depend on the atom's bonding state, which makes the assignment of the oxidation state a somewhat circular argument. For example, some scales may turn out unusual oxidation states, such as −6 for platinum in PtH 2− 4 , for Pauling and Mulliken scales. [ 7 ] The dipole moments would sometimes also turn out abnormal oxidation numbers, such as in CO and NO , which
are oriented with their positive end towards oxygen. Therefore, this leaves the atom's contribution to the
bonding MO, the atomic-orbital energy, and from quantum-chemical calculations of charges, as the only viable criteria with cogent values for ionic approximation. However, for a simple estimate for the ionic approximation, we can use Allen electronegativities , [ 7 ] as only that electronegativity scale is truly independent of the oxidation state, as it relates to the average valence‐electron energy of the free atom:
While introductory levels of chemistry teaching use postulated oxidation states, the IUPAC recommendation [ 6 ] and the Gold Book entry [ 8 ] list two entirely general algorithms for the calculation of the oxidation states of elements in chemical compounds.
Introductory chemistry uses postulates: the oxidation state for an element in a chemical formula is calculated from the overall charge and postulated oxidation states for all the other atoms.
A simple example is based on two postulates,
where OS stands for oxidation state. This approach yields correct oxidation states in oxides and hydroxides of any single element, and in acids such as sulfuric acid ( H 2 SO 4 ) or dichromic acid ( H 2 Cr 2 O 7 ). Its coverage can be extended either by a list of exceptions or by assigning priority to the postulates. The latter works for hydrogen peroxide ( H 2 O 2 ) where the priority of rule 1 leaves both oxygens with oxidation state −1.
Additional postulates and their ranking may expand the range of compounds to fit a textbook's scope. As an example, one postulatory algorithm from many possible; in a sequence of decreasing priority:
This set of postulates covers oxidation states of fluorides, chlorides, bromides, oxides, hydroxides, and hydrides of any single element. It covers all oxoacids of any central atom (and all their fluoro-, chloro-, and bromo-relatives), as well as salts of such acids with group 1 and 2 metals. It also covers iodides , sulfides , and similar simple salts of these metals.
This algorithm is performed on a Lewis structure (a diagram that shows all valence electrons ). Oxidation state equals the charge of an atom after each of its heteronuclear bonds has been assigned to the more electronegative partner of the bond ( except when that partner is a reversibly bonded Lewis-acid ligand ) and homonuclear bonds have been divided equally:
where each "—" represents an electron pair (either shared between two atoms or solely on one atom), and "OS" is the oxidation state as a numerical variable.
After the electrons have been assigned according to the vertical red lines on the formula, the total number of valence electrons that now "belong" to each atom is subtracted from the number N of valence electrons of the neutral atom (such as 5 for nitrogen in group 15 ) to yield that atom's oxidation state.
This example shows the importance of describing the bonding. Its summary formula, HNO 3 , corresponds to two structural isomers ; the peroxynitrous acid in the above figure and the more stable nitric acid . With the formula HNO 3 , the simple approach without bonding considerations yields −2 for all three oxygens and +5 for nitrogen, which is correct for nitric acid. For the peroxynitrous acid, however, both oxygens in the O–O bond have OS = −1, and the nitrogen has OS = +3, which requires a structure to understand.
Organic compounds are treated in a similar manner; exemplified here on functional groups occurring in between methane ( CH 4 ) and carbon dioxide ( CO 2 ):
Analogously for transition-metal compounds; CrO(O 2 ) 2 on the left has a total of 36 valence electrons (18 pairs to be distributed), and hexacarbonylchromium ( Cr(CO) 6 ) on the right has 66 valence electrons (33 pairs):
A key step is drawing the Lewis structure of the molecule (neutral, cationic, anionic): Atom symbols are arranged so that pairs of atoms can be joined by single two-electron bonds as in the molecule (a sort of "skeletal" structure), and the remaining valence electrons are distributed such that sp atoms obtain an octet (duet for hydrogen) with a priority that increases in proportion with electronegativity. In some cases, this leads to alternative formulae that differ in bond orders (the full set of which is called the resonance formulas ). Consider the sulfate anion ( SO 2− 4 ) with 32 valence electrons; 24 from oxygens, 6 from sulfur, 2 of the anion charge obtained from the implied cation. The bond orders to the terminal oxygens do not affect the oxidation state so long as the oxygens have octets. Already the skeletal structure, top left, yields the correct oxidation states, as does the Lewis structure, top right (one of the resonance formulas):
The bond-order formula at the bottom is closest to the reality of four equivalent oxygens each having a total bond order of 2. That total includes the bond of order 1 / 2 to the implied cation and follows the 8 − N rule [ 7 ] requiring that the main-group atom's bond-order total equals 8 − N valence electrons of the neutral atom, enforced with a priority that proportionately increases with electronegativity.
This algorithm works equally for molecular cations composed of several atoms. An example is the ammonium cation of 8 valence electrons (5 from nitrogen, 4 from hydrogens, minus 1 electron for the cation's positive charge):
Drawing Lewis structures with electron pairs as dashes emphasizes the essential equivalence of bond pairs and lone pairs when counting electrons and moving bonds onto atoms. Structures drawn with electron dot pairs are of course identical in every way:
The algorithm contains a caveat, which concerns rare cases of transition-metal complexes with a type of ligand that is reversibly bonded as a Lewis acid (as an acceptor of the electron pair from the transition metal); termed a "Z-type" ligand in Green's covalent bond classification method . The caveat originates from the simplifying use of electronegativity instead of the MO -based electron allegiance to decide the ionic sign. [ 6 ] One early example is the O 2 S−RhCl(CO)( PPh 3 ) 2 complex [ 13 ] with sulfur dioxide ( SO 2 ) as the reversibly-bonded acceptor ligand (released upon heating). The Rh−S bond is therefore extrapolated ionic against Allen electronegativities of rhodium and sulfur, yielding oxidation state +1 for rhodium:
This algorithm works on Lewis structures and bond graphs of extended (non-molecular) solids:
Oxidation state is obtained by summing the heteronuclear-bond orders at the atom as positive if that atom is the electropositive partner in a particular bond and as negative if not, and the atom’s formal charge (if any) is added to that sum. The same caveat as above applies.
An example of a Lewis structure with no formal charge,
illustrates that, in this algorithm, homonuclear bonds are simply ignored (the bond orders are in blue).
Carbon monoxide exemplifies a Lewis structure with formal charges :
To obtain the oxidation states, the formal charges are summed with the bond-order value taken positively at the carbon and negatively at the oxygen.
Applied to molecular ions, this algorithm considers the actual location of the formal (ionic) charge, as drawn in the Lewis structure. As an example, summing bond orders in the ammonium cation yields −4 at the nitrogen of formal charge +1, with the two numbers adding to the oxidation state of −3:
The sum of oxidation states in the ion equals its charge (as it equals zero for a neutral molecule).
Also in anions, the formal (ionic) charges have to be considered when nonzero. For sulfate this is exemplified with the skeletal or Lewis structures (top), compared with the bond-order formula of all oxygens equivalent and fulfilling the octet and 8 − N rules (bottom):
A bond graph in solid-state chemistry is a chemical formula of an extended structure, in which direct bonding connectivities are shown. An example is the AuORb 3 perovskite , the unit cell of which is drawn on the left and the bond graph (with added numerical values) on the right:
We see that the oxygen atom bonds to the six nearest rubidium cations, each of which has 4 bonds to the auride anion. The bond graph summarizes these connectivities. The bond orders (also called bond valences ) sum up to oxidation states according to the attached sign of the bond's ionic approximation (there are no formal charges in bond graphs).
Determination of oxidation states from a bond graph can be illustrated on ilmenite , FeTiO 3 . We may ask whether the mineral contains Fe 2+ and Ti 4+ , or Fe 3+ and Ti 3+ . Its crystal structure has each metal atom bonded to six oxygens and each of the equivalent oxygens to two irons and two titaniums , as in the bond graph below. Experimental data show that three metal-oxygen bonds in the octahedron are short and three are long (the metals are off-center). The bond orders (valences), obtained from the bond lengths by the bond valence method , sum up to 2.01 at Fe and 3.99 at Ti; which can be rounded off to oxidation states +2 and +4, respectively:
Oxidation states can be useful for balancing chemical equations for oxidation-reduction (or redox ) reactions, because the changes in the oxidized atoms have to be balanced by the changes in the reduced atoms. For example, in the reaction of acetaldehyde with Tollens' reagent to form acetic acid (shown below), the carbonyl carbon atom changes its oxidation state from +1 to +3 (loses two electrons). This oxidation is balanced by reducing two Ag + cations to Ag 0 (gaining two electrons in total).
An inorganic example is the Bettendorf reaction using tin dichloride ( SnCl 2 ) to prove the presence of arsenite ions in a concentrated HCl extract. When arsenic(III) is present, a brown coloration appears forming a dark precipitate of arsenic , according to the following simplified reaction:
Here three tin atoms are oxidized from oxidation state +2 to +4, yielding six electrons that reduce two arsenic atoms from oxidation state +3 to 0. The simple one-line balancing goes as follows: the two redox couples are written down as they react;
One tin is oxidized from oxidation state +2 to +4, a two-electron step, hence 2 is written in front of the two arsenic partners. One arsenic is reduced from +3 to 0, a three-electron step, hence 3 goes in front of the two tin partners. An alternative three-line procedure is to write separately the half-reactions for oxidation and reduction, each balanced with electrons, and then to sum them up such that the electrons cross out. In general, these redox balances (the one-line balance or each half-reaction) need to be checked for the ionic and electron charge sums on both sides of the equation being indeed equal. If they are not equal, suitable ions are added to balance the charges and the non-redox elemental balance.
A nominal oxidation state is a general term with two different definitions:
Lewis formulae are rule-based approximations of chemical reality, as are Allen electronegativities . Still, oxidation states may seem ambiguous when their determination is not straightforward. If only an experiment can determine the oxidation state, the rule-based determination is ambiguous (insufficient). There are also truly dichotomous values that are decided arbitrarily.
Seemingly ambiguous oxidation states are derived from a set of resonance formulas of equal weights for a molecule having heteronuclear bonds where the atom connectivity does not correspond to the number of two-electron bonds dictated by the 8 − N rule. [ 7 ] : 1027 An example is S 2 N 2 where four resonance formulas featuring one S=N double bond have oxidation states +2 and +4 for the two sulfur atoms, which average to +3 because the two sulfur atoms are equivalent in this square-shaped molecule.
Fractional oxidation states are often used to represent the average oxidation state of several atoms of the same element in a structure. For example, the formula of magnetite is Fe 3 O 4 , implying an average oxidation state for iron of + 8 / 3 . [ 17 ] : 81–82 However, this average value may not be representative if the atoms are not equivalent. In a Fe 3 O 4 crystal below 120 K (−153 °C), two-thirds of the cations are Fe 3+ and one-third are Fe 2+ , and the formula may be more clearly represented as FeO· Fe 2 O 3 . [ 18 ]
Likewise, propane , C 3 H 8 , has been described as having a carbon oxidation state of − 8 / 3 . [ 19 ] Again, this is an average value since the structure of the molecule is H 3 C−CH 2 −CH 3 , with the first and third carbon atoms each having an oxidation state of −3 and the central one −2.
An example with true fractional oxidation states for equivalent atoms is potassium superoxide , KO 2 . The diatomic superoxide ion O − 2 has an overall charge of −1, so each of its two equivalent oxygen atoms is assigned an oxidation state of − 1 / 2 . This ion can be described as a resonance hybrid of two Lewis structures, where each oxygen has an oxidation state of 0 in one structure and −1 in the other.
For the cyclopentadienyl anion C 5 H − 5 , the oxidation state of C is −1 + − 1 / 5 = − 6 / 5 . The −1 occurs because each carbon is bonded to one hydrogen atom (a less electronegative element), and the − 1 / 5 because the total ionic charge of −1 is divided among five equivalent carbons. Again this can be described as a resonance hybrid of five equivalent structures, each having four carbons with oxidation state −1 and one with −2.
Finally, fractional oxidation numbers are not used in the chemical nomenclature. [ 20 ] : 66 For example the red lead Pb 3 O 4 is represented as lead(II,IV) oxide, showing the oxidation states of the two nonequivalent lead atoms.
Most elements have more than one possible oxidation state. For example, carbon has nine possible integer oxidation states from −4 to +4:
Many compounds with luster and electrical conductivity maintain a simple stoichiometric formula, such as the golden TiO , blue-black RuO 2 or coppery ReO 3 , all of obvious oxidation state. Ultimately, assigning the free metallic electrons to one of the bonded atoms is not comprehensive and can yield unusual oxidation states. Examples are the LiPb and Cu 3 Au ordered alloys , the composition and structure of which are largely determined by atomic size and packing factors . Should oxidation state be needed for redox balancing, it is best set to 0 for all atoms of such an alloy.
This is a list of known oxidation states of the chemical elements , excluding nonintegral values . The most common states appear in bold. The table is based on that of Greenwood and Earnshaw, [ 21 ] with additions noted. Every element exists in oxidation state 0 when it is the pure non-ionized element in any phase, whether monatomic or polyatomic allotrope . The column for oxidation state 0 only shows elements known to exist in oxidation state 0 in compounds.
A figure with a similar format was used by Irving Langmuir in 1919 in one of the early papers about the octet rule . [ 165 ] The periodicity of the oxidation states was one of the pieces of evidence that led Langmuir to adopt the rule.
The oxidation state in compound naming for transition metals and lanthanides and actinides is placed either as a right superscript to the element symbol in a chemical formula, such as Fe III or in parentheses after the name of the element in chemical names, such as iron(III). For example, Fe 2 (SO 4 ) 3 is named iron(III) sulfate and its formula can be shown as Fe III 2 (SO 4 ) 3 . This is because a sulfate ion has a charge of −2, so each iron atom takes a charge of +3.
Oxidation itself was first studied by Antoine Lavoisier , who defined it as the result of reactions with oxygen (hence the name). [ 166 ] [ 167 ] The term has since been generalized to imply a formal loss of electrons. Oxidation states, called oxidation grades by Friedrich Wöhler in 1835, [ 168 ] were one of the intellectual stepping stones that Dmitri Mendeleev used to derive the periodic table . [ 169 ] William B. Jensen [ 170 ] gives an overview of the history up to 1938.
When it was realized that some metals form two different binary compounds with the same nonmetal, the two compounds were often distinguished by using the ending -ic for the higher metal oxidation state and the ending -ous for the lower. For example, FeCl 3 is ferric chloride and FeCl 2 is ferrous chloride . This system is not very satisfactory (although sometimes still used) because different metals have different oxidation states which have to be learned: ferric and ferrous are +3 and +2 respectively, but cupric and cuprous are +2 and +1, and stannic and stannous are +4 and +2. Also, there was no allowance for metals with more than two oxidation states, such as vanadium with oxidation states +2, +3, +4, and +5. [ 17 ] : 84
This system has been largely replaced by one suggested by Alfred Stock in 1919 [ 171 ] and adopted [ 172 ] by IUPAC in 1940. Thus, FeCl 2 was written as iron(II) chloride rather than ferrous chloride. The Roman numeral II at the central atom came to be called the " Stock number " (now an obsolete term), and its value was obtained as a charge at the central atom after removing its ligands along with the electron pairs they shared with it. [ 20 ] : 147
The term "oxidation state" in English chemical literature was popularized by Wendell Mitchell Latimer in his 1938 book about electrochemical potentials. [ 173 ] He used it for the value (synonymous with the German term Wertigkeit ) previously termed "valence", "polar valence" or "polar number" [ 174 ] in English, or "oxidation stage" or indeed [ 175 ] [ 176 ] the "state of oxidation". Since 1938, the term "oxidation state" has been connected with electrochemical potentials and electrons exchanged in redox couples participating in redox reactions. By 1948, IUPAC used the 1940 nomenclature rules with the term "oxidation state", [ 177 ] [ 178 ] instead of the original [ 172 ] valency . In 1948 Linus Pauling proposed that oxidation number could be determined by extrapolating bonds to being completely ionic in the direction of electronegativity . [ 179 ] A full acceptance of this suggestion was complicated by the fact that the Pauling electronegativities as such depend on the oxidation state and that they may lead to unusual values of oxidation states for some transition metals. In 1990 IUPAC resorted to a postulatory (rule-based) method to determine the oxidation state. [ 180 ] This was complemented by the synonymous term oxidation number as a descendant of the Stock number introduced in 1940 into the nomenclature. However, the terminology using " ligands " [ 20 ] : 147 gave the impression that oxidation number might be something specific to coordination complexes . This situation and the lack of a real single definition generated numerous debates about the meaning of oxidation state, suggestions about methods to obtain it and definitions of it. To resolve the issue, an IUPAC project (2008-040-1-200) was started in 2008 on the "Comprehensive Definition of Oxidation State", and was concluded by two reports [ 7 ] [ 6 ] and by the revised entries "Oxidation State" [ 8 ] and "Oxidation Number" [ 9 ] in the IUPAC Gold Book . The outcomes were a single definition of oxidation state and two algorithms to calculate it in molecular and extended-solid compounds, guided by Allen electronegativities that are independent of oxidation state. | https://en.wikipedia.org/wiki/Oxidation_state |
Oxidation state localized orbitals (OSLOs) is a new concept used to determine the oxidation states of each fragment for the coordination complexes . [ 1 ] Based on the result of density functional theory (DFT), all the occupied molecular orbitals are remixed to get the oxidation state localized orbitals. These orbitals are assigned to one of the fragments in this molecule based on the fragment orbital localization index (FOLI) . After all the electrons are assigned, the oxidation states of each fragment could be obtained by calculating the difference between the number of electrons and protons in each fragment. [ 1 ]
Oxidation state is an important index to evaluate the charge distribution within molecules. [ 2 ] The most common definition of oxidation state was established by IUPAC , [ 3 ] which let the atom with higher electronegativity takes all the bonding electrons and calculated the difference between the number of electrons and protons around each atom to assign the oxidation states. However, the definition doesn't thoroughly consider the distribution of the bonding electrons and further restricts the applicability of oxidation states. [ 4 ]
To precisely assign the oxidation state for each component in the molecule, especially for organometallic complexes , several different research groups, including Pedro Salvador and Martin Head-Gordon , have developed different methods to determine the oxidation states. [ 5 ] [ 6 ] [ 7 ] In 2009, Martin Head-Gordon group established a new method called localized orbitals bonding analysis (LOBA) to assign the electrons associated with each localized orbitals. However, this method failed to provide reasonable oxidation states since the orbitals cannot be localized for some complicated systems. [ 6 ]
To overcome this problem to get the correct assignment of oxidation states, in 2022, Martin Head-Gordon and Pedro Salvador decide to localize the electrons based on different fragments rather than atoms. Thus, they developed the method known as oxidation state localized orbitals (OSLOs) , [ 1 ] which can accurately assign electrons to different fragments to obtain the oxidation states of each fragment.
Based on the density functional theory , a full set of orbitals will compose the resulting OSLOs for each fragment. Then, these sets will be imported to the algorithm for further assignment of oxidation states and construction of OSLOs. [ 1 ]
The extent of delocalization could be quantified by using Pipek's delocalization measurement. [ 8 ] For orbitals which are highly localized, the Pipek's indexes will be very close to 1. On the other hand, for highly delocalized orbitals, the Pipek's indexes become larger.
However, this method cannot evaluate the localization extent on each fragment. Thus, a new measurement is necessary. The fragment orbital localization index (FOLI) is defined as the square root of the fragment population over the delocalization index: [ 1 ]
Based on this localization index, the localization extent on each fragment can be determined. with higher FOLI, it means the extent of localization on this fragment is relatively low, vice versa. Thus, after acquiring the FOLI, the electrons in each OSLO will be assigned to the fragment with the lowest FOLI.
First, based on the results of density functional theory calculations. The set with the minimal FOLI is selected for further analysis. Then, after calculating the FOLI for each set, the set with the minimal FOLI is selected. For the selected set, the OSLOs are removed and the oxidation states are assigned based on these OSLOs.In this method, the fragment with the higher electron population gets all the electrons in this orbital. For all the other sets, they become the input for the next-round analysis, and the process repeats until all OSLOs are constructed and all electrons are assigned. [ 1 ]
The valence OSLOs of the molecule can also be constructed using the method. The oxidation state of the ligand and metal are also determined and show consistency with the expected Lewis structure and can provide great insight for evaluating the redox reactivity.
last FOLI and Δ-FOLI are two important values to evaluate the quality of the localization result. With the last FOLI closer to 1, it means that the OSLOs are highly localized on one fragment. On the other hand, Δ-FOLI is the difference between the last FOLI and the second-last FOLI. With a larger Δ-FOLI, it means the selected set of OSLOs is much better than other options, which indicates the unambiguity of this result. [ 1 ]
For example, using the OSLOs for ferrocene shows great consistency with the prediction. The metal center was assigned the oxidation state of +2, and the Cp ligands were assigned the oxidation state of -1, which is quite consistent with the aromatic behavior of Cp . Furthermore, the last FOLI for ferrocene is 1.313 and the Δ-FOLI is 1.800, both indicating the unambiguity of the result. [ 1 ]
However, for some complicated species possessing noninnocent ligands, the results become ambiguous. For example, several copper-trifluoromethyl complexes show small Δ-FOLI, which means the result is no longer unique. Moreover, whether the copper has the oxidation state of +3 or +1 remain controversial. Besides, for the Grubbs catalyst , the result is also inconsistent with conventional Fischer and Schrock classifications. [ 1 ] | https://en.wikipedia.org/wiki/Oxidation_state_localized_orbitals |
Oxidation with dioxiranes refers to the introduction of oxygen into organic substrates using dioxiranes . Dioxiranes are well known for epoxidations (synthesis of epoxides from alkenes ). [ 1 ] Dioxiranes oxidize other unsaturated functionality, heteroatoms, and alkane C-H bonds. [ 2 ] Dioxiranes are metal-free oxidants.
Dioxiranes are electrophilic oxidants that react more quickly with electron-rich than electron-poor double bonds; however, both classes of substrates can be epoxidized within a reasonable time frame. The mechanism of epoxidation with dioxiranes likely involves concerted oxygen transfer through a spiro transition state. As oxygen transfer occurs, the plane of the oxirane is perpendicular to and bisects the plane of the alkene pi system. The configuration of the alkene is maintained in the product, ruling out long-lived radical intermediates. In addition, the spiro transition state has been used to explain the selectivity in enantioselective epoxidations with chiral ketones. [ 1 ]
Diastereoselective epoxidation may be achieved through the use of alkene starting materials with diastereotopic faces. When racemic 3-isopropylcyclohexene was subjected to DMD oxidation, the trans epoxide, which resulted from attack on the less hindered face of the double bond, was the major product. [ 1 ]
Epoxidations of electron-rich double bonds yield intermediates of Rubottom oxidation . Upon hydrolysis, these siloxyepoxides yield α-hydroxyketones. [ 1 ]
Electron-poor double bonds take much longer to epoxidize. Heating may be used to encourage oxidation, although the reaction temperature should never exceed 50 °C, to avoid decomposition of the dioxirane [ 1 ]
Alkenes bound to both electron-withdrawing and -donating groups tend to behave like the former, requiring long oxidation times and occasionally some heating. Like electron-poor epoxides, epoxide products from this class of substrates are often stable with respect to hydrolysis. [ 3 ]
In substrates containing multiple double bonds, the more electron-rich double bond tends to be epoxidized first. [ 4 ]
Epoxidations employing aqueous oxone and a catalytic amount of ketone are convenient if a specialized dioxirane must be used (as in asymmetric applications) or if isolation of the dioxirane is inconvenient. Hydrolytic decomposition of the epoxidation product may be used to good advantage. [ 5 ]
Diastereoselective DMD epoxidation of a chiral unsaturated ketone was applied to the synthesis of verrucosan-2β-ol. [ 6 ]
Enantioselective dioxirane epoxidation is critical in a synthetic sequence leading to an analogue of glabrescol. The sequence produced the glabrescol analogue in 31% overall yield in only two steps. [ 7 ]
Dioxirane epoxidation compares favorably to related peracid oxidations. Peracids generate acidic byproducts, meaning that acid-labile substrates and products must be avoided. [ 8 ]
Some methods are well-suited to the oxidation of electron-rich or electron-poor double bonds, but few are as effective for both classes of substrate as dioxiranes. Weitz-Scheffer conditions (NaOCl, H 2 O 2 /KOH, tBuO 2 H/KOH) work well for oxidations of electron-poor double bonds, [ 9 ] and sulfonyl-substituted oxaziridines are effective for electron-rich double bonds. [ 10 ]
Metal-based oxidants are often more efficient than dioxirane oxidations in the catalytic mode; however, environmentally unfriendly byproducts are typically generated. In the realm of asymmetric methods, both the Sharpless epoxidation [ 11 ] and Jacobsen epoxidation [ 12 ] surpass asymmetric dioxirane oxidations in enantioselectivity. Additionally, enzymatic epoxidations are more enantioselective than dioxirane-based methods; however, such methods often suffer from operational difficulties and low yields. [ 13 ]
Chiral ketones react with oxone to give chiral dioxiranes. This fact underpins enantioselective epoxidations. [ 14 ] A popular implementation is the Shi epoxidation , which uses a fructose-derived chiral ketone.
Dioxiranes may be prepared and isolated or generated in situ from ketones and potassium peroxymonosulfate ( Oxone ). In situ preparations may be catalytic in ketone. The functional group compatibility of dioxiranes is limited somewhat, as side oxidations of amines and sulfides are rapid. Baeyer-Villiger oxidation may compete with dioxirane formation.
Dioxirane itself (CH 2 O 2 ) is not useful. Instead, the substituted dioxiranes dimethyldioxirane (DMD) and methyl(trifluoromethyl)dioxirane (TFD) are commonly employed for synthesis.
DMD and TFD may be generated in situ using conventional glassware with a two-phase system consisting of a buffered aqueous solution of oxone and a solution of substrate in an organic solvent. Such a two-phase set up is called for since oxone is insoluble in organic solvents. Mechanical stirring and/or polar organic solvents such as acetone are employed often. [ 15 ]
The salt KHSO 5 is often referred to as oxone, but they are not the same. Oxone refers to the triple salt 2KHSO 5 ·KHSO 4 ·K 2 SO 4 , which is more shelf-stable than KHSO 5 . [ 16 ]
Epoxidations using isolated dioxirane (e.g. DMD or TFD) avoid the need for aqueous buffering. The volatile dioxiranes DMD and TFD can be isolated via distillation. Once isolated, dioxiranes can be kept in solutions of the corresponding ketones and dried with molecular sieves . These solutions are suitable when substrates or products are sensitive to hydrolysis. [ 17 ] Catalytic dioxirane oxidations do, however, require water.
Dioxiranes oxidize a wide variety of functional groups yielding epoxides or other oxidized products. [ 2 ] Oxidation of allenes affords allene dioxides or products of intramolecular participation. [ 2 ] (6)
The oxidations of heteroaromatic compounds can depend on conditions. Thus, at low temperatures, acetylated indoles are simply epoxidized in high yield (unprotected indoles undergo N-oxidation). However, when the temperature is raised to 0 °C, rearranged products are obtained. [ 2 ]
(7)
DMD may oxidize heteroatoms to the corresponding oxides (or products of oxide decomposition). Often, the results of these oxidations depend on reaction conditions. Tertiary amines cleanly give the corresponding N-oxides. [ 2 ] Primary amines give nitroalkanes upon treatment with 4 equivalents of DMD, but azoxy compounds upon treatment with only 2 equivalents. [ 2 ] Secondary amines afford either hydroxylamines or nitrones. [ 2 ]
(8)
Sulfide oxidation in the presence of a single equivalent of DMD leads to sulfoxides. [ 2 ] Increasing the amount of DMD used (2 or more equivalents) leads to sulfones. Both nitrogen and sulfur are more susceptible to oxidation than carbon-carbon multiple bonds.
(10)
Although alkanes are typically difficult to functionalize directly, C-H insertion with TFD is an efficient process in many cases. The order of reactivity of C-H bonds is: allylic > benzylic > tertiary > secondary > primary. Often, the intermediate alcohols produced are oxidized further to carbonyl compounds, although this can be prevented by trapping in situ with an anhydride. Chiral alkanes are functionalized with retention of configuration. [ 2 ]
(11)
Dioxiranes oxidize primary alcohols to either the aldehyde or carboxylic acid; however, DMD selectively oxidizes secondary over primary alcohols. Thus, vicinal diols may be transformed into α-hydroxy ketones with dioxirane oxidation. [ 2 ]
(12)
Epoxidation is usually more facile than C-H oxidation, although sterically hindered allyl groups may undergo selective C-H oxidation instead of epoxidation of the allylic double bond. [ 2 ]
A variety of alternative heteroatom oxidation reagents are known, including peroxides (often employed with a transition metal catalyst) and oxaziridines . These reagents do not suffer from the over-oxidation problems and decomposition issues associated with dioxiranes. Their substrate scope, however, tends to be more limited. Nucleophilic decomposition of dioxiranes to singlet oxygen is uniquely prolem associated with dioxirane heteroatom oxidations on the other hand. Although chiral dioxiranes do not provide the same levels of enantioselectivity as other protocols, such as Kagan's sulfoxidation system, [ 18 ] complexation to a chiral transition metal complex followed by oxidation affords optically active sulfoxides with good enantioselectivity.
Oxidation of arenes and cumulenes leads initially to epoxides. These substrates are resistant to many epoxidation reagents, including oxaziridines, hydrogen peroxide , and manganese oxo compounds. Organometallic oxidants also react sluggishly with these compounds, with the exception of methyltrioxorhenium . [ 19 ] Peracids also react with arenes and cumulenes, but cannot be employed with substrates containing acid-sensitive functionality. | https://en.wikipedia.org/wiki/Oxidation_with_dioxiranes |
Oxidation induction time or OIT is a standardized test performed in a DSC which measures the level of thermal stabilization of the material tested. The time between melting and the onset of decomposition in isothermal conditions is measured. The atmosphere is nitrogen up to melting and then oxygen. The typical temperature is 190-220 °C.
Oxidation-induction time can be known with the use of Differential Scanning Calorimetry measurements, which is done with the sample body and a substance that will be heated in a constant rate in an atmosphere of inert gas . Once the specified temperature is attained, its atmosphere will be replaced by an air atmosphere of the said rate or an oxygen atmosphere. The specimen will be then held at a constant temperature up to the indication of oxidative reaction by exothermal deviation of DSC heat flow curve. Time interval in the middle of the start of the air flow and the beginning of the oxidation reaction is called the isothermal OIT. The said method was also mentioned and discussed on several various technical standards like DIN EN ISO 11357-6. [ 1 ]
This test is routine when assessing the quality of organic materials or polymers , such as polyethylene pipes.
This article about polymer science is a stub . You can help Wikipedia by expanding it . | https://en.wikipedia.org/wiki/Oxidative-induction_time |
Oxidative/fermentation glucose test (OF glucose test) is a biological technique. It was developed in 1953 by Hugh and Leifson to be utilized in microbiology to determine the way a microorganism metabolizes a carbohydrate such as glucose (dextrose). [ 1 ] OF-glucose deeps contain glucose as a carbohydrate, peptones , bromothymol blue indicator for Hugh-Leifson's OF medium or phenol red for King's OF medium, and 0.5% agar .
To perform the OF-glucose test, two tubes of OF-glucose medium are inoculated with the test organism. A layer of mineral oil is added to the top of the deep in one of the tubes to create anaerobic conditions. Oil is not added to the other tube to allow for aerobic conditions. The tubes are then incubated for 24–48 hours. If the medium in the anaerobic tube turns yellow, then the bacteria are fermenting glucose. If the tube with oil doesn't turn yellow, but the open tube does turn yellow, then the bacterium is oxidizing glucose. If the tube with mineral oil doesn't change, and the open tube turns blue, then the organism neither ferments, nor oxidizes glucose. Instead it oxidizes peptones, which liberates ammonia, turning the indicator blue.
If only the aerobic tube has turned yellow then the organism is able to oxidase glucose aerobically ("O").
By-products: CO 2 and although organic acids may be present at low rates.
If both tubes are yellow then the organism is capable of fermentation ("F").
If there is, however, growth evident on the aerobic tube yet the medium has not turned yellow, either
(a) glucose has been respired and evolved CO 2 without significant production of acid, or
(b) the organism is respiring the peptone.
This microbiology -related article is a stub . You can help Wikipedia by expanding it . | https://en.wikipedia.org/wiki/Oxidative/fermentation_glucose_test |
Oxidative addition and reductive elimination are two important and related classes of reactions in organometallic chemistry . [ 1 ] [ 2 ] [ 3 ] [ 4 ] Oxidative addition is a process that increases both the oxidation state and coordination number of a metal centre. Oxidative addition is often a step in catalytic cycles , in conjunction with its reverse reaction, reductive elimination. [ 5 ]
For transition metals, oxidative reaction results in the decrease in the d n to a configuration with fewer electrons, often 2e fewer. Oxidative addition is favored for metals that are (i) basic and/or (ii) easily oxidized. Metals with a relatively low oxidation state often satisfy one of these requirements, but even high oxidation state metals undergo oxidative addition, as illustrated by the oxidation of Pt(II) with chlorine:
In classical organometallic chemistry , the formal oxidation state of the metal and the electron count of the complex both increase by two. [ 6 ] One-electron changes are also possible and in fact some oxidative addition reactions proceed via series of 1e changes. Although oxidative additions can occur with the insertion of a metal into many different substrates, oxidative additions are most commonly seen with H–H, H–X, and C–X bonds because these substrates are most relevant to commercial applications.
Oxidative addition requires that the metal complex have a vacant coordination site. For this reason, oxidative additions are common for four- and five-coordinate complexes.
Reductive elimination is the reverse of oxidative addition. [ 7 ] Reductive elimination is favored when the newly formed X–Y bond is strong. For reductive elimination to occur the two groups (X and Y) should be mutually adjacent on the metal's coordination sphere . Reductive elimination is the key product-releasing step of several reactions that form C–H and C–C bonds. [ 5 ]
Oxidative additions proceed by diverse pathways that depend on the metal center and the substrates.
Oxidative additions of nonpolar substrates such as hydrogen and hydrocarbons appear to proceed via concerted pathways. Such substrates lack π-bonds , consequently a three-centered σ complex is invoked, followed by intramolecular ligand bond cleavage of the ligand (probably by donation of electron pair into the sigma* orbital of the inter ligand bond) to form the oxidized complex. The resulting ligands will be mutually cis , [ 2 ] although subsequent isomerization may occur.
This mechanism applies to the addition of homonuclear diatomic molecules such as H 2 . Many C–H activation reactions also follow a concerted mechanism through the formation of an M–(C–H) agostic complex . [ 2 ]
A representative example is the reaction of hydrogen with Vaska's complex , trans -IrCl(CO)[P(C 6 H 5 ) 3 ] 2 . In this transformation, iridium changes its formal oxidation state from +1 to +3. The product is formally bound to three anions: one chloride and two hydride ligands. As shown below, the initial metal complex has 16 valence electrons and a coordination number of four whereas the product is a six-coordinate 18 electron complex.
Formation of a trigonal bipyramidal dihydrogen intermediate is followed by cleavage of the H–H bond, due to electron back donation into the H–H σ*-orbital, i.e. a sigma complex . [ 8 ] This system is also in chemical equilibrium , with the reverse reaction proceeding by the elimination of hydrogen gas with simultaneous reduction of the metal center. [ 9 ]
The electron back donation into the H–H σ*-orbital to cleave the H–H bond causes electron-rich metals to favor this reaction. [ 9 ] The concerted mechanism produces a cis dihydride, while the stereochemistry of the other oxidative addition pathways do not usually produce cis adducts.
Some oxidative additions proceed analogously to the well known bimolecular nucleophilic substitution reactions in organic chemistry . Nucleophilic attack by the metal center at the less electronegative atom in the substrate leads to cleavage of the R–X bond, to form an [M–R] + species. This step is followed by rapid coordination of the anion to the cationic metal center. For example, reaction of a square planar complex with methyl iodide :
This mechanism is often assumed in the addition of polar and electrophilic substrates, such as alkyl halides and halogens . [ 2 ]
The ionic mechanism of oxidative addition is similar to the S N 2 type in that it involves the stepwise addition of two distinct ligand fragments. The key difference being that ionic mechanisms involve substrates which are dissociated in solution prior to any interactions with the metal center. An example of ionic oxidative addition is the addition of hydrogen chloride . [ 2 ]
In addition to undergoing S N 2-type reactions, alkyl halides and similar substrates can add to a metal center via a radical mechanism, although some details remain controversial. [ 2 ] Reactions which are generally accepted to proceed by a radical mechanism are known however. One example was proposed by Lednor and co-workers. [ 10 ]
Oxidative addition and reductive elimination are invoked in many catalytic processes in homogeneous catalysis , e.g., hydrogenations , hydroformylations , hydrosilylations , etc. [ 5 ] Cross-coupling reactions like the Suzuki coupling , Negishi coupling , and the Sonogashira coupling also proceed by oxidative addition. [ 11 ] [ 12 ] | https://en.wikipedia.org/wiki/Oxidative_addition |
Oxidative carbonylation is a class of reactions that use carbon monoxide in combination with an oxidant to generate esters and carbonate esters . These transformations utilize transition metal complexes as homogeneous catalysts . [ 1 ] Many of these reactions employ palladium catalysts. Mechanistically, these reactions resemble the Wacker process .
Oxidative carbonylation, using palladium-based catalysts, allows certain alkenes to be converted into homologated esters:
Such reactions are assumed to proceed by the insertion of the alkene into the Pd(II)-CO 2 Me bond of a metallacarboxylic ester followed by beta-hydride elimination (Me = CH 3 ).
Arylboronic acids react with Pd(II) compounds to give Pd(II)-aryl species, which undergo carbonylation to give Pd(II)-C(O)aryl. These benzyl-Pd intermediates are intercepted by alkenes, which insert. Subsequent beta-hydride elimination gives the arylketone. [ 1 ]
The conversion of methanol to dimethylcarbonate by oxidative carbonylation is economically competitive with phosgenation . This reaction is practiced commercially using Cu(I) catalysts: [ 2 ]
The preparation of dimethyl oxalate by oxidative carbonylation has also attracted commercial interest. It requires only C1 precursors : [ 3 ] | https://en.wikipedia.org/wiki/Oxidative_carbonylation |
Oxidative coupling in chemistry is a coupling reaction of two molecular entities through an oxidative process . Usually oxidative couplings are catalysed by a transition metal complex like in classical cross-coupling reactions, although the underlying mechanism is different due to the oxidation process that requires an external (or internal) oxidant. [ 1 ] [ 2 ] Many such couplings utilize dioxygen as the stoichiometric oxidant but proceed by electron transfer . [ 3 ]
Many oxidative couplings generate new C-C bonds. Early examples involve coupling of terminal alkynes: [ 4 ]
In oxidative aromatic coupling the reactants are electron-rich aromatic compounds . Typical substrates are phenols and typical catalysts are copper and iron compounds and enzymes, [ 6 ] although Scholl demonstrated that high heat and a Lewis acid suffice. The first reported synthetic application dates back to 1868 with Julius Löwe and the synthesis of ellagic acid by heating gallic acid with arsenic acid or silver oxide . [ 7 ] Another reaction is the synthesis of 1,1'-Bi-2-naphthol from 2-naphthol by iron chloride , discovered in 1873 by Alexander Dianin [ 8 ] ( S )-BINOL can be prepared directly from an asymmetric oxidative coupling of 2-naphthol with copper(II) chloride . [ 9 ]
Coupling reactions involving methane are highly sought, related to C1 chemistry because C 2 derivatives are far more valuable than methane. The oxidative coupling of methane gives ethylene: [ 10 ] [ 11 ]
The oxygen evolution reaction entails, in effect, the oxidative coupling of water molecules to give O 2 . | https://en.wikipedia.org/wiki/Oxidative_coupling |
The oxidative coupling of methane ( OCM ) is a potential chemical reaction studied in the 1980s for the direct conversion of natural gas , primarily consisting of methane , into value-added chemicals. Although the reaction would have strong economics if practicable, no effective catalysts are known, and thermodynamic arguments suggest none can exist.
The principal desired product of OCM is ethylene , the world's largest commodity chemical and the chemical industry's fundamental building block. While converting methane to ethylene would offer enormous economic benefits, it is a major scientific challenge. Thirty years of research failed to produce a commercial OCM catalyst, preventing this process from commercial applications.
Ethylene derivatives are found in food packaging, eyeglasses, cars, medical devices, lubricants, engine coolants and liquid crystal displays. Ethylene production by steam cracking consumes large amounts of energy and uses oil and natural gas fractions such as naphtha and ethane .
The oxidative coupling of methane to ethylene is written below: [ 1 ] [ 2 ]
The reaction is exothermic (∆H = -280 kJ/mol) and occurs at high temperatures (750–950 ˚C). [ 3 ] In the reaction, methane ( CH 4 ) is activated heterogeneously on the catalyst surface, forming methyl free radicals , which then couple in the gas phase to form ethane ( C 2 H 6 ). The ethane subsequently undergoes dehydrogenation to form ethylene ( C 2 H 4 ). The yield of the desired C 2 products is reduced by non-selective reactions of methyl radicals with the surface and oxygen in the gas phase, which produce (undesirable) carbon monoxide and carbon dioxide.
Direct conversion of methane into other useful products is one of the most challenging subjects to be studied in heterogeneous catalysis . [ 4 ] Methane activation is difficult because of its thermodynamic stability with a noble gas like electronic configuration . The tetrahedral arrangement of strong C–H bonds (435 kJ/mol) offer no functional group , magnetic moments or polar distributions to undergo chemical attack. This makes methane less reactive than nearly all of its conversion products, limiting efficient utilization of natural gas, the world's most abundant petrochemical resource.
The economic promise of OCM has attracted significant industrial interest. In the 1980s and 1990s multiple research efforts were pursued by academic investigators and petrochemical companies. Hundreds of catalysts have been tested, and several promising candidates were extensively studied. Researchers were unable to achieve the required chemoselectivity for economic operation. Instead of producing ethylene, the majority of methane was non-selectively oxidized to carbon dioxide .
The lack of selectivity was related to the poor C-H activation of known catalysts, requiring high reaction temperatures (750 ˚C and 950 ˚C) to activate the C-H bond. This high reaction temperature establishes a secondary gas-phase reaction mechanism pathway, whereby the desired reaction of methyl radical coupling to C 2 products (leading to ethylene) strongly competes with CO x side reactions. [ 3 ]
The high temperature also presents a challenge for the reaction engineering. Among the process engineering challenges are the requirements for expensive metallurgy , lack of industry experience with high temperature catalytic processes and the potential need for new reactor design to manage heat transfer efficiently. [ 5 ]
Labinger postulated an inherent limit to OCM selectivity, concluding that "expecting substantial improvements in the OCM performance might not be wise". [ 6 ] Labinger's argument, later demonstrated experimentally by Mazanec et al., is based on the mechanism of methane activation, which is a radical mechanism, forming H and CH3 radicals by the homolytic cleavage of the C-H bond. Ethylene and ethane that are proposed products have C-H bonds of similar strength. Thus, any catalyst that can activate methane can also activate the products. The yield of ethylene (and/or ethane) is limited by the relative rates of the methane and ethylene reactions, and these rates are very similar. Reactions of the products lead to higher homologues, and eventually to aromatics and coke. The same limitation applies to direct pyrolysis of methane, which is also a radical process. [ 7 ] Nevertheless, some recent work have shown that the mechanism of the OCM could be initiated by an heterolytic cleavage of the C-H bond on magnesium oxide in the presence of O 2 atmosphere. [ 8 ]
Eventually, the inability to discover a selective catalyst led to a gradual loss of interest in OCM. Beginning in the mid-1990s, research activity in this area began to decline significantly, as evidenced by the decreasing number of patents filed and peer-reviewed publications. The research company Siluria attempted to develop a commercially viable OCM process, but did not succeed. The company sold their OCM technology to McDermott in 2019. [ 9 ] | https://en.wikipedia.org/wiki/Oxidative_coupling_of_methane |
Oxidative deamination is a form of deamination that generates α-keto acids and other oxidized products from amine-containing compounds, and occurs primarily in the liver . [ 1 ] Oxidative deamination is stereospecific, meaning it contains different stereoisomers as reactants and products; this process is either catalyzed by L or D- amino acid oxidase and L-amino acid oxidase is present only in the liver and kidney. [ 2 ] Oxidative deamination is an important step in the catabolism of amino acids, generating a more metabolizable form of the amino acid, and also generating ammonia as a toxic byproduct. The ammonia generated in this process can then be neutralized into urea via the urea cycle .
Much of the oxidative deamination occurring in cells involves the amino acid glutamate , which can be oxidatively deaminated by the enzyme glutamate dehydrogenase (GDH), using NAD or NADP as a coenzyme . This reaction generates α-ketoglutarate (α-KG) and ammonia. Glutamate can then be regenerated from α-KG via the action of transaminases or aminotransferase, which catalyze the transfer of an amino group from an amino acid to an α-keto acid. In this manner, an amino acid can transfer its amine group to glutamate, after which GDH can then liberate ammonia via oxidative deamination. This is a common pathway during amino acid catabolism. [ 3 ]
Another enzyme responsible for oxidative deamination is monoamine oxidase , which catalyzes the deamination of monoamines via addition of oxygen. This generates the corresponding ketone- or aldehyde-containing form of the molecule, and generates ammonia. Monoamine oxidases MAO-A and MAO-B play vital roles in the degradation and inactivation of monoamine neurotransmitters such as serotonin and epinephrine . [ 4 ] Monoamine oxidases are important drug targets, targeted by MAO inhibitors (MAOIs) such as selegiline . Glutamate dehydrogenase play an important role in oxidative deamination.
This biochemistry article is a stub . You can help Wikipedia by expanding it . | https://en.wikipedia.org/wiki/Oxidative_deamination |
Silver nanoparticles (AgNPs) act primarily through a process known as oxidative dissolution , wherein Ag+ ions are released through an oxidative mechanism. AgNPs have potentially vast applications within the fields of medicine, science, and food and drug industries due to their antimicrobial properties, low cytotoxicity in humans, and inexpensive cost.
Silver is stable in water and needs an oxidizing element to achieve oxidative dissolution. When oxidizing agents such as hydrogen peroxide or oxygen are present, they dissolute AgNPs to release Ag + . The release of Ag + leads to creation of reactive oxygen species ( ROS ) inside cells, which can further dissolute the nanoparticles . Some nano silver particles develop protective Ag 3 OH surface groups [ 1 ] and it is thought that dissolution removes these groups and forms oxygen radicals, which attenuate reactivity of the AgNPs by entering into the lattice to form a highly stable Ag 6 O octahedral structure. [ 1 ] It has been thought AgNP efficacy can mainly be attributed to shape, as nanoprisms and naorods have proven more active than nanospheres because they possess more highly exposed facets , thus leading to a faster release of Ag+ ions.
Environmental factors that play a role in particle dissolution:
AgNPs are synthesized using microwave irradiation , [ 4 ] gamma irradiation [ 6 ] UV activation, [ 7 ] or conventional heating [ 8 ] of the precursor silver nitrate, AgNO 3 using an alginate solution as a stabilizing and reducing agent. [ 4 ] [ 9 ] The carboxyl or hydroxyl groups on the alginate reagent form complexes during the synthesis of the AgNPs that stabilize the reaction. [ 4 ] Nanoparticle size and shape can be specified by changing the ratio of alginate to silver nitrate used and/or the pH . [ 4 ] A coating such as PVP may be added to the nanoparticles by heating and subsequent slow cooling . [ 3 ]
Stopped-flow spectrometry has been used to characterize the chemical mechanism and kinetics of AgNPs. Oxidative dissolution of AgNPs has been shown to be a first order reaction with respect to both silver and hydrogen peroxide and is independent of particle size. [ 5 ]
Antibacterial , [ 10 ] [ 11 ] antiviral [ 12 ] and anti-fungal [ 13 ] properties have been investigated in response to AgNP dissolution . Antibacterial activities of AgNPs are much stronger in oxygenic conditions than anoxic conditions. [ 10 ] [ 14 ] [ 15 ] [ 16 ] [ 17 ] Through their oxidative dissolution in biological systems, AgNPs can target important biomolecules such as “ DNA , peptides , and cofactors ” as well as absorb into nonspecific moieties and simultaneously disrupt several metabolic pathways . [ 10 ] They have been known to act as a bridging agent between thiols , to have affinity for organic amines and phosphates . [ 10 ] The combination of silver ions ’ reaction with biomolecules with oxidative stress , ultimately leads to toxicity in biological environment. [ 18 ]
Oxidative dissolution of AgNPs, which gives rise to Ag + , potentially inhibits nitrification within Ammonia oxidizing bacteria . A key step in nitrification is the oxidation of ammonia to hydroxylamine (NH 2 OH) catalyzed by the enzyme ammonia monooxyganase (AMO). [ 19 ] The enzymatic activity of AMO is highly vulnerable to interference due to its intracytoplasmic location and its abundance of copper . It is speculated that Ag + ions from AgNPs interfere with AMO's copper bonds by replacing copper with Ag + causing a decrease in enzymatic activity, and thus nitrification. [ 20 ] | https://en.wikipedia.org/wiki/Oxidative_dissolution_of_silver_nanoparticles |
Oxidative stress reflects an imbalance between the systemic manifestation of reactive oxygen species and a biological system's ability to readily detoxify the reactive intermediates or to repair the resulting damage. [ 1 ] Disturbances in the normal redox state of cells can cause toxic effects through the production of peroxides and free radicals that damage all components of the cell, including proteins , lipids , and DNA . Oxidative stress from oxidative metabolism causes base damage, as well as strand breaks in DNA . Base damage is mostly indirect and caused by the reactive oxygen species generated, e.g., O − 2 ( superoxide radical), OH ( hydroxyl radical) and H 2 O 2 ( hydrogen peroxide ). [ 2 ] Further, some reactive oxidative species act as cellular messengers in redox signaling. Thus, oxidative stress can cause disruptions in normal mechanisms of cellular signaling . [ citation needed ]
In humans, oxidative stress is thought to be involved in the development of attention deficit hyperactivity disorder , [ 3 ] cancer , [ 4 ] Parkinson's disease , [ 5 ] Lafora disease , [ 6 ] Alzheimer's disease , [ 7 ] atherosclerosis , [ 8 ] heart failure , [ 9 ] myocardial infarction , [ 10 ] [ 11 ] fragile X syndrome , [ 12 ] sickle-cell disease , [ 13 ] lichen planus , [ 14 ] vitiligo , [ 15 ] autism , [ 16 ] infection , chronic fatigue syndrome , [ 17 ] and depression ; [ 18 ] however, reactive oxygen species can be beneficial, as they are used by the immune system as a way to attack and kill pathogens . [ 19 ] Oxidative stress due to noise was estimated at cell level using model of growing lymphocytes. Exposure of sound with frequency 1 KHz and intensity 110 dBA for 4 hours and eight hours per day may induce oxidative stress in growing lymphocytes causing the difference in viable cell count. However the catalase activity depends on duration of exposure. In case of noise exposure of 8 hours per day, it declines significantly as compared to noise exposure of 4 hours per day. [ 20 ]
Short-term oxidative stress may also be important in prevention of aging by induction of a process named mitohormesis , [ 21 ] and is required to initiate stress response processes in plants. [ 22 ]
Chemically, oxidative stress is associated with increased production of oxidizing species or a significant decrease in the effectiveness of antioxidant defenses, such as glutathione . [ 23 ] The effects of oxidative stress depend upon the size of these changes, with a cell being able to overcome small perturbations and regain its original state. However, more severe oxidative stress can cause cell death, and even moderate oxidation can trigger apoptosis , while more intense stresses may cause necrosis . [ 24 ]
Production of reactive oxygen species is a particularly destructive aspect of oxidative stress. Such species include free radicals and peroxides . Some of the less reactive of these species (such as superoxide ) can be converted by oxidoreduction reactions with transition metals or other redox cycling compounds (including quinones ) into more aggressive radical species that can cause extensive cellular damage. [ 25 ] Most long-term effects are caused by damage to DNA. [ 26 ] DNA damage induced by ionizing radiation is similar to oxidative stress, and these lesions have been implicated in aging and cancer. Biological effects of single-base damage by radiation or oxidation, such as 8-oxoguanine and thymine glycol , have been extensively studied. Recently the focus has shifted to some of the more complex lesions. Tandem DNA lesions are formed at substantial frequency by ionizing radiation and metal- catalyzed H 2 O 2 reactions. Under anoxic conditions , the predominant double-base lesion is a species in which C8 of guanine is linked to the 5-methyl group of an adjacent 3'-thymine (G[8,5- Me]T). [ 27 ] Most of these oxygen -derived species are produced by normal aerobic metabolism . Normal cellular defense mechanisms destroy most of these. Repair of oxidative damages to DNA is frequent and ongoing, largely keeping up with newly induced damages. In rat urine, about 74,000 oxidative DNA adducts per cell are excreted daily. [ 28 ] There is also a steady state level of oxidative damages in the DNA of a cell. There are about 24,000 oxidative DNA adducts per cell in young rats and 66,000 adducts per cell in old rats. [ 28 ] Likewise, any damage to cells is constantly repaired. However, under the severe levels of oxidative stress that cause necrosis, the damage causes ATP depletion, preventing controlled apoptotic death and causing the cell to simply fall apart. [ 29 ] [ 30 ]
Polyunsaturated fatty acids , particularly arachidonic acid and linoleic acid , are primary targets for free radical and singlet oxygen oxidations. For example, in tissues and cells, the free radical oxidation of linoleic acid produces racemic mixtures of 13-hydroxy-9 Z ,11 E -octadecadienoic acid, 13-hydroxy-9 E ,11 E -octadecadienoic acid, 9-hydroxy-10 E ,12- E -octadecadienoic acid (9-EE-HODE), and 11-hydroxy-9 Z ,12- Z -octadecadienoic acid as well as 4-Hydroxynonenal while singlet oxygen attacks linoleic acid to produce (presumed but not yet proven to be racemic mixtures of) 13-hydroxy-9 Z ,11 E -octadecadienoic acid, 9-hydroxy-10 E ,12- Z -octadecadienoic acid, 10-hydroxy-8 E ,12 Z -octadecadienoic acid, and 12-hydroxy-9 Z -13- E -octadecadienoic (see 13-Hydroxyoctadecadienoic acid and 9-Hydroxyoctadecadienoic acid ). [ 31 ] [ 32 ] [ 33 ] Similar attacks on arachidonic acid produce a far larger set of products including various isoprostanes , hydroperoxy- and hydroxy- eicosatetraenoates, and 4-hydroxyalkenals. [ 32 ] [ 34 ] While many of these products are used as markers of oxidative stress, the products derived from linoleic acid appear far more predominant than arachidonic acid products and therefore easier to identify and quantify in, for example, atheromatous plaques. [ 35 ] Certain linoleic acid products have also been proposed to be markers for specific types of oxidative stress. For example, the presence of racemic 9-HODE and 9-EE-HODE mixtures reflects free radical oxidation of linoleic acid whereas the presence of racemic 10-hydroxy-8 E ,12 Z -octadecadienoic acid and 12-hydroxy-9 Z -13- E -octadecadienoic acid reflects singlet oxygen attack on linoleic acid. [ 33 ] [ 31 ] In addition to serving as markers, the linoleic and arachidonic acid products can contribute to tissue and/or DNA damage but also act as signals to stimulate pathways which function to combat oxidative stress. [ 32 ] [ 36 ] [ 37 ] [ 38 ] [ 39 ]
Table adapted from. [ 40 ] [ 41 ] [ 42 ]
One source of reactive oxygen under normal conditions in humans is the leakage of activated oxygen from mitochondria during oxidative phosphorylation . E. coli mutants that lack an active electron transport chain produce as much hydrogen peroxide as wild-type cells, indicating that other enzymes contribute the bulk of oxidants in these organisms. [ 43 ] One possibility is that multiple redox-active flavoproteins all contribute a small portion to the overall production of oxidants under normal conditions. [ 44 ] [ 45 ]
Other enzymes capable of producing superoxide are xanthine oxidase , NADPH oxidases and cytochromes P450 . Hydrogen peroxide is produced by a wide variety of enzymes including several oxidases. Reactive oxygen species play important roles in cell signalling, a process termed redox signaling . Thus, to maintain proper cellular homeostasis , a balance must be struck between reactive oxygen production and consumption. [ citation needed ]
The best studied cellular antioxidants are the enzymes superoxide dismutase (SOD), catalase , and glutathione peroxidase . Less well studied (but probably just as important) enzymatic antioxidants are the peroxiredoxins and the recently discovered sulfiredoxin . Other enzymes that have antioxidant properties (though this is not their primary role) include paraoxonase, glutathione-S transferases, and aldehyde dehydrogenases. [ citation needed ]
The amino acid methionine is prone to oxidation, but oxidized methionine can be reversible. Oxidation of methionine is shown to inhibit the phosphorylation of adjacent Ser/Thr/Tyr sites in proteins. [ 46 ] This gives a plausible mechanism for cells to couple oxidative stress signals with cellular mainstream signaling such as phosphorylation.
Oxidative stress is suspected to be important in neurodegenerative diseases including Lou Gehrig's disease (aka MND or ALS), Parkinson's disease , Alzheimer's disease , Huntington's disease , depression , multiple sclerosis and multiple system atrophy . [ 47 ] [ 48 ] It is also indicated in Neurodevelopmental conditions such as Autism Spectrum Disorder . [ 49 ] Indirect evidence via monitoring biomarkers such as reactive oxygen species, and reactive nitrogen species production indicates oxidative damage may be involved in the pathogenesis of these diseases, [ 50 ] [ 51 ] while cumulative oxidative stress with disrupted mitochondrial respiration and mitochondrial damage are related to Alzheimer's disease, Parkinson's disease, and other neurodegenerative diseases. [ 52 ]
Oxidative stress is thought to be linked to certain cardiovascular disease , since oxidation of LDL in the vascular endothelium is a precursor to plaque formation. Oxidative stress also plays a role in the ischemic cascade due to oxygen reperfusion injury following hypoxia . This cascade includes both strokes and heart attacks . Oxidative stress has also been implicated in chronic fatigue syndrome (ME/CFS). [ 53 ] Oxidative stress also contributes to tissue injury following irradiation and hyperoxia , as well as in diabetes. In hematological cancers, such as leukemia, the impact of oxidative stress can be bilateral. Reactive oxygen species can disrupt the function of immune cells, promoting immune evasion of leukemic cells. On the other hand, high levels of oxidative stress can also be selectively toxic to cancer cells. [ 54 ] [ 55 ]
Oxidative stress is likely to be involved in age-related development of cancer. The reactive species produced in oxidative stress can cause direct damage to the DNA and are therefore mutagenic , and it may also suppress apoptosis and promote proliferation, invasiveness and metastasis . [ 4 ] Infection by Helicobacter pylori which increases the production of reactive oxygen and nitrogen species in human stomach is also thought to be important in the development of gastric cancer . [ 56 ]
Oxidative stress can cause DNA damage in neurons. [ 57 ] In neuronal progenitor cells , DNA damage is associated with increased secretion of amyloid beta proteins Aβ40 and Aβ42. [ 57 ] This association supports the existence of a causal relationship between oxidative DNA damage and Aβ accumulation and suggests that oxidative DNA damage may contribute to Alzheimer's disease (AD) pathology. [ 57 ] AD is associated with an accumulation of DNA damage (double-strand breaks) in vulnerable neuronal and glial cell populations from early stages onward, [ 58 ] and DNA double-strand breaks are increased in the hippocampus of AD brains compared to non-AD control brains. [ 59 ]
The use of antioxidants to prevent some diseases is controversial. [ 60 ] In a high-risk group like smokers, high doses of beta carotene increased the rate of lung cancer since high doses of beta-carotene in conjunction of high oxygen tension due to smoking results in a pro-oxidant effect and an antioxidant effect when oxygen tension is not high. [ 61 ] [ 62 ] In less high-risk groups, the use of vitamin E appears to reduce the risk of heart disease . [ 63 ] However, while consumption of food rich in vitamin E may reduce the risk of coronary heart disease in middle-aged to older men and women, using vitamin E supplements also appear to result in an increase in total mortality, heart failure, and hemorrhagic stroke . The American Heart Association therefore recommends the consumption of food rich in antioxidant vitamins and other nutrients, but does not recommend the use of vitamin E supplements to prevent cardiovascular disease. [ 64 ] In other diseases, such as Alzheimer's , the evidence on vitamin E supplementation is also mixed. [ 65 ] [ 66 ] Since dietary sources contain a wider range of carotenoids and vitamin E tocopherols and tocotrienols from whole foods, ex post facto epidemiological studies can have differing conclusions than artificial experiments using isolated compounds. AstraZeneca 's radical scavenging nitrone drug NXY-059 shows some efficacy in the treatment of stroke. [ 67 ]
Oxidative stress (as formulated in Denham Harman 's free-radical theory of aging ) is also thought to contribute to the aging process. While there is good evidence to support this idea in model organisms such as Drosophila melanogaster and Caenorhabditis elegans , [ 68 ] [ 69 ] recent evidence from Michael Ristow 's laboratory suggests that oxidative stress may also promote life expectancy of Caenorhabditis elegans by inducing a secondary response to initially increased levels of reactive oxygen species. [ 70 ] The situation in mammals is even less clear. [ 71 ] [ 72 ] [ 73 ] Recent epidemiological findings support the process of mitohormesis , but a 2007 meta-analysis finds that in studies with a low risk of bias (randomization, blinding, follow-up), some popular antioxidant supplements (vitamin A, beta carotene, and vitamin E) may increase mortality risk (although studies more prone to bias reported the reverse). [ 74 ]
The USDA removed the table showing the Oxygen Radical Absorbance Capacity (ORAC) of Selected Foods Release 2 (2010) table due to the lack of evidence that the antioxidant level present in a food translated into a related antioxidant effect in the body. [ 75 ]
Metals such as iron , copper , chromium , vanadium , and cobalt are capable of redox cycling in which a single electron may be accepted or donated by the metal. This action catalyzes production of reactive radicals and reactive oxygen species. [ 76 ] The presence of such metals in biological systems in an uncomplexed form (not in a protein or other protective metal complex) can significantly increase the level of oxidative stress. These metals are thought to induce Fenton reactions and the Haber-Weiss reaction, in which hydroxyl radical is generated from hydrogen peroxide. [ 77 ] The hydroxyl radical then can modify amino acids. For example, meta- tyrosine and ortho- tyrosine form by hydroxylation of phenylalanine . Other reactions include lipid peroxidation and oxidation of nucleobases. Metal-catalyzed oxidations also lead to irreversible modification of arginine, lysine, proline, and threonine. Excessive oxidative-damage leads to protein degradation or aggregation. [ 78 ] [ 79 ]
The reaction of transition metals with proteins oxidated by reactive oxygen or nitrogen species can yield reactive products that accumulate and contribute to aging and disease. For example, in Alzheimer's patients, peroxidized lipids and proteins accumulate in lysosomes of the brain cells. [ 80 ]
Certain organic compounds in addition to metal redox catalysts can also produce reactive oxygen species. One of the most important classes of these is the quinones . Quinones can redox cycle with their conjugate semiquinones and hydroquinones , in some cases catalyzing the production of superoxide from dioxygen or hydrogen peroxide from superoxide. [ citation needed ]
The immune system uses the lethal effects of oxidants by making the production of oxidizing species a central part of its mechanism of killing pathogens; with activated phagocytes producing both reactive oxygen and nitrogen species. These include superoxide (•O − 2 ) , nitric oxide (•NO) and their particularly reactive product, peroxynitrite (ONOO-). [ 81 ] Although the use of these highly reactive compounds in the cytotoxic response of phagocytes causes damage to host tissues, the non-specificity of these oxidants is an advantage since they will damage almost every part of their target cell. [ 42 ] This prevents a pathogen from escaping this part of immune response by mutation of a single molecular target.
Sperm DNA fragmentation appears to be an important factor in the cause of male infertility , since men with high DNA fragmentation levels have significantly lower odds of conceiving. [ 82 ] Oxidative stress is the major cause of DNA fragmentation in spermatozoa . [ 82 ] A high level of the oxidative DNA damage 8-oxo-2'-deoxyguanosine is associated with abnormal spermatozoa and male infertility. [ 83 ]
The great oxygenation event began with the biologically induced appearance of oxygen in the Earth's atmosphere about 2.45 billion years ago. The rise of oxygen levels due to cyanobacterial photosynthesis in ancient microenvironments was probably highly toxic to the surrounding biota. Under these conditions, the selective pressure of oxidative stress is thought to have driven the evolutionary transformation of an archaeal lineage into the first eukaryotes . [ 84 ] Oxidative stress might have acted in synergy with other environmental stresses (such as ultraviolet radiation and/or desiccation ) to drive this selection. Selective pressure for efficient repair of oxidative DNA damages may have promoted the evolution of eukaryotic sex involving such features as cell- cell fusions , cytoskeleton -mediated chromosome movements and emergence of the nuclear membrane . [ 84 ] Thus, the evolution of meiotic sex and eukaryogenesis may have been inseparable processes that evolved in large part to facilitate repair of oxidative DNA damages. [ 84 ] [ 85 ] [ 86 ]
It has been proposed that oxidative stress may play a major role in determining cardiac complications in COVID-19 . [ 87 ] [ 88 ] | https://en.wikipedia.org/wiki/Oxidative_stress |
Oxide dispersion strengthened alloys ( ODS ) are alloys that consist of a metal matrix with small oxide particles dispersed within it. They have high heat resistance, strength, and ductility . Alloys of nickel are the most common but includes iron aluminum alloys. [ 1 ]
Applications include high temperature turbine blades and heat exchanger tubing, [ 2 ] while steels are used in nuclear applications. [ 3 ] ODS materials are used on spacecraft to protect the vehicle, especially during re-entry . Noble metal ODS alloys, for example, platinum-based alloys, are used in glass production.
When it comes to re-entry at hypersonic speeds, the properties of gases change dramatically. Shock waves that can cause serious damage on any structure are created. At these speeds and temperatures, oxygen becomes aggressive.
Oxide dispersion strengthening is based on incoherency of the oxide particles within the lattice of the material. Coherent particles have a continuous lattice plane from the matrix to the particles whereas incoherent particles do not have this continuity and therefore both lattice planes end at the interface. This mismatch in interfaces results in a high interfacial energy, which impedes dislocation. [ 4 ] The oxide particles instead are stable in the matrix, which helps prevent creep. Particle stability implies little dimensional change, embrittlement, effects on properties, stable particle spacing, and general resistance to change at high temperatures. [ 5 ]
Since the oxide particles are incoherent, dislocations can only overcome the particles by climb . If instead the particles are semi-coherent or coherent with the lattice, dislocations can simply cut the particles by a more favourable process that requires less energy called dislocation glide or by Orowan bowing between particles, both of which are athermal mechanisms. Dislocation climb is a diffusional process, which is less energetically favourable, and mostly occurs at higher temperatures that provide enough energy to advance via the addition and removal of atoms. [ 6 ] Because the particles are incoherent, glide mechanisms alone are not enough and the more energetically exhausting climb process is dominant, meaning that dislocations are stopped more effectively. Climb can occur either at the particle-dislocation interface (local climb) or by overcoming multiple particles at once (general climb). In local climb, the part of the dislocation that is between two particles stays in the glide plane while the rest of the dislocation is climbing along the surface of the particle. For general climb, the dislocations all come out the glide plane. General climb requires less energy because the mechanism decreases the dislocation line length which reduces the elastic strain energy and therefore is the common climb mechanism. [ 7 ] For γ’ volume fractions of 0.4 to 0.6 in nickel-based alloys, the threshold stress for local climb is only about 1.25 to 1.40 times higher than general climb. [ 8 ]
Dislocations are not limited to either all local or all general climb as the path that requires less energy is taken. Cooperative climb is an example of a more nuanced mechanism where a dislocation travels around a group of particles rather than climbing past each particle individually. McLean stated that the dislocation is most relaxed when climbing over multiple particles because of the skipping of some of the abrupt interfaces between segments in the glide plane to segments that travel along the particle surface. [ 9 ]
The presence of incoherent particles introduces a threshold stress (σ t ), since an additional stress will have to be applied for the dislocations to move past the oxides by climb. After overcoming a particle by climb, dislocations can remain pinned at the particle-matrix interface with an attractive phenomenon called interfacial pinning, [ 10 ] [ 11 ] which requires additional threshold stress to free a dislocation out of this pinning, which must be overcome for plastic deformation to occur. [ 12 ] This detachment phenomenon is a result of the interaction between the particle and the dislocation where total elastic strain energy is reduced. [ 13 ] Schroder and Arzt explain that the additional stress required is due to the relaxation caused by the reduction in the stress field as the dislocation climbs and accommodates the shear traction. [ 14 ] The following equations represent the strain rate and stress as a result of oxide introduction.
Strain Rate:
ϵ . = A ′ ( σ − σ t μ ) n {\displaystyle {\overset {.}{\epsilon }}=A'({\frac {\sigma -\sigma _{t}}{\mu }})^{n}}
Threshold Shear Stress:
τ t h = α 2 G b l {\displaystyle \tau _{th}={\frac {\alpha }{2}}{\frac {Gb}{l}}}
ODS steels creep properties are dependent on the characteristics of the oxide particles in the metal matrix, specifically their ability to prevent dislocation motion as well as the size and distribution of the particles. Hoelzer and coworkers showed that an alloy containing a homogeneous dispersion of 1-5 nm Y 2 Ti 2 O 7 nanoclusters has superior creep properties to an alloy with a heterogeneous dispersion of 5-20 nm nanoclusters of the same composition. [ 15 ]
ODS steels are commonly produced through ball-milling an oxide of interest (e.g. Y 2 O 3 , Al 2 O 3 ) with pre-alloyed metal powders followed by compression and sintering. It is believed that the oxides enter into solid solution with the metal during ball-milling and subsequently precipitate during the thermal treatment. This process seems simple but many parameters need to be carefully controlled to produce a successful alloy. Leseigneur and coworkers carefully controlled some of these parameters and achieved more consistent and better microstructures. [ 16 ] In this two step method the oxide is ball-milled for longer periods to ensure a homogeneous solid solution of the oxide. The powder is annealed at higher temperatures to begin a controlled nucleation of the oxide clusters. Finally the powder is again compressed and sintered to yield the final material.
NASA used ResonantAcoustic mixing and additive manufacturing to synthesize an alloy they termed GRX-810, which survived temperatures over 1,090 °C (1,990 °F). The alloy also featured improved strength, malleability, and durability. The printer dispersed oxide particles uniformly throughout the metal matrix. The alloy was identified using 30 simulations of thermodynamic modeling. [ 17 ] [ 18 ] [ 19 ]
Advantages:
Disadvantages: | https://en.wikipedia.org/wiki/Oxide_dispersion-strengthened_alloy |
The expansive force of rusting , which may be called oxide jacking or rust burst , is a phenomenon that can cause damage to structures made of stone, masonry, concrete or ceramics, and reinforced with metal components. A definition is "the displacement of building elements due to the expansion of iron and steel products as the metal rusts and becomes iron oxide". [ 1 ] Corrosion of other metals such as aluminum can also cause oxide jacking.
According to metallurgist Jack Harris, "Oxidation is usually accompanied by a net expansion so that when it occurs in a confined space stresses are generated in the metal component itself or in any surrounding medium such as stone or cement. So much energy is released by oxidation that the stresses generated are of sufficient magnitude to deform or fracture all known materials." [ 2 ]
As early as 1915, it was recognized that certain modern metal alloys are more susceptible to excessive oxidation when subjected to weathering than other metals. At that time, there was a trend to replace wrought iron fasteners with mild steel equivalents, which were less expensive. Unexpectedly, the mild steel fasteners failed in real world use much more quickly than anticipated, leading to a return to use of wrought iron in certain applications where length of service was important. [ 3 ]
In a 1987 article in New Scientist , Jack Harris reported that oxide jacking has caused significant damage to many historic structures in the United Kingdom, including St Paul's Cathedral , the British Museum and the Albert Memorial in London, Gloucester Cathedral , St. Margaret's Church in King's Lynn , Winchester Cathedral , and Blackburn Cathedral . [ 4 ]
Harris also wrote that oxide jacking also damaged the ancient Horses of Saint Mark on the exterior of St. Mark's Basilica in Venice . Expansive rusting of iron and steel bolts and reinforcements affected the structural integrity of the copper horse sculptures, which were relocated indoors and replaced with replicas. Poorly-designed early 20th-century renovations also led to oxide jacking damage to the Acropolis of Athens . [ 4 ]
In the United States, rusting of iron pegs inserted into holes in the stone entrance stair in order to support handrails resulted in cracking of the steps at the Basilica of the Sacred Heart in Notre Dame, Indiana . [ 5 ]
Oxide jacking damaged the terra cotta cornice on the Land Title Building in Philadelphia , designed in 1897 and expanded in 1902 by pioneer skyscraper architect Daniel Burnham . [ 6 ] The Land Title complex, with its two interconnected towers, is on the National Register of Historic Places . By 1922, experts on architectural terra cotta were warning that the rusting of embedded iron fasteners could cause decorative building components to fail. [ 7 ] This 1902 cornice is nearly 9 feet (2.7 m) high, projects 7 feet (2.1 m) from the facade of the building and is 465 feet (142 m) long. The cornice was stabilized, steel anchors subject to rusting were replaced with new stainless steel anchors, and the cornice was completely renovated. The project was completed in 1991. [ 6 ]
Flooding in 2007 damaged the modernist Farnsworth House in Plano, Illinois , designed in 1945 by Ludwig Mies van der Rohe , and now owned by the National Trust for Historic Preservation . Among the damage discovered by an architect inspecting the house in 2007 was oxide jacking at the corners of the house's steel framework. [ 8 ] The house flooded again in 2008.
Structures built of concrete and reinforced with metal rebar are also subject to damage by oxide jacking. Expansion of corroded rebar causes spalling of the concrete. Structures exposed to a marine environment, or where salt is used for de-icing purposes, are especially susceptible to this type of damage. [ 4 ] This may also be caused by concrete having been installed without sufficient cover for the rebars, allowing moisture to reach the metal and cause oxidation.
Research in the 1960s showed that 22 percent of concrete bridge decks in Pennsylvania showed signs of spalling due to oxide jacking within four years of construction. Oxide jacking caused widespread damage to concrete council houses built in the United Kingdom in the post World War II era. [ 4 ]
According to an expert in the field, the problem resulted in "intensive worldwide research into the causes and repair of reinforcement corrosion, which in turn led to a vast output of research papers, conferences and publications on the subject." [ 9 ]
Countertop components fabricated out of granite and other natural stones are sometimes reinforced with metal rods inserted into grooves cut into the underside of the stone, and bonded in place with various resins. This procedure is called "rodding" by countertop fabricators. Most commonly, these rods will be placed near sink cutouts to prevent cracking of the brittle stone countertop during transportation and installation. [ 10 ] Data published by the Marble Institute of America shows that this technique results in a 600% increase in the deflection strength of the component. [ 11 ]
However, if a metal rod subject to oxidation or other forms of corrosion is used, and moisture from a sink or faucet reaches the rod, oxide jacking can crack the countertop directly above the rod. [ 12 ] Mild steel and some grades of aluminium rods are known to cause oxide jacking failures in granite countertops. Skilled stone repair professionals can disassemble the cracked stone, remove the metal rod, and reassemble the stone using various resins tinted to match the colors of the stone. [ 11 ] This type of problem can be prevented by using reinforcing rods made of stainless steel or fiberglass in the rodding procedure. [ 11 ] | https://en.wikipedia.org/wiki/Oxide_jacking |
An oxidizing agent (also known as an oxidant , oxidizer , electron recipient , or electron acceptor ) is a substance in a redox chemical reaction that gains or " accepts "/"receives" an electron from a reducing agent (called the reductant , reducer , or electron donor ). In other words, an oxidizer is any substance that oxidizes another substance. The oxidation state , which describes the degree of loss of electrons , of the oxidizer decreases while that of the reductant increases; this is expressed by saying that oxidizers "undergo reduction" and "are reduced" while reducers "undergo oxidation" and "are oxidized".
Common oxidizing agents are oxygen , hydrogen peroxide , and the halogens .
In one sense, an oxidizing agent is a chemical species that undergoes a chemical reaction in which it gains one or more electrons. In that sense, it is one component in an oxidation–reduction (redox) reaction. In the second sense, an oxidizing agent is a chemical species that transfers electronegative atoms, usually oxygen, to a substrate. Combustion , many explosives, and organic redox reactions involve atom-transfer reactions.
Electron acceptors participate in electron-transfer reactions . In this context, the oxidizing agent is called an electron acceptor and the reducing agent is called an electron donor . A classic oxidizing agent is the ferrocenium ion Fe(C 5 H 5 ) + 2 , which accepts an electron to form Fe(C 5 H 5 ) 2 . One of the strongest acceptors commercially available is " Magic blue ", the radical cation derived from N(C 6 H 4 -4-Br) 3 . [ 2 ]
Extensive tabulations of ranking the electron accepting properties of various reagents (redox potentials) are available, see Standard electrode potential (data page) .
In more common usage, an oxidizing agent transfers oxygen atoms to a substrate. In this context, the oxidizing agent can be called an oxygenation reagent or oxygen-atom transfer (OAT) agent. [ 3 ] Examples include MnO − 4 ( permanganate ), CrO 2− 4 ( chromate ), OsO 4 ( osmium tetroxide ), and especially ClO − 4 ( perchlorate ). Notice that these species are all oxides .
In some cases, these oxides can also serve as electron acceptors, as illustrated by the conversion of MnO − 4 to MnO 2− 4 ,ie permanganate to manganate .
The dangerous goods definition of an oxidizing agent is a substance that can cause or contribute to the combustion of other material. [ 4 ] By this definition some materials that are classified as oxidizing agents by analytical chemists are not classified as oxidizing agents in a dangerous materials sense. An example is potassium dichromate , which does not pass the dangerous goods test of an oxidizing agent.
The U.S. Department of Transportation defines oxidizing agents specifically. There are two definitions for oxidizing agents governed under DOT regulations. These two are Class 5 ; Division 5.1(a)1 and Class 5; Division 5.1(a)2. Division 5.1 "means a material that may, generally by yielding oxygen, cause or enhance the combustion of other materials." Division 5.(a)1 of the DOT code applies to solid oxidizers "if, when tested in accordance with the UN Manual of Tests and Criteria (IBR, see § 171.7 of this subchapter), its mean burning time is less than or equal to the burning time of a 3:7 potassium bromate/cellulose mixture." 5.1(a)2 of the DOT code applies to liquid oxidizers "if, when tested in accordance with the UN Manual of Tests and Criteria, it spontaneously ignites or its mean time for a pressure rise from 690 kPa to 2070 kPa gauge is less than the time of a 1:1 nitric acid (65 percent)/cellulose mixture." [ 5 ]
SO 3 Sulphur trioxide
H 2 SO 3 Sulphurous acid (In aqueous solution) | https://en.wikipedia.org/wiki/Oxidizing_agent |
In biochemistry , an oxidoreductase is an enzyme that catalyzes the transfer of electrons from one molecule, the reductant , also called the electron donor , to another, the oxidant , also called the electron acceptor . This group of enzymes usually utilizes NADP+ or NAD+ as cofactors . [ 1 ] [ 2 ] Transmembrane oxidoreductases create electron transport chains in bacteria, chloroplasts and mitochondria , including respiratory complexes I , II and III . Some others can associate with biological membranes as peripheral membrane proteins or be anchored to the membranes through a single transmembrane helix . [ 3 ]
For example, an enzyme that catalyzed this reaction would be an oxidoreductase:
In this example, A is the reductant (electron donor) and B is the oxidant (electron acceptor).
In biochemical reactions, the redox reactions are sometimes more difficult to see, such as this reaction from glycolysis :
In this reaction, NAD + is the oxidant (electron acceptor), and glyceraldehyde-3-phosphate is the reductant (electron donor).
Proper names of oxidoreductases are formed as " donor:acceptor oxidoreductase"; however, other names are much more common. [ citation needed ]
Oxidoreductases are classified as EC 1 in the EC number classification of enzymes. Oxidoreductases can be further classified into 21 subclasses: | https://en.wikipedia.org/wiki/Oxidoreductase |
In organic chemistry , an oxime is an organic compound belonging to the imines , with the general formula RR’C=N−OH , where R is an organic side-chain and R' may be hydrogen , forming an aldoxime , or another organic group , forming a ketoxime . O-substituted oximes form a closely related family of compounds. Amidoximes are oximes of amides ( R 1 C(=O)NR 2 R 3 ) with general structure R 1 C(=NOH)NR 2 R 3 .
Oximes are usually generated by the reaction of hydroxylamine with aldehydes ( R−CH=O ) or ketones ( RR’C=O ). The term oxime dates back to the 19th century, a combination of the words oxygen and imine . [ 1 ]
If the two side-chains on the central carbon are different from each other—either an aldoxime, or a ketoxime with two different "R" groups—the oxime can often have two different geometric stereoisomeric forms according to the E / Z configuration . An older terminology of syn and anti was used to identify especially aldoximes according to whether the R group was closer or further from the hydroxyl. Both forms are often stable enough to be separated from each other by standard techniques.
Oximes have three characteristic bands in the infrared spectrum , whose wavelengths corresponding to the stretching vibrations of its three types of bonds: 3600 cm −1 (O−H), 1665 cm −1 (C=N) and 945 cm −1 (N−O). [ 2 ]
In aqueous solution, aliphatic oximes are 10 2 - to 10 3 -fold more resistant to hydrolysis than analogous hydrazones. [ 3 ]
Oximes can be synthesized by condensation of an aldehyde or a ketone with hydroxylamine . The condensation of aldehydes with hydroxylamine gives aldoximes, and ketoximes are produced from ketones and hydroxylamine. In general, oximes exist as colorless crystals or as thick liquids and are poorly soluble in water. Therefore, oxime formation can be used for the identification of ketone or aldehyde functional groups.
Oximes can also be obtained from reaction of nitrites such as isoamyl nitrite with compounds containing an acidic hydrogen atom. Examples are the reaction of ethyl acetoacetate and sodium nitrite in acetic acid , [ 4 ] [ 5 ] the reaction of methyl ethyl ketone with ethyl nitrite in hydrochloric acid . [ 6 ] and a similar reaction with propiophenone , [ 7 ] the reaction of phenacyl chloride , [ 8 ] the reaction of malononitrile with sodium nitrite in acetic acid [ 9 ]
A conceptually related reaction is the Japp–Klingemann reaction .
The hydrolysis of oximes proceeds easily by heating in the presence of various inorganic acids , and the oximes decompose into the corresponding ketones or aldehydes, and hydroxylamines. The reduction of oximes by sodium metal, [ 10 ] sodium amalgam , hydrogenation , or reaction with hydride reagents produces amines . [ 11 ] Typically the reduction of aldoximes gives both primary amines and secondary amines; however, reaction conditions can be altered (such as the addition of potassium hydroxide in a 1/30 molar ratio) to yield solely primary amines. [ 12 ]
In general, oximes can be changed to the corresponding amide derivatives by treatment with various acids. This reaction is called Beckmann rearrangement . [ 13 ] In this reaction, a hydroxyl group is exchanged with the group that is in the anti position of the hydroxyl group. The amide derivatives that are obtained by Beckmann rearrangement can be transformed into a carboxylic acid by means of hydrolysis (base or acid catalyzed). Beckmann rearrangement is used for the industrial synthesis of caprolactam (see applications below).
The Ponzio reaction (1906) [ 14 ] concerning the conversion of m -nitrobenzaldoxime to m -nitrophenyldinitromethane using dinitrogen tetroxide was the result of research into TNT analogues: [ 15 ]
Gentler oxidants give mono-nitro compounds. [ 16 ]
In the Neber rearrangement certain oximes are converted to the corresponding alpha-amino ketones.
Oximes can be dehydrated using acid anhydrides to yield corresponding nitriles .
Certain amidoximes react with benzenesulfonyl chloride to make substituted ureas in the Tiemann rearrangement : [ 17 ] [ 18 ]
In their largest application, an oxime is an intermediate in the industrial production of caprolactam , a precursor to Nylon 6 . About half of the world's supply of cyclohexanone , more than a million tonnes annually, is converted to the oxime. In the presence of sulfuric acid catalyst , the oxime undergoes the Beckmann rearrangement to give the cyclic amide caprolactam: [ 19 ]
Oximes are commonly used as ligands and sequestering agents for metal ions. Dimethylglyoxime (dmgH 2 ) is a reagent for the analysis of nickel and a popular ligand in its own right. In the typical reaction, a metal reacts with two equivalents of dmgH 2 concomitant with ionization of one proton. Salicylaldoxime is a chelator in hydrometallurgy . [ 20 ]
Amidoximes such as polyacrylamidoxime can be used to capture trace amounts of uranium from sea water. [ 21 ] [ 22 ] In 2017 researchers announced a configuration that absorbed up to nine times as much uranyl as previous fibers without saturating. [ 23 ] | https://en.wikipedia.org/wiki/Oxime |
An oxo-Diels–Alder reaction (also called an oxa-Diels–Alder reaction ) is an organic reaction and a variation of the Diels–Alder reaction in which a suitable diene reacts with an aldehyde to form a dihydropyran ring. This reaction is of some importance to synthetic organic chemistry.
The oxo-DA reaction was first reported in 1949 [ 1 ] using a 2-methylpenta-1,3-diene and formaldehyde as reactants.
Asymmetric oxo-DA reactions (including catalytic reactions) are well known. [ 2 ] Many strategies rely on coordinating a chiral Lewis acid to the carbonyl group. | https://en.wikipedia.org/wiki/Oxo-Diels–Alder_reaction |
Oxo-degradation refers to the breakdown mechanism caused by heat, light or oxygen on plastics that contain additives that accelerate the process of breaking them into smaller fragments called microplastics . [ 1 ] These plastics contrast biodegradable or compostable plastics, which decompose at the molecular or polymer level. [ 2 ] Oxo-degradable plastics are currently banned in the EU, [ 3 ] but are still permitted in other jurisdictions such as the UK. [ 4 ]
The specific definitions are found in CEN (European Committee for Standardisation) Technical report CEN/TR 15351. "'Oxo-degradation' is degradation identified as resulting from oxidative cleavage of macromolecules". [ citation needed ] It describes ordinary plastics which abiotically degrade by oxidation in the open environment and create microplastics, but do not become biodegradable except over a very long period of time.
Oxo-degradable plastics are intended to fragment if they are introduced into the open environment as litter and should not be confused with plastics intended to biodegrade in the special conditions found in an industrial composting unit. These compostable plastics use an entirely different technology, but confusion is caused by the fact that they are so often referred to in discussions of oxo-degradable plastic.
Oxo-degradable plastic packaging has been promoted as a potential solution to plastic pollution, with claims that it can degrade over time. [ 5 ] However, questions have been raised regarding its actual performance and environmental impact. Some studies suggest that instead of fully biodegrading, oxo-degradable plastics tend to fragment into smaller pieces, including microplastics , which can persist in the environment. These microplastics may take longer to degrade than initially anticipated depending on environmental conditions. [ 6 ]
Concerns have also been raised about the potential effects of microplastics on ecosystems, as well as the risk of bioaccumulation in food chains, which could impact both human health and the environment. [ 7 ]
From a reuse and recycling perspective, oxo-degradable plastics are generally not considered suitable for long-term applications. They are designed to break down over time, making them less suitable for reuse. In addition, recyclers have expressed concerns that oxo-degradable plastics may reduce the quality and value of recycled materials. They are also difficult to detect and sort out in recycling streams, presenting challenges for recycling at scale. [ 8 ]
Regarding composting, oxo-degradable plastics typically do not meet the requirements of international composting standards, as their degradation process is slower than required, and plastic fragments can remain in the compos, which has led to concerns about their compatibility with composting systems and their potential to affect compost quality. [ 9 ]
Since 2017 there has been a move towards regulating or banning the use of oxo-degradable plastics, when the Ellen MacArthur Foundation published a statement supported by more than 150 organizations calling for a ban. [ 10 ]
Effective July 2021 oxo-degradable plastics have been banned in the EU with Directive 2019/904 (also known as the Single-Use Plastics Directive). Oxo-degradable plastics were particularly targeted; the rationale behind this focus was that oxo-degradable plastics often do not break down completely but instead fragment into microplastics, which persist in the environment and contribute to pollution. [ 3 ]
In December 2020 Symphony Environmental Technologies filed a lawsuit against the European Commission, arguing that the prohibition was arbitrary and unlawful. [ 11 ] In January 2024 the European Court of Justice dismissed the suit, ruling that none of the commission's actions had been improper. [ 12 ]
As of April 2022, Switzerland banned oxo-degradable plastics, bringing their rules in line with those in the EU. [ 13 ]
Although oxo-degradable plastics are not illegal in the US, the Federal Trade Commission (FTC) has taken the stance that oxo-degradable plastics cannot be called "degradable" or "biodegradable" without strong scientific evidence. In 2014 the FTC advised 14 firms to either remove their oxo-degradable claims or provide reliable scientific evidence. [ 14 ]
Scotland is contemplating a ban on oxo-degradable plastics. [ 15 ]
In October 2022, a comprehensive bill banning single-use plastics also banned plastics with pro-degradant additives such as oxo and photo degradable plastics. [ 16 ]
As of July 15, 2024, British Columbia businesses can no longer sell or distribute any packaging or single-use disposable products that contain oxo-degradable plastics. [ 17 ]
Oxo-degradable plastics were banned in South Australia in March 2022 and Western Australia in September 2023. [ 18 ]
Hong Kong banned the manufacture of oxo-degradable plastics as of April 22, 2024. [ 19 ]
UAE, Jordan, Saudi Arabia, Bahrain among others require the use of oxo-degradable plastic for disposable plastic bags. [ 20 ] | https://en.wikipedia.org/wiki/Oxo-degradation |
In organic chemistry , an oxocarbenium ion (alternatively spelled oxacarbenium ) is a chemical species characterized by a central sp 2 -hybridized atom of carbon , a substituent atom of oxygen , and an overall positive charge that is delocalized between the central carbon and oxygen atoms ( R 2 [CO] + R ). [ 1 ] An oxocarbenium ion is represented by two limiting resonance structures , one in the form of a carbenium ion with the positive charge on carbon ( >C + −O− ) and the other in the form of an oxonium species with the formal charge on oxygen ( >C=O + − ). As a resonance hybrid, the true structure falls between the two.
Compared to neutral carbonyl ( C=O ) compounds like ketones ( >C=O ) or esters , the carbenium ion form is a larger contributor to the structure. They are common reactive intermediates in the hydrolysis of glycosidic bonds , and are a commonly used strategy for chemical glycosylation . These ions have since been proposed as reactive intermediates in a wide range of chemical transformations, and have been utilized in the total synthesis of several natural products. In addition, they commonly appear in mechanisms of enzyme -catalyzed biosynthesis and hydrolysis of carbohydrates in nature. Anthocyanins are natural flavylium dyes, which are stabilized oxocarbenium compounds. Anthocyanins are responsible for the colors of a wide variety of common flowers such as pansies and edible plants such as eggplant and blueberry .
The best Lewis structure for an oxocarbenium ion contains an oxygen–carbon double bond , with the oxygen atom attached to an additional group and consequently taking on a formal positive charge. In the language of canonical structures (or "resonance"), the polarization of the π bond is described by a secondary carbocationic resonance form, with a formal positive charge on carbon (see above). In terms of frontier molecular orbital theory , the Lowest Unoccupied Molecular Orbital (LUMO) of the oxocarbenium ion is a π* orbital that has the large lobe on the carbon atom; the more electronegative oxygen contributes less to the LUMO. Consequently, in an event of a nucleophilic attack , the carbon is the electrophilic site . Compared to a ketone , the polarization of an oxocarbenium ion is accentuated: they more strongly resemble a "true" carbocation, and they are more reactive toward nucleophiles. In organic reactions, ketones are commonly activated by the coordination of a Lewis acid or Brønsted acid to the oxygen to generate an oxocarbenium ion as an intermediate.
Numerically, a typical partial charge (derived from Hartree-Fock computations) for the carbonyl carbon of a ketone R 2 C=O (like acetone) is δ+ = 0.51. With the addition of an acidic hydrogen to the oxygen atom to produce [R 2 C=OH] + , the partial charge increases to δ+ = 0.61. In comparison, the nitrogen analogues of ketones and oxocarbenium ions, imines ( R 2 C=NR ) and iminium ions ( [R 2 C=NRH] + ), respectively, have partial charges of δ+ = 0.33 and δ+ = 0.54, respectively. The order of partial positive charge on the carbonyl carbon is therefore imine < ketone < iminium < oxocarbenium.
This is also the order of electrophilicity for species containing C=X (X = O, NR) bonds. This order is synthetically significant and explains, for example, why reductive aminations are often best carried out at pH = 5 to 6 using sodium cyanoborohydride ( Na + [H 3 B(CN)] − ) or sodium triacetoxyborohydride ( Na + [HB(OAc) 3 ] − ) as a reagent. Bearing an electron-withdrawing group, sodium cyanoborohydride and sodium triacetoxyborohydride are poorer reducing agents than sodium borohydride, and their direct reaction with ketones is generally a slow and inefficient process. However, the iminium ion (but not the imine itself) formed in situ during a reductive amination reaction is a stronger electrophile than the ketone starting material and will react with the hydride source at a synthetically useful rate. Importantly, the reaction is conducted under mildly acidic conditions that protonate the imine intermediate to a significant extent, forming the iminium ion, while not being strongly acidic enough to protonate the ketone, which would form the even more electrophilic oxocarbenium ion. Thus, the reaction conditions and reagent ensure that amine is formed selectively from iminium reduction, instead of direct reduction of the carbonyl group (or its protonated form) to form an alcohol.
Formation of oxocarbenium ions can proceed through several different pathways. Most commonly, the oxygen of a ketone will bind to a Lewis Acid , which activates the ketone, making it a more effective electrophile. The Lewis acid can be a wide range of molecules, from a simple hydrogen atom to metal complexes. The remainder of this article will focus on alkyl oxocarbenium ions, however, where the atom added to the oxygen is a carbon. One way that this sort of ion will form is the elimination of a leaving group . In carbohydrate chemistry, this leaving group is often an ether or ester . An alternative to elimination is direct deprotonation of the molecule to form the ion, however, this can be difficult and require strong bases to achieve.
The stereochemistry involved in the reactions of five-membered rings can be predicted by an envelope transition state model. Nucleophiles favor addition from the "inside" of the envelope, or from the top of the figure on the right. The "inside" addition produces a results in a staggered conformation , rather than the eclipsed conformation that results from the "outside" addition. [ 2 ]
The transition state model for a six-membered oxocarbenium ring was proposed earlier in 1992 by Woods et al. [ 3 ] The general strategy for determining the stereochemistry of a nucleophilic addition to a six-membered ring follows a similar procedure to the case of the five-membered ring. The assumption that one makes for this analysis is that the ring is in the same conformation as cyclohexene , with three carbons and the oxygen in a plane with the two other carbon atome puckered out of the plane, with one above and one below (see the figure to the right). Based on the substituients present on the ring, the lowest energy conformation is determined, keeping in mind steric and stereoelectronic effects (see the section below for a discussion of stereoelectronic effects in oxocarbenium rings). Once this conformation is established, one can consider the nucleophilic addition. The addition will proceed through the low energy chair transition state, rather than the relatively high energy twist-boat. An example of this type of reaction can be seen below. The example also highlights how the stereoelectronic effect exerted by an electronegative substituent flips the lowest energy conformation and leads to opposite selectivity. [ 4 ]
In an alkene ring that does not contain an oxygen atom, any large substituent prefers to be in an equatorial position, in order to minimize steric effects . It has been observed in rings containing oxocarbenium ions that electronegative substituents prefer the axial or pseudo-axial positions. When the electronegative atom is in the axial position, its electron density can be donated through space to the positively charged oxygen atom in the ring. [ 5 ] This electronic interaction stabilizes the axial conformation. Hydroxyl groups, ethers and halogens are examples of substituents that exhibit this phenomenon. Stereoelectronic effects must be taken into consideration when determining the lowest energy conformation in the analysis for nucleophilic addition to an oxocarbenium ion. [ 4 ] [ 6 ]
In organic synthesis, vinyl oxocarbenium ions (structure on right) can be utilized in a wide range of cycloaddition reactions. They are commonly employed as dienophiles in the Diels–Alder reaction . An electron withdrawing ketone is often added to the dienophile to increase the rate of the reaction, [ 7 ] and these ketones are often converted to vinyl oxocarbenium ions during the reaction. [ 8 ] It is not clear that an oxocarbenium ion necessarily will form, but Roush and co-workers demonstrated the oxocarbenium intermediate in the cyclization shown below. Two products were observed in this reaction, which could only form if the oxocarbenium ring is present as an intermediate. [ 9 ] [4+3], [2+2], [3+2] and [5+2] cycloadditions with oxocarbenium intermediates have also been reported. [ 8 ]
Chiral oxocarbenium ions have been exploited to carry out highly diastereoselective and enantioselective acetate aldol
addition reactions. [ 10 ] The oxocarbenium ion is used as an electrophile in the reaction. When the methyl group increases in size, the diastereoselevtivity increases.
Oxocarbenium ions have been utilized in total synthesis on several occasions. A major subunit of (+)-clavosolide was synthesized with a reduction of a six-membered oxocarbenium ring. All the large substituents were found in an equatorial position, and the transformation went through the chair transition state, as predicted. [ 11 ]
A second example is seen in the key step of the synthesis of (−)-neopeltolide, which uses another six-membered oxocarbenium ring reduction for a diastereoselective hydride addition. [ 12 ]
In biological systems, oxocarbenium ions are mostly seen during reactions of carbohydrates . Since sugars are present in the structure of nucleic acids , with a ribose sugar present in RNA and a deoxyribose present in the structure of DNA , their chemistry plays an important role in wide range of cellular functions of nucleic acids. In addition to their functions in nucleotides, sugars are also used for structural components of organisms, as energy storage molecules, cell signaling molecules, protein modification and play key roles in the immune system , fertilization , preventing pathogenesis , blood clotting , and development . [ 13 ] The abundance of sugar chemistry in biological processes leads many reaction mechanisms to proceed through oxocarbenium ions. Several important biological reactions that utilize oxocarbenium ions are outlined in this section.
Nucleotides can undergo enzyme-catalyzed intramolecular cyclization in order to produce several important biological molecules. These cyclizations typically proceed through an oxocarbenium intermediate. An example of this reaction can be seen in the cyclization cyclic ADP ribose , which is an important molecule for intracellular calcium signaling . [ 14 ]
A glycosidase is an enzyme that catalyzes the breakdown of a glycosidic linkage to produce two smaller sugars. This process has important implications in the utilization of stored energy, like glycogen in animals, as well as in the breakdown of cellulose by organisms that feed on plants. In general, aspartic or glutamic acid residues in the active site of the enzyme catalyze the hydrolysis of the glycosidic bond. The mechanism of these enzymes involves an oxocarbenium ion intermediate, a general example of which is shown below. [ 15 ] | https://en.wikipedia.org/wiki/Oxocarbenium |
Oxodipine is a calcium channel blocker .
This biochemistry article is a stub . You can help Wikipedia by expanding it . | https://en.wikipedia.org/wiki/Oxodipine |
The oxoeicosanoids are nonclassic eicosanoids , derived from arachidonic acid (AA).
For example, Lipoxygenase produces 5-HETE from AA; a dehydrogenase then produces 5-oxo-eicosatetraenoic acid , an oxoeicosanoid, from 5-HETE.
They are similar to the leukotrienes in their actions, but they act via a different receptor. [ 1 ]
This biochemistry article is a stub . You can help Wikipedia by expanding it . | https://en.wikipedia.org/wiki/Oxoeicosanoid |
Oxophilicity is the tendency of certain chemical compounds to form oxides by hydrolysis or abstraction of an oxygen atom from another molecule, often from organic compounds. The term is often used to describe metal centers, commonly the early transition metals such as titanium , niobium , and tungsten . Oxophilicity is often stated to be related to the hardness of the element, within the HSAB theory ( hard and soft (Lewis) acids and bases ), but it has been shown that oxophilicity depends more on the electronegativity and effective nuclear charge of the element than on its hardness. [ 1 ] This explains why the early transition metals, whose electronegativities and effective nuclear charges are low, are very oxophilic. Many main group compounds are also oxophilic, such as derivatives of aluminium , silicon , and phosphorus (III). The handling of oxophilic compounds often requires air-free techniques .
Complexes of oxophilic metals typically are prone to hydrolysis . For example, the high valent chlorides hydrolyze rapidly to give oxides:
These reactions proceed via oxychloride intermediates. For example, WOCl 4 results from the partial hydrolysis of tungsten hexachloride . Hydroxide-containing intermediates are rarely observed for oxophilic metals. In contrast, the anhydrous halides of the later metals tend to hydrate , not hydrolyze, and they often form hydroxides .
Reduced complexes of oxophilic metals tend to generate oxides by reaction with oxygen. Typically the oxide-ligand is bridging , e.g.
Only in rare cases do the products of oxygenation feature terminal oxo ligands. [ 2 ]
Oxophilic reagents are often used to extract or exchange oxygen centers in organic substrates, especially carbonyls (esters, ketones, amides) and epoxides. The highly oxophilic reagent generated from tungsten hexachloride and butyl lithium is useful for the deoxygenation of epoxides . [ 3 ] Such conversions are sometimes valuable in organic synthesis . In the McMurry reaction , ketones are converted into alkenes using oxophilic reagents:
Similarly, Tebbe's reagent is used in olefination reactions: [ 4 ]
Oxophilic main group compounds are also well known and useful. The highly oxophilic reagent Si 2 Cl 6 stereospecifically deoxygenates phosphine oxides . [ 5 ] Phosphorus pentasulfide and the related Lawesson's reagent convert certain organic carbonyls to the corresponding sulfur derivatives:
Owing to the high stability of carbon dioxide , many carbon compounds such as phosgene are oxophilic. This reactivity is used for recycling of triphenylphosphine oxide : [ 6 ] | https://en.wikipedia.org/wiki/Oxophilicity |
Oxopropalines are novel cytocidal beta-carbolines isolated from Streptomyces . [ 1 ]
This biochemistry article is a stub . You can help Wikipedia by expanding it . | https://en.wikipedia.org/wiki/Oxopropaline |
In organic chemistry , the oxy-Cope rearrangement is a chemical reaction . It involves reorganization of the skeleton of certain unsaturated alcohols. It is a variation of the Cope rearrangement in which 1,5-dien-3-ols are converted to unsaturated carbonyl compounds by a mechanism typical for such a [3,3]- sigmatropic rearrangement . [ 1 ] [ 2 ]
The reaction is highly general: a wide variety of precursors undergo the reorganization predictably and with ease, rendering it a highly useful synthetic tool. [ 3 ] Further, production of the required starting material is often straightforward. The modification was first proposed in 1964 by Berson and Jones, who coined the term. The driving force is the formation of a carbonyl via spontaneous keto-enol tautomerization . [ 4 ]
Base accelerates the reaction by 10 10 -10 17 , the anionic oxy-Cope rearrangement. [ 3 ]
The formation of an enolate renders the reaction irreversible in most cases. [ 3 ] [ 4 ] [ 5 ]
Sigmatropic rearrangements are useful organic synthesis. [ 5 ] In an effort to demonstrate the versatility of the Cope rearrangement by demonstrating its tolerance of an alcohol situated at C-3 of a 1,5-diene, Berson and Jones heated a bicyclic diene alcohol in the gas phase to give cis -∆ 5,6 -octalone in fair yield. [ 1 ]
The modification is immensely appealing as a result of the two new disparately placed functional groups that lend themselves well to a variety of previously unavailable synthetic manipulations.
The next development occurred in 1975, when Evans and Golob reported the tremendous rate enhancements by base. Their use of potassium hydride in the cation's corresponding crown ether became the default approach for most applications. Indeed, in some cases, anionic assistance is intentionally forgone to accommodate for the production of overly sensitive enolate product. For example, in the following reaction only tar was obtained, a result that the authors attributed to the product's ostensible intolerance to base. [ 6 ] The original oxy-Cope modification thus to this day occupies a relevant niche in synthetic chemistry.
Both the neutral and anionic variants of the oxy-Cope rearrangement may occur via either concerted or stepwise radical pathways, although the former mode is generally favored. [ 7 ] [ 8 ] The preferred intermediate is characterized by a chair-like conformation . [ 9 ] Chirality transfer is effected by a highly ordered transition state . [ 4 ] [ 9 ] The positioning of the double bonds in the most readily accessible transition state determines the stereochemical course of the reaction [ 3 ] A boat transition state is disfavored, but rearrangements occur via this path to an appreciable extent as well, resulting in the production of diastereomeric mixtures.
Steric effects can be significant. [ 10 ]
Rearrangements for which a chair transition state is geometrically impossible nonetheless occur. In fact, enolate formation provides enough of a driving force to overcome the energetic barrier associated with both dearomatization and the boat conformer. [ 11 ]
The concerted, synchronous pathways presented above generally predominate; it was calculated for anionic oxy-Cope processes that a divide of 17-34 kcal/mol favors heterolysis over homolysis. [ 12 ] Several factors may bridge this energetic gap. [ 9 ]
The large degree of strain and the presence of a methyl group's bulk favored the (Z)- instead of the expected (E)-cyclooctenone isomer, suggesting that the intermediate is not formed synchronously. Only with fragmentation and subsequent isomerization steps could the observed product be rationalized. [ 9 ]
A study on the anionic oxy-Cope rearrangement carried out entirely in the gaseous phase reported that the rate enhancement stems not from solvent interactions, but from those within the structure itself. [ 13 ]
In general, decreasing the stability of the oxy-Cope or anionic oxy-Cope substrate relative to that of the product results in increased rate of reaction by the principle of ground state destabilization. This desirable outcome is readily achieved in a variety of ways. Ionic interactions between metal and alkoxide are important: dissociative character causes rate acceleration. [ 14 ] Use of 15-crown-5 in conjunction with sodium hydride afforded a 1.27-fold rate enhancement over the course of a bicyclic diene alkoxide's sigmatropic conversion to enolate product, while the same reaction with HMPT in 15-crown-5's place did not appreciably affect the rate. The use of potassium hydride in conjunction with 18-crown-6 to achieve the same end afforded a 180-fold maximum rate acceleration. From the above results it was concluded that rate increases as counterions more poorly approximate point charges—and with the addition of counterion-sequestering species.
The inclusion of more polar solvents and catalytic quantities of phase transfer salts has also been demonstrated to exert the same rate-enhancing effect. [ 15 ]
Finally, the relief of ring strain over the course of a rearrangement will drive a reaction more forcibly to completion, thereby increasing its rate.
Because there exist multiple classes of natural products containing eight-membered rings, the syntheses of which having proved difficult, the anionic oxy-Cope rearrangement has been highlighted as a suitable alternative pathway. Its application here offers great stereochemical control, and its use is far more general than the relatively unsuccessful routes that had been employed before its development. [ 16 ]
In spite of possible geometrical constraints, the required unsaturated substrates may contain triple bonds in place of either of the double bonds. Such an alkynol was effectively manipulated in the elegant synthesis of both poitediol and dactylol. [ 5 ] These interesting sigmatropic rearrangements can occur either with anionic assistance or under thermal conditions. [ 17 ]
Of particular interest is the application of the oxy-Cope to situations in which the immediate product reacts further in a predictable manner to furnish a desired final product. This goal was achieved in the synthesis of the cis-hydroazulenone below, in which the oxy-Cope intermediate was characterized by a stereoelectronic configuration amenable to remote S N displacement. [ 18 ]
Potassium hydride , a frequently utilized reagent for the anionic oxy-Cope rearrangement, is occasionally contaminated with trace impurities that have been suggested to destroy the dienolate intermediate, resulting in putative polymerization. Macdonald et al., who documented the occurrence, prescribed pre-treatment with iodine to eliminate any potassium superoxide that may persist within a purchased batch of the material. This simple preparatory step, as they describe in their paper, effects dramatic improvement in both yield and reproducibility of results. [ 19 ]
Important side reactions include heterolytic cleavage, in which the homoallylic alcohol decomposes into a carbonyl and an allylic system. [ 20 ]
Suppression of this phenomenon is readily achievable by decreasing the ionic nature of the metal-alkoxide bond. Specifically, the use of more electronegative alkali metals or solvents less amenable to cation solvation generates the desired effect. [ 21 ] In keeping with the above discussion, the rate of reaction may be diminished but should not approach an unsatisfactory level. | https://en.wikipedia.org/wiki/Oxy-Cope_rearrangement |
Oxy-fuel combustion is the process of burning a fuel using pure oxygen, or a mixture of oxygen and recirculated flue gas, instead of air. Since the nitrogen component of air is not heated, fuel consumption is reduced, and higher flame temperatures are possible. Historically, the primary use of oxy-fuel combustion has been in welding and cutting of metals, especially steel, since oxy-fuel allows for higher flame temperatures than can be achieved with an air-fuel flame. [ 1 ] It has also received a lot of attention in recent decades as a potential carbon capture and storage technology. [ 2 ]
There is currently research being done in firing fossil fuel power plants with an oxygen-enriched gas mix instead of air. Almost all of the nitrogen is removed from input air, yielding a stream that is approximately 95% oxygen. [ 3 ] Firing with pure oxygen would result in too high a flame temperature, so the mixture is diluted by mixing with recycled flue gas , or staged combustion . The recycled flue gas can also be used to carry fuel into the boiler and ensure adequate convective heat transfer to all boiler areas. Oxy-fuel combustion produces approximately 75% less flue gas than air fueled combustion and produces exhaust consisting primarily of CO 2 and H 2 O (see figure).
The justification for using oxy-fuel is to produce a CO 2 rich flue gas ready for sequestration . Oxy-fuel combustion has significant advantages over traditional air-fired plants. Among these are:
Economically speaking this method costs more than a traditional air-fired plant. The main problem has been separating oxygen from the air. This process requires much energy, nearly 15% of production by a coal-fired power station can be consumed for this process. However, a new technology which is not yet practical called chemical looping combustion [ 4 ] can be used to reduce this cost. In chemical looping combustion, the oxygen required to burn the coal is produced internally by oxidation and reduction reactions, as opposed to using more expensive methods of generating oxygen by separating it from air. [ 5 ]
At present in the absence of any need to reduce CO 2 emissions, oxy-fuel is not competitive. However, oxy-fuel is a viable alternative to removing CO 2 from the flue gas from a conventional air-fired fossil fuel plant. However, an oxygen concentrator might be able to help, as it simply removes nitrogen.
In industries other than power generation, oxy-fuel combustion can be competitive due to higher sensible heat availability. Oxy-fuel combustion is common in various aspects of metal production.
The glass industry has been converting to oxy-fuel since the early 1990s because glass furnaces require a temperature of approximately 1500 degrees C, which is not economically attainable at adiabatic flame temperatures for air-fuel combustion unless heat is regenerated between the flue stream and the incoming air stream. Developed in the mid-19th century, glass furnace regenerators are large and expensive high temperature brick ducts filled with brick arranged in a checkerboard pattern to capture heat as flue gas exits the furnace. When the flue duct is thoroughly heated, air flow is reversed and the flue duct becomes the air inlet, releasing its heat into the incoming air, and allowing for higher furnace temperatures than can be attained with air-fuel only. Two sets of regenerative flue ducts allowed for the air flow to be reversed at regular intervals, and thus maintain a high temperature in the incoming air. By allowing new furnaces to be built without the expense of regenerators, and especially with the added benefit of nitrogen oxide reduction, which allows glass plants to meet emission restrictions, oxy-fuel is cost effective without the need to reduce CO 2 emissions. Oxy-fuel combustion also reduces CO 2 release at the glass plant location, although this may be offset by CO 2 production due to electric power generation which is necessary to produce oxygen for the combustion process.
Oxy-fuel combustion may also be cost effective in the incineration of low BTU value hazardous waste fuels. It is often combined with staged combustion for nitrogen oxide reduction, since pure oxygen can stabilize combustion characteristics of a flame.
There are pilot plants undergoing initial proof-of-concept testing to evaluate the technologies for scaling up to commercial plants, including
One case study of oxy-fuel combustion is the attempted White Rose plant in North Yorkshire, United Kingdom. The planned project was an oxy-fuel power plant coupled with air separation to capture two million tons of carbon dioxide per year. The carbon dioxide would then be delivered by pipeline to be sequestered in a saline aquifer beneath the North Sea. [ 9 ] However, in late 2015 and early 2016, following withdrawal of funding by the Drax Group and the U.K. government, construction was halted. [ 10 ] The unforeseen loss of the funding from the UK government's CCS Commercialisation Programme, along with decreased subsidies for renewable energy, left the White Rose Plant with insufficient funds to continue development. [ 9 ]
One of the major environmental impacts of burning fossil fuels is the release of CO 2 , which contributes to climate change . Because oxyfuel combustion results in flue gas that already has a high concentration of CO 2 , it makes it easier to purify and store the CO 2 rather than releasing it to the atmosphere. [ 2 ]
Many fossil fuels, such as coal and oil shale , produce ash as a result of combustion. This ash also needs to be disposed of, which may impact the environment. So far studies indicate that, in general, oxyfuel combustion does not significantly affect the composition of ash produced. Measurements have shown similar mineral and heavy metal concentrations regardless of whether an air or oxyfuel environment was used. [ 11 ] [ 12 ] However, one notable exception is that oxyfuel ashes often have lower concentrations of calcium oxide or calcium hydroxide (free lime). Free lime forms when carbonate minerals in fuels like coal and oil shale decompose at the high temperatures occurring during combustion ( calcination ). Calcination is an equilibrium reaction and a higher partial pressure of CO 2 shifts the equilibrium in favor of CaCO 3 and MgCO 3 respectively. Free lime is reactive and can potentially affect the environment, for instance by increasing the alkalinity of the ash. Because oxyfuel combustion takes place in a CO 2 -rich atmosphere, decomposition is reduced and the ash generally contains less free lime. [ 11 ] [ 12 ] Flue gas desulfurization is usually employed to increase the pH of flue gases or their product when reacting with atmospheric moisture ( acid rain ). Besides sulfur and its oxides, another potential acid rain component is formed from nitric and nitrous oxide interacting with water - eliminating nitrogen from combustion reduces this factor altogether. | https://en.wikipedia.org/wiki/Oxy-fuel_combustion_process |
Oxy-fuel welding (commonly called oxyacetylene welding , oxy welding , or gas welding in the United States) and oxy-fuel cutting are processes that use fuel gases (or liquid fuels such as gasoline or petrol, diesel, biodiesel, kerosene , etc) and oxygen to weld or cut metals. French engineers Edmond Fouché and Charles Picard became the first to develop oxygen- acetylene welding in 1903. [ 1 ] Pure oxygen, instead of air , is used to increase the flame temperature to allow localized melting of the workpiece material (e.g. steel) in a room environment.
A common propane /air flame burns at about 2,250 K (1,980 °C; 3,590 °F), [ 2 ] a propane/oxygen flame burns at about 2,526 K (2,253 °C; 4,087 °F), [ 3 ] an oxyhydrogen flame burns at 3,073 K (2,800 °C; 5,072 °F) and an acetylene /oxygen flame burns at about 3,773 K (3,500 °C; 6,332 °F). [ 4 ]
During the early 20th century, before the development and availability of coated arc welding electrodes in the late 1920s that were capable of making sound welds in steel, oxy-acetylene welding was the only process capable of making welds of exceptionally high quality in virtually all metals in commercial use at the time. These included not only carbon steel but also alloy steels, cast iron , aluminium , and magnesium . In recent decades it has been superseded in almost all industrial uses by various arc welding methods offering greater speed and, in the case of gas tungsten arc welding , the capability of welding very reactive metals such as titanium .
Oxy-acetylene welding is still used for metal-based artwork and in smaller home-based shops, as well as situations where accessing electricity (e.g., via an extension cord or portable generator) would present difficulties. The oxy-acetylene (and other oxy-fuel gas mixtures) welding torch remains a mainstay heat source for manual brazing , as well as metal forming , preparation, and localized heat treating. In addition, oxy-fuel cutting is still widely used, both in heavy industry and light industrial and repair operations.
In oxy-fuel welding , a welding torch is used to weld metals. Welding metal results when two pieces are heated to a temperature that produces a shared pool of molten metal. The molten pool is generally supplied with additional metal called filler. Filler material selection depends upon the metals to be welded.
In oxy-fuel cutting , a torch is used to heat metal to its kindling temperature . A stream of oxygen is then trained on the metal, burning it into a metal oxide that flows out of the kerf as dross . [ 5 ]
Torches that do not mix fuel with oxygen (combining, instead, atmospheric air) are not considered oxy-fuel torches and can typically be identified by a single tank (oxy-fuel cutting requires two isolated supplies, fuel and oxygen). Most metals cannot be melted with a single-tank torch. Consequently, single-tank torches are typically suitable for soldering and brazing but not for welding.
Oxy-fuel torches are or have been used for:
In short, oxy-fuel equipment is quite versatile, not only because it is preferred for some sorts of iron or steel welding but also because it lends itself to brazing, braze-welding, metal heating (for annealing or tempering, bending or forming), rust, or scale removal, the loosening of corroded nuts and bolts, and is a ubiquitous means of cutting ferrous metals.
The apparatus used in gas welding consists basically of an oxygen source and a fuel gas source (usually contained in cylinders ), two pressure regulators and two flexible hoses (one for each cylinder), and a torch. This sort of torch can also be used for soldering and brazing . The cylinders are often carried in a special wheeled trolley .
There have been examples of oxyhydrogen cutting sets with small ( scuba -sized) gas cylinders worn on the user's back in a backpack harness, for rescue work, and similar.
There are also examples of both non-pressurized and pressurized liquid fuel cutting torches, usually using gasoline (petrol). These are used for their increased cutting power over gaseous fuel systems and also greater portability compared to systems requiring two high pressure tanks.
The regulator ensures that pressure of the gas from the tanks matches the required pressure in the hose. The flow rate is then adjusted by the operator using needle valves on the torch. Accurate flow control with a needle valve relies on a constant inlet pressure.
Most regulators have two stages. The first stage is a fixed-pressure regulator, which releases gas from the cylinder at a constant intermediate pressure, despite the pressure in the cylinder falling as the gas in it is consumed. This is similar to the first stage of a scuba-diving regulator . The adjustable second stage of the regulator controls the pressure reduction from the intermediate pressure to the low outlet pressure. The regulator has two pressure gauges, one indicating cylinder pressure, the other indicating hose pressure. The adjustment knob of the regulator is sometimes roughly calibrated for pressure, but an accurate setting requires observation of the gauge.
Some simpler or cheaper oxygen-fuel regulators have only a single-stage regulator, or only a single gauge. A single-stage regulator will tend to allow a reduction in outlet pressure as the cylinder is emptied, requiring manual readjustment. For low-volume users, this is an acceptable simplification. Welding regulators, unlike simpler LPG heating regulators, retain their outlet (hose) pressure gauge and do not rely on the calibration of the adjustment knob. The cheaper single-stage regulators may sometimes omit the cylinder contents gauge, or replace the accurate dial gauge with a cheaper and less precise "rising button" gauge.
The hoses are designed for use in welding and cutting metal. A double-hose or twinned design can be used, meaning that the oxygen and fuel hoses are joined. If separate hoses are used, they should be clipped together at intervals approximately 3 feet (1 m) apart, although that is not recommended for cutting applications, because beads of molten metal given off by the process can become lodged between the hoses where they are held together, and burn through, releasing the pressurized gas inside, which in the case of fuel gas usually ignites.
The hoses are color-coded for visual identification. The color of the hoses varies between countries. In the United States, the oxygen hose is green and the fuel hose is red. [ 6 ] In the UK and other countries, the oxygen hose is blue (black hoses may still be found on old equipment), and the acetylene (fuel) hose is red. [ 7 ] If liquefied petroleum gas (LPG) fuel, such as propane , is used, the fuel hose should be orange, indicating that it is compatible with LPG. LPG will damage an incompatible hose, including most acetylene hoses.
The threaded connectors on the hoses are handed to avoid accidental mis-connection: the thread on the oxygen hose is right-handed (as normal), while the fuel gas hose has a left-handed thread. [ 6 ] The left-handed threads also have an identifying groove cut into their nuts.
Gas-tight connections between the flexible hoses and rigid fittings are made by using crimped hose clips or ferrules , often referred to as 'O' clips, over barbed spigots. The use of worm-drive hose clips or Jubilee Clips is specifically forbidden in the UK and other countries. [ 8 ]
Acetylene is not just flammable; in certain conditions it is explosive . Although it has an upper flammability limit in air of 81%, [ 9 ] acetylene's explosive decomposition behaviour makes this irrelevant. If a detonation wave enters the acetylene tank, the tank will be blown apart by the decomposition. Ordinary check valves that normally prevent backflow cannot stop a detonation wave because they are not capable of closing before the wave passes around the gate. For that reason a flashback arrestor is needed. It is designed to operate before the detonation wave makes it from the hose side to the supply side.
Between the regulator and hose, and ideally between hose and torch on both oxygen and fuel lines, a flashback arrestor and/or non-return valve (check valve) should be installed to prevent flame or oxygen-fuel mixture being pushed back into either cylinder and damaging the equipment or causing a cylinder to explode.
European practice is to fit flashback arrestors at the regulator and check valves at the torch. US practice is to fit both at the regulator.
The flashback arrestor prevents shock waves from downstream coming back up the hoses and entering the cylinder, possibly rupturing it, as there are quantities of fuel/oxygen mixtures inside parts of the equipment (specifically within the mixer and blowpipe/nozzle) that may explode if the equipment is incorrectly shut down, and acetylene decomposes at excessive pressures or temperatures. In case the pressure wave has created a leak downstream of the flashback arrestor, it will remain switched off until someone resets it.
A check valve lets gas flow in one direction only. It is usually a chamber containing a ball that is pressed against one end by a spring. Gas flow one way pushes the ball out of the way, and a lack of flow or a reverse flow allows the spring to push the ball into the inlet, blocking it. Not to be confused with a flashback arrestor, a check valve is not designed to block a shock wave. The shock wave could occur while the ball is so far from the inlet that the wave will get past the ball before it can reach its off position.
The torch is the tool that the welder holds and manipulates to make the weld. It has a connection and valve for the fuel gas and a connection and valve for the oxygen, a handle for the welder to grasp, and a mixing chamber (set at an angle) where the fuel gas and oxygen mix, with a tip where the flame forms. Two basic types of torches are positive pressure type and low pressure or injector type.
A welding torch head is used to weld metals. It can be identified by having only one or two pipes running to the nozzle, no oxygen-blast trigger, and two valve knobs at the bottom of the handle letting the operator adjust the oxygen and fuel flow respectively.
A cutting torch head is used to cut materials. It is similar to a welding torch, but can be identified by the oxygen blast trigger or lever.
When cutting, the metal is first heated by the flame until it is cherry red. Once this temperature is attained, oxygen is supplied to the heated parts by pressing the oxygen-blast trigger. This oxygen reacts with the metal, producing more heat and forming an oxide which is then blasted out of the cut. It is the heat that continues the cutting process. The cutting torch only heats the metal to start the process; further heat is provided by the burning metal.
The melting point of the iron oxide is around half that of the metal being cut. As the metal burns, it immediately turns to liquid iron oxide and flows away from the cutting zone. However, some of the iron oxide remains on the workpiece, forming a hard "slag" which can be removed by gentle tapping and/or grinding.
A rose bud torch is used to heat metals for bending, straightening, etc. where a large area needs to be heated. It is so-called because the flame at the end looks like a rose bud . A welding torch can also be used to heat small areas such as rusted nuts and bolts.
A typical oxy-fuel torch, called an equal-pressure torch, merely mixes the two gases. In an injector torch, high-pressure oxygen comes out of a small nozzle inside the torch head which drags the fuel gas along with it, using the Venturi effect .
Oxy-fuel processes may use a variety of fuel gases (or combustible liquids), the most common being acetylene . Other gases that may be used are propylene , liquified petroleum gas (LPG), propane, natural gas , hydrogen , and MAPP gas . Liquid fuel cutting systems use such fuels as Gasoline (Petrol) Diesel, Kerosene and possibly some aviation fuels.
Acetylene is the primary fuel for oxy-fuel welding and is the fuel of choice for repair work and general cutting and welding. Acetylene gas is shipped in special cylinders designed to keep the gas dissolved. The cylinders are packed with porous materials (e.g. kapok fibre, diatomaceous earth , or (formerly) asbestos ), then filled to around 50% capacity with acetone , as acetylene is soluble in acetone. This method is necessary because above 207 kilopascals (30 pounds per square inch ) (absolute pressure), acetylene is unstable and may explode .
There is about 1,700 kPa (247 psi) pressure in the tank when full. When combined with oxygen , acetyline burns at 3,200 to 3,500 degrees Celsius (5,790 to 6,330 degrees Fahrenheit ), the highest among commonly used gaseous fuels. As a fuel, acetylene's primary disadvantage in comparison to other fuels is its high price.
As acetylene is unstable at a pressure roughly equivalent to 33 ft (10 m) underwater, water-submerged cutting and welding is reserved for hydrogen rather than acetylene.
Tests [ citation needed ] showed that an oxy-gasoline torch can cut steel plate up to 0.5 in (13 mm) thick at the same rate as oxy-acetylene. In plate thicknesses greater than 0.5 in (13 mm) the cutting rate was better than that of oxy-acetylene; at 4.5 in (110 mm) it was three times faster. [ 10 ] Additionally the liquid fuel vapour is about 4x the density of a gaseous fuel. A high velocity cutting flame is produced by the huge volume expansion while the liquid transitions to a vapour so the cutting flame can cut across voids (air space between plates).
Oxy-gasoline torches can also cut through paint, dirt, rust and other contaminating surface materials coating old steel. This system provides almost 100% oxidation during cutting, leaving almost no molten steel in the slag to prevent "sticking" together cut material. Operating cost for a gasoline torch is typically 75-90% less than using propane or Acetylene.
The gasoline can be fed either from a pressurized tank (whose pressure can be hand-pumped or fed from a gas cylinder) or a non-pressurized tank, with the fuel being drawn into the torch by a venturi action created by the pressurized oxygen flow. [ 10 ] Another low cost approach commonly used by jewelry makers in Asia is using air bubbled through a gasoline container by a foot-operated air pump, and burning the fuel-air mixture in a specialized welding torch.
Diesel is a new option in the liquid fuel cutting torch market. Diesel torches claim several advantages over gaseous fuels and gasoline. Diesel is inherently safer and more powerful than gasoline or gaseous fuel such as acetylene and propane, and will cut steel faster and cheaper than either of those gases. In addition, the liquid fuel vapor is about 5 times the density of a gaseous fuel, providing much greater "punch". A high velocity cutting flame is produced by the huge volume expansion when the liquid transitions to a vapor, so the cutting flame will easily cut across air voids between plates. A diesel/oxygen torch can cut through paint, dirt, rust and other surface contaminants on steel. This system provides almost 100% oxidation during cutting so it leaves virtually no molten steel in the slag, preventing the "sticking together" of the cut materials. The operating cost for a diesel torch is typically 75-90% less than using propane or acetylene. Growing use in the demolition or scrap industries
Hydrogen has a clean flame and is good for use on aluminium . It can be used at a higher pressure than acetylene and is therefore useful for underwater welding and cutting. It is a good type of flame to use when heating large amounts of material. The flame temperature is high, about 2,000 °C for hydrogen gas in air at atmospheric pressure, [ 11 ] and up to 2800 °C when pre-mixed in a 2:1 ratio with pure oxygen (oxyhydrogen). Hydrogen is not used for welding steels and other ferrous materials, because it causes hydrogen embrittlement .
For some oxyhydrogen torches the oxygen and hydrogen are produced by electrolysis of water in an apparatus which is connected directly to the torch. Types of this sort of torch:
Methylacetylene-propadiene (MAPP) gas and LPG gas are similar fuels, because LPG gas is liquefied petroleum gas mixed with MPS. It has the storage and shipping characteristics of LPG and has a heat value a little lower than that of acetylene. Because it can be shipped in small containers for sale at retail stores, it is used by hobbyists and large industrial companies and shipyards because it does not polymerize at high pressures — above 15 psi or so (as acetylene does) and is therefore much less dangerous than acetylene.
Further, more of it can be stored in a single place at one time, as the increased compressibility allows for more gas to be put into a tank. MAPP gas can be used at much higher pressures than acetylene, sometimes up to 40 or 50 psi in high-volume oxy-fuel cutting torches which can cut up to 12-inch-thick (300 mm) steel. Other welding gases that develop comparable temperatures need special procedures for safe shipping and handling. MPS and MAPP are recommended for cutting applications in particular, rather than welding applications.
On 30 April 2008 the Petromont Varennes plant closed its methylacetylene/propadiene crackers. As it was the only North American plant making MAPP gas, many substitutes were introduced by companies that had repackaged the Dow and Varennes product(s) - most of these substitutes are propylene, see below.
Propylene is used in production welding and cutting. It cuts similarly to propane. When propylene is used, the torch rarely needs tip cleaning. There is often a substantial advantage to cutting with an injector torch (see the propane section) rather than an equal-pressure torch when using propylene. Quite a few North American suppliers have begun selling propylene under proprietary trademarks such as FG2 and Fuel-Max.
Butane , like propane , is a saturated hydrocarbon. Butane and propane do not react with each other and are regularly mixed. Butane boils at 0.6 °C. Propane is more volatile, with a boiling point of -42 °C. Vaporization is rapid at temperatures above the boiling points. The calorific (heat) values of the two are almost equal. Both are thus mixed to attain the vapor pressure that is required by the end user and depending on the ambient conditions. If the ambient temperature is very low, propane is preferred to achieve higher vapor pressure at the given temperature. [ citation needed ]
Propane does not burn as hot as acetylene in its inner cone, and so it is rarely used for welding. [ 12 ] Propane, however, has a very high number of BTUs per cubic foot in its outer cone, and so with the right torch ( injector style ) can make a faster and cleaner cut than acetylene, and is much more useful for heating and bending than acetylene.
The maximum neutral flame temperature of propane in oxygen is 2,822 °C (5,112 °F). [ 13 ]
Propane is cheaper than acetylene and easier to transport. [ 14 ]
The following is a comparison of operating costs in cutting 1 ⁄ 2 in (13 mm) plate. Costing is based on an average cost for oxygen and different fuels in May 2012. [ obsolete source ] The opex with Gasoline was 25% that of propane and 10% that of acetylene. Numbers will vary depending on source of oxygen or fuel and on the type of cutting and the cutting environment or situation. [ 15 ]
Oxygen is not the fuel. It is the oxidizing agent , which chemically combines with the fuel to produce the heat for welding. This is called 'oxidation', but the more specific and more commonly used term in this context is ' combustion '. In the case of hydrogen, the product of combustion is simply water. For the other hydrocarbon fuels, water and carbon dioxide are produced. The heat is released because the molecules of the products of combustion have a lower energy state than the molecules of the fuel and oxygen. In oxy-fuel cutting, oxidation of the metal being cut (typically iron) produces nearly all of the heat required to "burn" through the workpiece.
Oxygen is usually produced elsewhere by distillation of liquefied air and shipped to the welding site in high-pressure vessels (commonly called "tanks" or "cylinders") at a pressure of about 21,000 kPa (3,000 lbf/in² = 200 atmospheres). It is also shipped as a liquid in Dewar type vessels (like a large Thermos jar) to places that use large amounts of oxygen.
It is also possible to separate oxygen from air by passing the air, under pressure, through a zeolite sieve that selectively adsorbs the nitrogen and lets the oxygen (and argon ) pass. This gives a purity of oxygen of about 93%. This method works well for brazing, but higher-purity oxygen is necessary to produce a clean, slag-free kerf when cutting.
The welder can adjust the oxy-acetylene flame to be carburizing (aka reducing), neutral, or oxidizing. Adjustment is made by adding more or less oxygen to the acetylene flame. The neutral flame is the flame most generally used when welding or cutting. The welder uses the neutral flame as the starting point for all other flame adjustments because it is so easily defined. This flame is attained when welders, as they slowly open the oxygen valve on the torch body, first see only two flame zones. At that point, the acetylene is being completely burned in the welding oxygen and surrounding air. [ 5 ] The flame is chemically neutral.
The two parts of this flame are the light blue inner cone and the darker blue to colorless outer cone. The inner cone is where the acetylene and the oxygen combine. The tip of this inner cone is the hottest part of the flame. It is approximately 6,000 °F (3,320 °C) and provides enough heat to easily melt steel. [ 5 ] In the inner cone the acetylene breaks down and partly burns to hydrogen and carbon monoxide , which in the outer cone combine with more oxygen from the surrounding air and burn.
An excess of acetylene creates a reducing (sometimes called carbonizing) flame. This flame is characterized by three flame zones; the hot inner cone, a white-hot "acetylene feather", and the blue-colored outer cone. This is the type of flame observed when oxygen is first added to the burning acetylene. The feather is adjusted and made ever smaller by adding increasing amounts of oxygen to the flame. A welding feather is measured as 2X or 3X, with X being the length of the inner flame cone.
The unburned carbon insulates the flame and drops the temperature to approximately 5,000 °F (2,760 °C). The reducing flame is typically used for hardfacing operations or backhand pipe welding techniques. The feather is caused by incomplete combustion of the acetylene to cause an excess of carbon in the flame. Some of this carbon is dissolved by the molten metal to carbonize it. The carbonizing flame will tend to remove the oxygen from iron oxides which may be present, a fact which has caused the flame to be known as a "reducing flame". [ 5 ]
The oxidizing flame is the third possible flame adjustment. It occurs when the ratio of oxygen to acetylene required for a neutral flame has been changed to give an excess of oxygen. This flame type is observed when welders add more oxygen to the neutral flame. This flame is hotter than the other two flames because the combustible gases will not have to search so far to find the necessary amount of oxygen, nor heat up as much thermally inert carbon. [ 5 ] It is called an oxidizing flame because of its effect on metal. This flame adjustment is generally not preferred. The oxidizing flame creates undesirable oxides to the structural and mechanical detriment of most metals. In an oxidizing flame, the inner cone acquires a purplish tinge and gets pinched and smaller at the tip, and the sound of the flame gets harsh. A slightly oxidizing flame is used in braze-welding and bronze-surfacing while a more strongly oxidizing flame is used in fusion welding certain brasses and bronzes [ 5 ]
The size of the flame can be adjusted to a limited extent by the valves on the torch and by the regulator settings, but in the main it depends on the size of the orifice in the tip. In fact, the tip should be chosen first according to the job at hand, and then the regulators set accordingly.
The flame is applied to the base metal and held until a small puddle of molten metal is formed. The puddle is moved along the path where the weld bead is desired. Usually, more metal is added to the puddle as it is moved along by dipping metal from a welding rod or filler rod into the molten metal puddle. The metal puddle will travel towards where the metal is the hottest. This is accomplished through torch manipulation by the welder.
The amount of heat applied to the metal is a function of the welding tip size, the speed of travel, and the welding position. The flame size is determined by the welding tip size. The proper tip size is determined by the metal thickness and the joint design.
Welding gas pressures using oxy-acetylene are set in accordance with the manufacturer's recommendations. The welder will modify the speed of welding travel to maintain a uniform bead width. Uniformity is a quality attribute indicating good workmanship. Trained welders are taught to keep the bead the same size at the beginning of the weld as at the end. If the bead gets too wide, the welder increases the speed of welding travel. If the bead gets too narrow or if the weld puddle is lost, the welder slows down the speed of travel. Welding in the vertical or overhead positions is typically slower than welding in the flat or horizontal positions.
The welder must add the filler rod to the molten puddle. The welder must also keep the filler metal in the hot outer flame zone when not adding it to the puddle to protect filler metal from oxidation. Do not let the welding flame burn off the filler metal. The metal will not wet into the base metal and will look like a series of cold dots on the base metal. There is very little strength in a cold weld. When the filler metal is properly added to the molten puddle, the resulting weld will be stronger than the original base metal.
Welding lead or ' lead burning ' was much more common in the 19th century to make some pipe connections and tanks. Great skill is required, but it can be quickly learned. [ 16 ] In building construction today some lead flashing is welded but soldered copper flashing is much more common in America. In the automotive body repair industry before the 1980s, oxyacetylene gas torch welding was seldom used to weld sheet metal, since warping was a byproduct as well as excess heat. Automotive body repair methods at the time were crude and yielded improprieties until MIG welding became the industry standard. Since the 1970s, when high strength steel became the standard for automotive manufacturing, electric welding became the preferred method. After the 1980s, oxyacetylene torches fell out of use for sheet metal welding in the industrialized world.
For cutting, the setup is a little different. A cutting torch has a 60- or 90-degree angled head with orifices placed around a central jet. The outer jets are for preheat flames of oxygen and acetylene. The central jet carries only oxygen for cutting. The use of several preheating flames rather than a single flame makes it possible to change the direction of the cut as desired without changing the position of the nozzle or the angle which the torch makes with the direction of the cut, as well as giving a better preheat balance. [ 5 ] Manufacturers have developed custom tips for Mapp, propane, and propylene gases to optimize the flames from these alternate fuel gases.
The flame is not intended to melt the metal, but to bring it to its ignition temperature .
The torch's trigger blows extra oxygen at higher pressures down the torch's third tube out of the central jet into the workpiece, causing the metal to burn and blowing the resulting molten oxide through to the other side. The ideal kerf is a narrow gap with a sharp edge on either side of the workpiece; overheating the workpiece and thus melting through it causes a rounded edge.
Cutting is initiated by heating the edge or leading face (as in cutting shapes such as round rod) of the steel to the ignition temperature (approximately bright cherry red heat) using the pre-heat jets only, then using the separate cutting oxygen valve to release the oxygen from the central jet. [ 5 ] The oxygen chemically combines with the iron in the ferrous material to oxidize the iron quickly into molten iron oxide , producing the cut. Initiating a cut in the middle of a workpiece is known as piercing.
It is worth noting several things at this point:
For a basic oxy-acetylene rig, the cutting speed in light steel section will usually be nearly twice as fast as a petrol -driven cut-off grinder. The advantages when cutting large sections are obvious: an oxy-fuel torch is light, small and quiet and needs very little effort to use, whereas an angle grinder is heavy and noisy and needs considerable operator exertion and may vibrate severely, leading to stiff hands and possible long-term vibration white finger . Oxy-acetylene torches can easily cut through ferrous materials in excess of 200 mm (7.9 in). Oxygen lances are used in scrapping operations and cut sections thicker than 200 mm. Cut-off grinders are useless for these kinds of application.
Robotic oxy-fuel cutters sometimes use a high-speed divergent nozzle. This uses an oxygen jet that opens slightly along its passage. This allows the compressed oxygen to expand as it leaves, forming a high-velocity jet that spreads less than a parallel-bore nozzle, allowing a cleaner cut. These are not used for cutting by hand since they need very accurate positioning above the work. Their ability to produce almost any shape from large steel plates gives them a secure future in shipbuilding and in many other industries.
Oxy-propane torches are usually used for cutting up scrap to save money, as LPG is far cheaper joule for joule than acetylene, although propane does not produce acetylene's very neat cut profile. Propane also finds a place in production, for cutting very large sections.
Oxy-acetylene can cut only low- to medium- carbon steels and wrought iron . High-carbon steels are difficult to cut because the melting point of the slag is closer to the melting point of the parent metal, so that the slag from the cutting action does not eject as sparks but rather mixes with the clean melt near the cut. This keeps the oxygen from reaching the clean metal and burning it. In the case of cast iron , graphite between the grains and the shape of the grains themselves interfere with the cutting action of the torch. Stainless steels cannot be cut either because the material does not burn readily. [ 17 ]
Oxyacetylene welding/cutting is generally considered not to be difficult, but there are a good number of subtle safety points that should be learned such as:
Proper protection such as welding goggles should be worn at all times, including to protect the eyes against glare and flying sparks. Special safety eyewear must be used—both to protect the welder and to provide a clear view through the yellow-orange flare given off by the incandescing flux. In the 1940s cobalt melters’ glasses were borrowed from steel foundries and were still available until the 1980s.
However, the lack of protection from impact, ultra-violet, infrared and blue light caused severe eyestrain and eye damage. Didymium eyewear, developed for glassblowers in the 1960s, was also borrowed—until many complained of eye problems from excessive infrared, blue light, and insufficient shading. Today very good eye protection can be found designed especially for gas-welding aluminum that cuts the sodium orange flare completely and provides the necessary protection from ultraviolet, infrared, blue light and impact, according to ANSI Z87-1989 safety standards for a Special Purpose Lens. [ 18 ]
Fuel and oxygen tanks should be fastened securely and upright to a wall, post, or portable cart. An oxygen tank is especially dangerous because the gas is stored at a pressure of 21 MPa (3,000 psi ; 210 atm ) when full. If the tank falls over and damages the valve, the tank can be jettisoned by the compressed oxygen escaping the cylinder at high speed. Tanks in this state are capable of breaking through a brick wall. [ 19 ] For this reason, an oxygen tank should never be moved around without its valve cap screwed in place.
On an oxyacetylene torch system there are three types of valves : the tank valve, the regulator valve, and the torch valve. Each gas in the system will have each of these three valves. The regulator converts the high pressure gas inside of the tanks to a low pressure stream suitable for welding. Acetylene cylinders must be maintained in an upright position to prevent the internal acetone and acetylene from separating in the filler material. [ 20 ]
A less obvious hazard of welding is exposure to harmful chemicals. Exposure to certain metals, metal oxides, or carbon monoxide can often lead to severe medical conditions. Damaging chemicals can be produced from the fuel, from the work-piece, or from a protective coating on the work-piece. By increasing ventilation around the welding environment, exposure to harmful chemicals are significantly reduced from any source.
The most common fuel used in welding is acetylene, which has a two-stage reaction. The primary chemical reaction involves the acetylene disassociating in the presence of oxygen to produce heat, carbon monoxide, and hydrogen gas: C 2 H 2 + O 2 → 2CO + H 2 . A secondary reaction follows where the carbon monoxide and hydrogen combine with more oxygen to produce carbon dioxide and water vapor. When the secondary reaction does not burn all of the reactants from the primary reaction, the welding process can often produce large amounts of carbon monoxide. Carbon monoxide is also the byproduct of many other incomplete fuel reactions.
Almost every piece of metal is an alloy of one type or another. Copper , aluminum, and other base metals are occasionally alloyed with beryllium , which is a highly toxic metal. When a metal like this is welded or cut, high concentrations of toxic beryllium fumes are released. Long-term exposure to beryllium may result in shortness of breath, chronic cough, and significant weight loss, accompanied by fatigue and general weakness. Other alloying elements such as arsenic , manganese , silver , and aluminum can cause sickness to those who are exposed.
More common are the anti-rust coatings on many manufactured metal components. Zinc , cadmium , and fluorides are often used to protect irons and steels from oxidizing . Galvanized metals have a very heavy zinc coating. Exposure to zinc oxide fumes can lead to a sickness named " metal fume fever ". This condition rarely lasts longer than 24 hours, but severe cases can be fatal. [ 21 ] Not unlike common influenza , fevers, chills, nausea, cough, and fatigue are common effects of high zinc oxide exposure.
Flashback is the condition of the flame propagating down the hoses of an oxy-fuel welding and cutting system. To prevent such a situation a flashback arrestor is usually employed. [ 22 ] The flame burns backwards into the hose, causing a popping or squealing noise. It can cause an explosion in the hose with the potential to injure or kill the operator. Using a lower pressure than recommended can cause a flashback. | https://en.wikipedia.org/wiki/Oxy-fuel_welding_and_cutting |
An oxyanion , or oxoanion , is an ion with the generic formula A x O z − y (where A represents a chemical element and O represents an oxygen atom). Oxyanions are formed by a large majority of the chemical elements . [ 1 ] The formulae of simple oxyanions are determined by the octet rule . The corresponding oxyacid of an oxyanion is the compound H z A x O y . The structures of condensed oxyanions can be rationalized in terms of AO n polyhedral units with sharing of corners or edges between polyhedra. The oxyanions (specifically, phosphate and polyphosphate esters) adenosine monophosphate ( AMP ), adenosine diphosphate ( ADP ) and adenosine triphosphate (ATP) are important in biology.
The formula of monomeric oxyanions, AO m − n , is dictated by the oxidation state of the element A and its position in the periodic table . Elements of the first row are limited to a maximum coordination number of 4. However, none of the first row elements has a monomeric oxyanion with that coordination number. Instead, carbonate ( CO 2− 3 ) and nitrate ( NO − 3 ) have a trigonal planar structure with π bonding between the central atom and the oxygen atoms. This π bonding is favoured by the similarity in size of the central atom and oxygen.
The oxyanions of second-row elements in the group oxidation state are tetrahedral . Tetrahedral SiO 4 units are found in olivine minerals, (Mg,Fe) 2 SiO 4 , but the anion does not have a separate existence as the oxygen atoms are surrounded tetrahedrally by cations in the solid state. Phosphate ( PO 3− 4 ), sulfate ( SO 2− 4 ), and perchlorate ( ClO − 4 ) ions can be found as such in various salts. Many oxyanions of elements in lower oxidation state obey the octet rule and this can be used to rationalize the formulae adopted. For example, chlorine(V) has two valence electrons so it can accommodate three electron pairs from bonds with oxide ions. The charge on the ion is +5 − 3 × 2 = −1, and so the formula is ClO − 3 . The structure of the ion is predicted by VSEPR theory to be pyramidal, with three bonding electron pairs and one lone pair. In a similar way,
The oxyanion of chlorine(III) has the formula ClO − 2 , and is bent with two lone pairs and two bonding pairs.
In the third and subsequent rows of the periodic table, 6-coordination is possible, but isolated octahedral oxyanions are not known because they would carry too high an electrical charge. Thus molybdenum(VI) does not form MoO 6− 6 , but forms the tetrahedral molybdate anion, MoO 2− 4 . MoO 6 units are found in condensed molybdates. Fully protonated oxyanions with an octahedral structure are found in such species as Sn(OH) 2− 6 and Sb(OH) − 6 . In addition, orthoperiodate can be only partially deprotonated, [ Note 1 ] with
The naming of monomeric oxyanions follows the following rules.
Here the halogen group (group 7 A, 17) is referred to as group VII and the noble gases group (group 8 A) is referred to as group VIII.
In aqueous solution, oxyanions with high charge can undergo condensation reactions, such as in the formation of the dichromate ion, Cr 2 O 2− 7 :
The driving force for this reaction is the reduction of electrical charge density on the anion and the elimination of the hydronium ( H + ) ion. The amount of order in the solution is decreased, releasing a certain amount of entropy which makes the Gibbs free energy more negative and favors the forward reaction. It is an example of an acid–base reaction with the monomeric oxyanion acting as a base and the condensed oxyanion acting as its conjugate acid . The reverse reaction is a hydrolysis reaction, as a water molecule , acting as a base, is split. Further condensation may occur, particularly with anions of higher charge, as occurs with adenosine phosphates.
The conversion of ATP to ADP is a hydrolysis reaction and is an important source of energy in biological systems.
The formation of most silicate minerals can be viewed as the result of a de-condensation reaction in which silica reacts with a basic oxide, an acid–base reaction in the Lux–Flood sense.
A polyoxyanion is a polymeric oxyanion in which multiple oxyanion monomers, usually regarded as MO n polyhedra, are joined by sharing corners or edges. [ 4 ] When two corners of a polyhedron are shared the resulting structure may be a chain or a ring. Short chains occur, for example, in polyphosphates . Inosilicates, such as pyroxenes , have a long chain of SiO 4 tetrahedra each sharing two corners. The same structure occurs in so-called meta-vanadates, such as ammonium metavanadate , NH 4 VO 3 .
The formula of the oxyanion SiO 2− 3 is obtained as follows: each nominal silicon ion ( Si 4+ ) is attached to two nominal oxide ions ( O 2− ) and has a half share in two others. Thus the stoichiometry and charge are given by:
A ring can be viewed as a chain in which the two ends have been joined. Cyclic triphosphate , P 3 O 3− 9 is an example.
When three corners are shared the structure extends into two dimensions. In amphiboles , (of which asbestos is an example) two chains are linked together by sharing of a third corner on alternate places along the chain. This results in an ideal formula Si 4 O 6− 11 and a linear chain structure which explains the fibrous nature of these minerals. Sharing of all three corners can result in a sheet structure, as in mica , Si 2 O 2− 5 , in which each silicon has one oxygen to itself and a half-share in three others. Crystalline mica can be cleaved into very thin sheets.
The sharing of all four corners of the tetrahedra results in a 3-dimensional structure, such as in quartz . Aluminosilicates are minerals in which some silicon is replaced by aluminium. However, the oxidation state of aluminium is one less than that of silicon, so the replacement must be accompanied by the addition of another cation. The number of possible combinations of such a structure is very large, which is, in part, the reason why there are so many aluminosilicates.
Octahedral MO 6 units are common in oxyanions of the larger transition metals. Some compounds, such as salts of the chain-polymeric ion, Mo 2 O 2− 7 even contain both tetrahedral and octahedral units. [ 5 ] [ 6 ] Edge-sharing is common in ions containing octahedral building blocks and the octahedra are usually distorted to reduce the strain at the bridging oxygen atoms. This results in 3-dimensional structures called polyoxometalates . Typical examples occur in the Keggin structure of the phosphomolybdate ion. Edge sharing is an effective means of reducing electrical charge density, as can be seen with the hypothetical condensation reaction involving two octahedra:
Here, the average charge on each M atom is reduced by 2. The efficacy of edge-sharing is demonstrated by the following reaction, which occurs when an alkaline aqueous solution of molybdate is acidified.
The tetrahedral molybdate ion is converted into a cluster of 7 edge-linked octahedra [ 6 ] [ 7 ] giving an average charge on each molybdenum of 6 ⁄ 7 . The heptamolybdate cluster is so stable that clusters with between 2 and 6 molybdate units have not been detected even though they must be formed as intermediates.
The pKa of the related acids can be guessed from the number of double bonds to oxygen. Thus perchloric acid is a very strong acid while hypochlorous acid is very weak. A simple rule usually works to within about 1 pH unit.
Most oxyanions are weak bases and can be protonated to give acids or acid salts. For example, the phosphate ion can be successively protonated to form phosphoric acid.
The extent of protonation in aqueous solution will depend on the acid dissociation constants and pH . For example, AMP (adenosine monophosphate) has a p K a value of 6.21, [ 8 ] so at pH 7 it will be about 10% protonated. Charge neutralization is an important factor in these protonation reactions. By contrast, the univalent anions perchlorate and permanganate ions are very difficult to protonate and so the corresponding acids are strong acids .
Although acids such as phosphoric acid are written as H 3 PO 4 , the protons are attached to oxygen atoms forming hydroxyl groups, so the formula can also be written as OP(OH) 3 to better reflect the structure. Sulfuric acid may be written as O 2 S(OH) 2 ; this is the molecule observed in the gas phase.
The phosphite ion, PO 3− 3 , is a strong base , and so always carries at least one proton. In this case the proton is attached directly to the phosphorus atom with the structure HPO 2− 3 . In forming this ion, the phosphite ion is behaving as a Lewis base and donating a pair of electrons to the Lewis acid, H + .
As mentioned above, a condensation reaction is also an acid–base reaction. In many systems, both protonation and condensation reactions can occur. The case of the chromate ion provides a relatively simple example. In the predominance diagram for chromate, shown at the right, pCr stands for the negative logarithm of the chromium concentration and pH stands for the negative logarithm of H + ion concentration. There are two independent equilibria. Equilibrium constants are defined as follows. [ 9 ]
The predominance diagram is interpreted as follows.
The species H 2 CrO 4 and HCr 2 O − 7 are not shown as they are formed only at very low pH.
Predominance diagrams can become very complicated when many polymeric species can be formed, [ 10 ] such as in vanadates , molybdates , and tungstates . Another complication is that many of the higher polymers are formed extremely slowly, such that equilibrium may not be attained even in months, leading to possible errors in the equilibrium constants and the predominance diagram. | https://en.wikipedia.org/wiki/Oxyanion |
In chemistry , oxychlorination is a process for generating the equivalent of chlorine gas (Cl 2 ) from hydrogen chloride and oxygen . [ 1 ] This process is attractive industrially because hydrogen chloride is less expensive than chlorine. [ 2 ]
The reaction is usually initiated by copper(II) chloride (CuCl 2 ), which is the most common catalyst in the production of 1,2-dichloroethane . In some cases, CuCl 2 is supported on silica in presence of KCl, LaCl 3 , or AlCl 3 as cocatalysts. Aside from silica, a variety of supports have also been used including various types of alumina , diatomaceous earth , or pumice . Because this reaction is highly exothermic (238 kJ/mol), the temperature is monitored, to guard against thermal degradation of the catalyst. The reaction is as follows:
The copper(II) chloride is regenerated by sequential reactions of the cuprous chloride with oxygen and then hydrogen chloride :
Oxychlorination is employed in the conversion of ethylene into vinyl chloride. In the first step in this process, ethylene undergoes oxychlorination to give ethylene chloride:
Oxychlorination is of special importance in the making of 1,2-dichloroethane, which is then converted into vinyl chloride . As can be seen in the following reaction, 1,2-dichloroethane is cracked:
The HCl from this cracking process is recycled by oxychlorination in order to reduce the consumption of raw material HCl (or Cl 2 , if direct chlorination of ethylene is chosen as main way to produce 1,2-dichloroethane). [ 3 ]
Iron(III) chloride is produced commercially by oxychlorination (and other methods). For example, dissolution of iron ores in hydrochloric acid gives a mixture of ferrous and ferric chlorides: [ 4 ]
The iron(II) chloride is converted to the iron(III) derivative by treatment with oxygen and hydrochloric acid: | https://en.wikipedia.org/wiki/Oxychlorination |
Oxyfedrine , sold under the brand names Ildamen and Myofedrin among others, is a sympathomimetic agent and coronary vasodilator which is used in the treatment of coronary heart disease , angina pectoris , and acute myocardial infarction . [ 1 ] [ 3 ] [ 4 ] [ 5 ] [ 6 ] [ 7 ] It is taken by mouth or intravenously . [ 1 ]
The drug acts as a β-adrenergic receptor partial agonist . [ 1 ] [ 7 ] It may also act as a norepinephrine releasing agent via its major active metabolite norephedrine . [ 2 ] Oxyfedrine is a phenethylamine and amphetamine derivative . [ 6 ] [ 7 ]
Oxyfedrine has been marketed in Europe , Hong Kong , India , Central America , and elsewhere. [ 4 ] [ 8 ] [ 9 ] It appears to remain marketed only in India. [ 9 ]
Oxyfedrine is a β-adrenergic receptor partial agonist . [ 1 ] [ 7 ] It appears to be non-selective for the β 1 - and β 2 -adrenergic receptors . [ 7 ] It is selective for the β-adrenergic receptors over the α-adrenergic receptors . [ 7 ] However, it has also been reported to interact with the α-adrenergic receptors at high concentrations, acting as a partial agonist or antagonist of these receptors. [ 7 ] Norephedrine , a norepinephrine releasing agent , is a major active metabolite of oxyfedrine, and hence oxyfedrine may additionally act as an indirectly acting sympathomimetic. [ 2 ]
It has been found to depress the tonicity of coronary vessels, improve myocardial metabolism (so that heart can sustain hypoxia better) and also exert a positive chronotropic and inotropic effects, [ 1 ] thereby not precipitating angina pectoris. The latter property (positive chronotropic and inotropic effects) is particularly important, because other vasodilators used in angina may be counter productive causing coronary steal phenomenon. [ additional citation(s) needed ]
The drug is chemically and pharmacologically unrelated to any other antianginal drugs. [ 1 ]
Oxyfedrine's oral bioavailability is 85%. [ 1 ] The plasma protein binding is almost 100%. [ 1 ] Its elimination half-life is 4.2 hours. [ 1 ] Norephedrine is a major active metabolite of oxyfedrine. [ 2 ] The excretion of the active metabolites of oxyfedrine is 90% in urine . [ 1 ] About 75 to 100% of oxyfedrine is excreted as norephedrine. [ 2 ]
Oxyfedrine is a substituted phenethylamine and amphetamine derivative . [ 7 ] It is l -norephedrine with a bulky and lipophilic 3-methoxypropiophenone substituent at the nitrogen atom. [ 7 ]
Mannich condensation of phenylpropanolamine ( 1 ) with formaldehyde and m - acetanisole (3-acetylanisole) ( 2 ) yields oxyfedrine ( 3 ). [ 10 ]
Synergistic effects of oxyfedrine with antibiotics against bacteria have been suggested. [ 11 ] | https://en.wikipedia.org/wiki/Oxyfedrine |
Oxygen-17 ( 17 O ) is a low-abundance, natural, stable isotope of oxygen (0.0373% in seawater; approximately twice as abundant as deuterium ).
As the only stable isotope of oxygen possessing a nuclear spin (+ 5 ⁄ 2 ) and a favorable characteristic of field-independent relaxation in liquid water, 17 O enables NMR studies of oxidative metabolic pathways through compounds containing 17 O (i.e. metabolically produced H 2 17 O water by oxidative phosphorylation in mitochondria [ 3 ] ) at high magnetic fields.
Water used as nuclear reactor coolant is subjected to intense neutron flux . Natural water starts out with 373 ppm of 17 O; heavy water starts out incidentally enriched to about 550 ppm of oxygen-17. The neutron flux slowly converts 16 O in the cooling water to 17 O by neutron capture , increasing its concentration. The neutron flux slowly converts 17 O (with much greater cross section ) in the cooling water to carbon-14 , an undesirable product that can escape to the environment:
Some tritium removal facilities make a point of replacing the oxygen of the water with natural oxygen (mostly 16 O) to give the added benefit of reducing 14 C production. [ 4 ] [ 5 ]
The isotope was first hypothesized and subsequently imaged by Patrick Blackett in Rutherford's lab in 1925: [ 6 ]
Of the nature of the integrated nucleus little can be said without further data. It must however have a mass 17, and provided no other nuclear electrons are gained or lost in the process, an atomic number 8. It ought therefore to be an isotope of oxygen. If it is stable it should exist on the earth.
It was a product out of the first man-made transmutation of 14 N and 4 He 2+ conducted by Frederick Soddy and Ernest Rutherford in 1917–1919. [ 7 ] Its natural abundance in Earth's atmosphere was later detected in 1929 by Giauque and Johnson in absorption spectra. [ 8 ] | https://en.wikipedia.org/wiki/Oxygen-17 |
Oxygen-18 ( 18 O , Ω [ 1 ] ) is a natural, stable isotope of oxygen and one of the environmental isotopes .
18 O is an important precursor for the production of fluorodeoxyglucose (FDG) used in positron emission tomography (PET). Generally, in the radiopharmaceutical industry, enriched water ( H 2 Ω ) is bombarded with hydrogen ions in either a cyclotron or linear accelerator , producing fluorine-18 . This is then synthesized into FDG and injected into a patient. It can also be used to make an extremely heavy version of water when combined with tritium ( hydrogen -3): 3 H 2 18 O or T 2 Ω . This compound has a density almost 30% greater than that of natural water. [ 2 ]
The accurate measurements of 18 O rely on proper procedures of analysis, sample preparation and storage. [ 3 ]
In ice cores, mainly Arctic and Antarctic , the ratio of 18 O to 16 O (known as δ 18 O ) can be used to determine the temperature of precipitation through time. Assuming that atmospheric circulation and elevation has not changed significantly over the poles, the temperature of ice formation can be calculated as equilibrium fractionation between phases of water that is known for different temperatures. Water molecules are also subject to Rayleigh fractionation [ 4 ] as atmospheric water moves from the equator poleward which results in progressive depletion of 18 O , or lower δ 18 O values. In the 1950s, Harold Urey performed an experiment in which he mixed both normal water and water with oxygen-18 in a barrel, and then partially froze the barrel's contents. The ratio 18 O / 16 O (δ 18 O ) can also be used to determine paleothermometry in certain types of fossils. The fossils in question have to show progressive growth in the animal or plant that the fossil represents. The fossil material used is generally calcite or aragonite , however oxygen isotope paleothermometry has also been done of phosphatic fossils using SHRIMP . [ 5 ] For example, seasonal temperature variations may be determined from a single sea shell from a scallop . As the scallop grows, an extension is seen on the surface of the shell. Each growth band can be measured, and a calculation is used to determine the probable sea water temperature in comparison to each growth. The equation for this is:
Where T is temperature in Celsius and A and B are constants.
For determination of ocean temperatures over geologic time, multiple fossils of the same species in different stratigraphic layers would be measured, and the difference between them would indicate long term changes. [ 6 ]
In the study of plants' photorespiration , the labeling of atmosphere by oxygen-18 allows for the measurement of oxygen uptake by the photorespiration pathway. Labeling by 18 O 2 gives the unidirectional flux of O 2 uptake, while there is a net photosynthetic 16 O 2 evolution. It was demonstrated that, under preindustrial atmosphere, most plants reabsorb, by photorespiration, half of the oxygen produced by photosynthesis . Then, the yield of photosynthesis was halved by the presence of oxygen in atmosphere. [ 7 ] [ 8 ]
Fluorine-18 is usually produced by irradiation of 18 O-enriched water (H 2 18 O) with high-energy (about 18 MeV ) protons prepared in a cyclotron or a linear accelerator , yielding an aqueous solution of 18 F fluoride. This solution is then used for rapid synthesis of a labeled molecule, often with the fluorine atom replacing a hydroxyl group. The labeled molecules or radiopharmaceuticals have to be synthesized after the radiofluorine is prepared, as the high energy proton radiation would destroy the molecules.
Large amounts of oxygen-18 enriched water are used in positron emission tomography centers, for on-site production of 18 F-labeled fludeoxyglucose (FDG).
An example of the production cycle is a 90-minute irradiation of 2 milliliters of 18 O-enriched water in a titanium cell, through a 25 μm thick window made of Havar (a cobalt alloy ) foil, with a proton beam having an energy of 17.5 MeV and a beam current of 30 microamperes .
The irradiated water has to be purified before another irradiation, to remove organic contaminants, traces of tritium produced by a 18 O(p,t) 16 O reaction, and ions leached from the target cell and sputtered from the Havar foil. [ 9 ] | https://en.wikipedia.org/wiki/Oxygen-18 |
The oxygen-evolving complex (OEC), also known as the water-splitting complex , is a water-oxidizing enzyme involved in the photo-oxidation of water during the light reactions of photosynthesis . [ 3 ] OEC is surrounded by 4 core proteins of photosystem II at the membrane-lumen interface. The mechanism for splitting water involves absorption of three photons before the fourth provides sufficient energy for water oxidation . [ 4 ] Based on a widely accepted theory from 1970 by Kok, the complex can exist in 5 states, denoted S 0 to S 4 , with S 0 the most reduced and S 4 the most oxidized. Energy from the photons captured by photosystem II moves the system from state S 0 to S 1 to S 2 to S 3 and finally to S 4 . S 4 reacts with water producing free oxygen :
This conversion resets the catalyst to the S 0 state.
The active site of the OEC consists of a cluster of manganese and calcium with the formula Mn 4 Ca 1 O x Cl 1–2 (HCO 3 ) y . This cluster is bound to D 1 and CP 43 subunits and stabilized by peripheral membrane proteins . Many characteristics of it have been examined by flash photolysis experiments, electron paramagnetic resonance (EPR), and X-ray spectroscopy . [ 5 ]
The mechanism of the complex is proposed to involve an Mn-oxide which couples by O-O bond formation to a calcium oxide/hydroxide. [ 6 ] [ 7 ] [ 8 ]
This biochemistry article is a stub . You can help Wikipedia by expanding it . | https://en.wikipedia.org/wiki/Oxygen-evolving_complex |
Oxygen balance ( OB , OB% , or Ω ) is an expression that is used to indicate the degree to which an explosive can be oxidized , [ 1 ] to determine whether the molecules of explosive substance or mixture contains enough oxygen to fully oxidize the other atoms in the molecules. For example, fully oxidized carbon forms carbon dioxide , hydrogen forms water, sulfur forms sulfur dioxide , and metals form metal oxides. A molecule is said to have a positive oxygen balance if it contains more oxygen than is needed and a negative oxygen balance if it contains less oxygen than is needed. [ 2 ]
An explosive with a negative oxygen balance will lead to incomplete combustion , which commonly produces carbon monoxide , which is a toxic gas. Explosives with negative or positive oxygen balance are commonly mixed with other energetic materials that are either oxygen positive or negative, respectively, to increase the explosive's power. For example, TNT is an oxygen negative explosive and is commonly mixed with oxygen positive energetic materials or fuels to increase its power. [ 3 ] [ 4 ]
Detonating a mixture of TNT (trinitrotoluene) and RDX (cyclotrimethylenetrinitramine), with its negative oxygen balance, in a closed chamber produces 5-nm detonation nanodiamonds .
The procedure for calculating oxygen balance in terms of 100 grams of the explosive material is to determine the number of moles of oxygen that are excess or deficient for 100 grams of the compound.
X = number of atoms of carbon, Y = number of atoms of hydrogen, Z = number of atoms of oxygen, and M = number of atoms of metal (metallic oxide produced).
In the case of TNT (C 6 H 2 (NO 2 ) 3 CH 3 ),
Molecular weight = 227.1
X = 7 (number of carbon atoms)
Y = 5 (number of hydrogen atoms)
Z = 6 (number of oxygen atoms)
Therefore,
Examples of materials with negative oxygen balance are nitromethane (−39%), trinitrotoluene (−74%), aluminium powder (−89%), sulfur (−100%), or carbon (−266.7%). Examples of materials with positive oxygen balance are ammonium nitrate (+20%), ammonium perchlorate (+34%), potassium chlorate (+39.2%), sodium chlorate (+45%), potassium nitrate (+47.5%), tetranitromethane (+49%), lithium perchlorate (+60%), or nitroglycerine (+3.5%). Ethylene glycol dinitrate has an oxygen balance of zero, as does the theoretical compound trinitrotriazine . [ citation needed ] [ 5 ]
Because sensitivity, brisance , and strength are properties resulting from a complex explosive chemical reaction, a simple relationship such as oxygen balance cannot be depended upon to yield universally consistent results. When using oxygen balance to predict properties of one explosive relative to another, it is to be expected that one with an oxygen balance closer to zero will be the more brisant, powerful, and sensitive; however, many exceptions to this rule do exist. [ citation needed ]
One area in which oxygen balance can be applied is in the processing of mixtures of explosives. The family of explosives called amatols are mixtures of ammonium nitrate and TNT . Ammonium nitrate has an oxygen balance of +20% and TNT has an oxygen balance of −74%, so it would appear that the mixture yielding an oxygen balance of zero would also result in the best explosive properties. In actual practice a mixture of 80% ammonium nitrate and 20% TNT by weight yields an oxygen balance of +1%, the best properties of all mixtures, and an increase in strength of 30% over TNT. [ citation needed ] | https://en.wikipedia.org/wiki/Oxygen_balance |
An oxygen bar is an establishment, or part of one, that sells oxygen for recreational use. Individual scents may be added to enhance the experience. The flavors in an oxygen bar come from bubbling oxygen through bottles containing aromatic solutions before it reaches the nostrils: most bars use food-grade particles to produce the scent, but some bars use aroma oils. [ 1 ] [ 2 ]
In 1776, Thomas Henry , an apothecary and Fellow of the Royal Society of England speculated tongue in cheek that Joseph Priestley ’s newly discovered dephlogisticated air (now called oxygen) might become "as fashionable as French wine at the fashionable taverns". He did not expect, however, that tavern goers would "relish calling for a bottle of Air, instead of Claret ." [ 3 ]
Another early reference to the recreational use of oxygen is found in Jules Verne 's 1870 novel Around the Moon . In this work, Verne states:
Do you know, my friends, that a curious establishment might be founded with rooms of oxygen, where people whose system is weakened could for a few hours live a more active life. Fancy parties where the room was saturated with this heroic fluid, theaters where it should be kept at high pressure; what passion in the souls of the actors and spectators! what fire, what enthusiasm! And if, instead of an assembly only a whole people could be saturated, what activity in its functions, what a supplement to life it would derive. From an exhausted nation they might make a great and strong one, and I know more than one state in old Europe which ought to put itself under the regime of oxygen for the sake of its health!
Modeled after the "air stations" in polluted downtown Tokyo and Beijing, the first oxygen bar (the O2 Spa Bar) opened in Toronto, Canada, in 1996. The trend continued in North America and by the late 1990s, bars were in use in New York, California, Florida, Las Vegas and the Rocky Mountain region. Customers in these bars breathe oxygen through a plastic nasal cannula inserted into their nostrils. [ 4 ] Oxygen bars can now be found in many venues such as nightclubs, salons, spas, health clubs, resorts, tanning salons, restaurants, coffee houses, bars, airports, ski chalets, yoga studios, chiropractors, and casinos. They can also be found at trade shows, conventions and corporate meetings, as well as at private parties and promotional events.
Oxygen bar guests pay about one U.S. dollar per minute to inhale a percentage of oxygen greater than the normal atmospheric content of 20.9% oxygen. This oxygen is gathered from the ambient air by an industrial (non-medical) oxygen concentrator and inhaled through a nasal cannula for up to about 20 minutes. [ 5 ]
The machines used by oxygen bars or oxygen vendors differ from the typical medical-issue machine, although customers use the cannula, the rubber tube apparatus that fits around the ears and inserts in the nostrils, to breathe in the oxygen. Customers can enhance their experience by using aromatherapy scents to be added to the oxygen, such as lavender or mint. [ 6 ]
It has been claimed by alternative medicine that the human body is oxygen-deprived, and that oxygen will remove "toxins" and even cure cancer. [ 7 ] Proponents claim this practice is not only safe, but enhances health and well-being, including strengthening the immune system , enhancing concentration, reducing stress, increasing energy and alertness, lessening the effects of hangovers, headaches, and sinus problems, and generally relaxing the body. [ 5 ] It has also been alleged to help with altitude sickness. However, no long-term, well-controlled scientific studies have confirmed any of the proponents' claims. [ 5 ] Furthermore, the human body is adapted to 21 percent oxygen, and the blood exiting the lungs already has about 97 percent of the oxygen that it could carry bound to hemoglobin . Having a higher oxygen fraction in the lungs serves no purpose, and may actually be detrimental. [ 7 ]
The medical profession warns that individuals with respiratory diseases such as asthma and emphysema should not inhale too much oxygen. [ 5 ] Higher than normal oxygen partial pressure can also indirectly cause carbon dioxide narcosis in patients with chronic obstructive pulmonary disease (COPD). [ 8 ] The FDA warns that in some situations, droplets of flavoring oil can be inhaled, which may contribute to an inflammation of the lungs . Some oxygen bar companies offer safe water-based aromas for flavoring in order to maintain compliance and stay within FDA guidelines. [ 5 ]
Oxygen may also cause serious side effects at excessive doses. Although the effects of oxygen toxicity at atmospheric pressure can cause lung damage, [ 9 ] the low fraction of oxygen (30–40%) [ 10 ] and relatively brief exposures make pulmonary toxicity unlikely. [ 11 ] Nevertheless, due caution should be exercised when consuming oxygen. In the UK, the Health and Safety Executive publishes guidance on equipment (including tubing) and on staff training, as well as warning on potential hazards, and makes several recommendations to ensure safe practice, principally to minimise fire risks. [ 12 ]
Another concern is the improper maintenance of oxygen equipment. Some oxygen concentrators use clay filters which cause micro-organisms to grow, creating an additional danger that can cause lung infections. [ 13 ]
Raised concentrations of oxygen increase the risk of ignition, the rate and heat of combustion , and the difficulty of extinguishing a fire. Many materials that will not burn in air will burn in a sufficiently high partial pressure of oxygen.
In the United States, the Federal Food, Drug, and Cosmetic Act defines any substance used for breathing and administered by another person as a prescription drug.
Melvin Szymanski, a consumer safety officer in the Food and Drug Administration's (FDA) Center for Drug Evaluation and Research, has explained that at one end of the hose is a source of oxygen, so the individual providing the hose and turning on the supply is dispensing a prescription drug. [ 14 ] He commented that "Although oxygen bars that dispense oxygen without a prescription violate FDA regulations, the agency applies regulatory discretion to permit the individual state boards of licensing to enforce the requirements pertaining to the dispensing of oxygen." [ 14 ]
In the state of Massachusetts , oxygen bars are illegal. [ 15 ] | https://en.wikipedia.org/wiki/Oxygen_bar |
In respiratory physiology , the oxygen cascade describes the flow of oxygen from air to mitochondria , where it is consumed in aerobic respiration to release energy. [ 1 ] Oxygen flows from areas with high partial pressure of oxygen (PO 2 , also known as oxygen tension ) to areas of lower PO 2 .
Air is typically around 21% oxygen, and at sea level , the PO 2 of air is typically around 159 mmHg . [ 2 ] Humidity dilutes the concentration of oxygen in air. As air is inhaled into the lungs, it mixes with water and exhaust gasses including CO 2 , further diluting the oxygen concentration and lowering the PO 2 . As oxygen continues to flow down the concentration gradient from areas of higher concentration to areas of lower concentration, it must pass through barriers such as the alveoli walls, capillary walls , capillary blood plasma , red blood cell membrane, interstitial space , other cell membranes , and cell cytoplasm . The partial pressure of oxygen drops across each barrier. [ 3 ]
Table 1 gives the example of a typical oxygen cascade for skeletal muscle of a healthy, adult male at rest who is breathing air at atmospheric pressure at sea level . Actual values in a person may vary widely due to ambient conditions, health status, tissue type, and metabolic demands. | https://en.wikipedia.org/wiki/Oxygen_cascade |
Oxygen compatibility is the issue of compatibility of materials for service in high concentrations of oxygen . It is a critical issue in space, aircraft, medical, underwater diving and industrial applications.
Aspects include effects of increased oxygen concentration on the ignition and burning of materials and components exposed to these concentrations in service.
Understanding of fire hazards is necessary when designing, operating, and maintaining oxygen systems so that fires can be prevented. Ignition risks can be minimized by controlling heat sources and using materials that will not ignite or will not support burning in the applicable environment. Some materials are more susceptible to ignition in oxygen-rich environments, and compatibility should be assessed before a component is introduced into an oxygen system. [ 1 ] Both partial pressure and concentration of oxygen affect the fire hazard.
The issues of cleaning and design are closely related to the compatibility of materials for safety and durability in oxygen service.
Fires occur when oxygen, fuel, and heat energy combine in a self-sustaining chemical reaction. In an oxygen system the presence of oxygen is implied, and in a sufficiently high partial pressure of oxygen, most materials can be considered fuel. Potential ignition sources are present in almost all oxygen systems, but fire hazards can be mitigated by controlling the risk factors associated with the oxygen, fuel, or heat, which can limit the tendency for a chemical reaction to occur.
Materials are easier to ignite and burn more readily as oxygen pressure or concentration increase, so operating oxygen systems at the lowest practicable pressure and concentration may be enough to avoid ignition and burning.
Use of materials which are inherently more difficult to ignite or are resistant to sustained burning, or which release less energy when they burn, can, in some cases, eliminate the possibility of fire or minimize the damage caused by a fire.
Although heat sources may be inherent in the operation of an oxygen system, initiation of the chemical reaction between the system materials and oxygen can be limited by controlling the ability of those heat sources to cause ignition. Design features which can limit or dissipate the heat generated to keep temperatures below the ignition temperatures of the system materials will prevent ignition.
An oxygen system should also be protected from external heat sources. [ 1 ]
The process of assessment of oxygen compatibility would generally include the following stages: [ 1 ]
Compatibility analysis would also consider the history of use of the component or material in similar conditions, or of a similar component.
Oxygen service implies use in contact with high partial pressures of oxygen. Generally this is taken to mean a higher partial pressure than possible from compressed air, but also can occur at lower pressures when the concentration is high.
Oxygen cleaning is preparation for oxygen service by ensuring that the surfaces that may come into contact with high partial pressures of oxygen while in use are free of contaminants that increase the risk of ignition. [ 2 ]
Oxygen cleaning is a necessary, but not always a sufficient condition for high partial pressure or high concentration oxygen service. The materials used must also be oxygen compatible at all expected service conditions. Aluminium and titanium components are specifically not suitable for oxygen service. [ 2 ]
In the case of diving equipment, oxygen cleaning generally involves the stripping down of the equipment into individual components which are then thoroughly cleaned of hydrocarbon and other combustible contaminants using non-flammable, non-toxic cleaners. Once dry, the equipment is reassembled under clean conditions. Lubricants are replaced by specifically oxygen- compatible substitutes during reassembly. [ 2 ]
The standard and requirements for oxygen cleaning of diving apparatus varies depending on the application and applicable legislation and codes of practice.
For scuba equipment, the industry standard is that breathing apparatus which will be exposed to concentrations in excess of 40% oxygen by volume should be oxygen cleaned before being put into such service. [ 2 ] Surface supplied equipment may be subject to more stringent requirements, as the diver may not be able to remove the equipment in an accident. Oxygen cleaning may be required for concentrations as low as 23% [ 3 ] Other common specifications for oxygen cleaning include ASTM G93 and CGA G-4.1. [ 4 ]
Cleaning agents used range from heavy-duty industrial solvents and detergents such as liquid freon , trichlorethylene and anhydrous trisodium phosphate , followed by rinsing in deionised water . These materials are now generally deprecated as being environmentally unsound and an unnecessary health hazard. Some strong all-purpose household detergents have been found to do the job adequately. They are diluted with water before use, and used hot for maximum efficacy. Ultrasonic agitation, shaking, pressure spraying and tumbling using glass or stainless steel beads or mild ceramic abrasives are effectively used to speed up the process where appropriate. Thorough rinsing and drying is necessary to ensure that the equipment is not contaminated by the cleaning agent. Rinsing should continue until the rinse water is clear and does not form a persistent foam when shaken. Drying using heated gas – usually hot air – is common and speeds up the process. Use of a low oxygen fraction drying gas can reduce flash-rusting of the interior of steel cylinders. [ 2 ]
After cleaning and drying, and before reassembly, the cleaned surfaces are inspected and where appropriate, tested for the presence of contaminants. Inspection under ultraviolet illumination can show the presence of fluorescent contaminants, but is not guaranteed to show all contaminants. [ 2 ]
Design for oxygen service includes several aspects:
As a general rule, oxygen compatibility is associated with a high ignition temperature, and a low rate of reaction once ignited. [ 6 ]
Organic materials generally have lower ignition temperatures than metals considered suitable for oxygen service. Therefore the use of organic materials in contact with oxygen should be avoided or minimised, particularly when the material is directly exposed to gas flow. When an organic material must be used for parts such as diaphragms, seals, packing or valve seats, the material with the highest ignition temperature for the required mechanical properties is usually chosen. Fluoroelastomers are preferred where large areas are in direct contact with oxygen flow. Other materials may be acceptable for static seals where the flow does not come into direct contact with the component. [ 6 ]
Only tested and certified oxygen compatible lubricants and sealants should be used, and in as small quantities as is reasonably practicable for effective function. Projection of excess sealant or contamination by lubricant into flow regions should be avoided. [ 5 ]
Commonly used engineering metals with a high resistance to ignition in oxygen include copper, copper alloys, and nickel-copper alloys, and these metals also do not normally propagate combustion, making them generally suitable for oxygen service. They are also available in free-cutting, castable or highly ductile alloys, and are reasonably strong, so are useful for a wide range of components for oxygen service. [ 6 ]
Aluminium alloys have a relatively low ignition temperature, and release a large amount of heat during combustion and are not considered suitable for oxygen service where they will be directly exposed to flow, but are acceptable for storage cylinders where the flow rate and temperatures are low. [ 5 ]
Hazards analyses are performed on materials, components, and systems; and failure analyses determine the cause of fires. Results are used in design and operation of safe oxygen systems. | https://en.wikipedia.org/wiki/Oxygen_compatibility |
An oxygen concentrator is a device that concentrates the oxygen from a gas supply (typically ambient air) by selectively removing nitrogen to supply an oxygen-enriched product gas stream. They are used industrially, to provide supplemental oxygen at high altitudes, and as medical devices for oxygen therapy . [ 1 ]
Oxygen concentrators are used widely for oxygen provision in healthcare applications, especially where liquid or pressurized oxygen is too dangerous or inconvenient, such as in homes or portable clinics, and can also provide an economical source of oxygen in industrial processes, where they are also known as oxygen gas generators or oxygen generation plants . Two methods in common use are pressure swing adsorption and membrane gas separation .
Pressure swing adsorption (PSA) oxygen concentrators use a molecular sieve to adsorb gases and operate on the principle of rapid pressure swing adsorption of atmospheric nitrogen onto zeolite minerals at high pressure. This type of adsorption system is therefore functionally a nitrogen scrubber, allowing the other atmospheric gases to pass through, leaving oxygen as the primary gas remaining. PSA technology is a reliable and economical technique for small to mid-scale oxygen generation. Cryogenic separation is more suitable at higher volumes. [ 2 ]
Gas separation across a membrane is a pressure-driven process, where the driving force is the difference in pressure between inlet of raw material and outlet of product. The membrane used in the process is a generally non-porous layer, so there will not be a severe leakage of gas through the membrane. The performance of the membrane depends on permeability and selectivity. Permeability is affected by the penetrant size. Larger gas molecules have a lower diffusion coefficient. The membrane gas separation equipment typically pumps gas into the membrane module and the targeted gases are separated based on difference in diffusivity and solubility. For example, oxygen will be separated from the ambient air and collected at the upstream side, and nitrogen at the downstream side. As of 2016, membrane technology was reported as capable of producing 10 to 25 tonnes of 25 to 40% oxygen per day. [ 3 ]
Home medical oxygen concentrators were invented in the early 1970s, with the manufacturing output of these devices increasing in the late 1970s. Union Carbide Corporation and Bendix Corporation were both early manufacturers. Before that era, home medical oxygen therapy required the use of heavy high-pressure oxygen cylinders or small cryogenic liquid oxygen systems. Both of these delivery systems required frequent home visits by suppliers to replenish oxygen supplies. In the United States, Medicare switched from fee-for-service payment to a flat monthly rate for home oxygen therapy in the mid-1980s, causing the durable medical equipment (DME) industry to rapidly embrace concentrators as a way to control costs. This reimbursement change dramatically decreased the number of primary high pressure and liquid oxygen delivery systems in use in homes in the United States at that time. Oxygen concentrators became the preferred and most common means of delivering home oxygen. The number of manufacturers entering the oxygen concentrator market increased greatly as a result of this change. Union Carbide Corporation invented the molecular sieve in the 1950s, which made these devices possible. It also invented the first cryogenic liquid home medical oxygen systems in the 1960s.
Oxygen concentrators using pressure swing adsorption (PSA) technology are used widely for oxygen provision in healthcare applications, especially where liquid or pressurized oxygen is too dangerous or inconvenient, such as in homes or portable clinics. For other purposes, there are also concentrators based on nitrogen separation membrane technology.
An oxygen concentrator takes in air and removes nitrogen from it, leaving an oxygen-enriched gas for use by people requiring medical oxygen due to low oxygen levels in their blood. [ 4 ] Oxygen concentrators provide an economical source of oxygen in industrial processes, where they are also known as oxygen gas generators or oxygen generation plants .
These oxygen concentrators utilize a molecular sieve to adsorb gases and operate on the principle of rapid pressure swing adsorption of atmospheric nitrogen onto zeolite minerals at high pressure. This type of adsorption system is therefore functionally a nitrogen scrubber, allowing the other atmospheric gases to pass through, leaving oxygen as the primary gas remaining. PSA technology is a reliable and economical technique for small- to mid-scale oxygen generation. Cryogenic separation is more suitable at higher volumes, and external delivery generally more suitable for small volumes. [ 5 ]
At high pressure, the porous zeolite adsorbs large quantities of nitrogen because of its large surface area and chemical characteristics. The oxygen concentrator compresses air and passes it over zeolite, causing the zeolite to adsorb the nitrogen from the air. It then collects the remaining gas, which is mostly oxygen, and the nitrogen desorbs from the zeolite under the reduced pressure to be vented.
An oxygen concentrator has an air compressor, two cylinders filled with zeolite pellets, a pressure-equalizing reservoir, and some valves and tubes. In the first half-cycle, the first cylinder receives air from the compressor, which lasts about 3 seconds. During that time, the pressure in the first cylinder rises from atmospheric to about 2.5 times normal atmospheric pressure (typically 20 psi/138 kPa gauge, or 2.36 atmospheres absolute) and the zeolite becomes saturated with nitrogen. As the first cylinder reaches near pure oxygen (there are small amounts of argon, CO 2 , water vapour, radon , and other minor atmospheric components) in the first half-cycle, a valve opens and the oxygen-enriched gas flows to the pressure-equalizing reservoir, which connects to the patient's oxygen hose. At the end of the first half of the cycle, there is another valve position change so that the air from the compressor is directed to the second cylinder. The pressure in the first cylinder drops as the enriched oxygen moves into the reservoir, allowing the nitrogen to be desorbed back into gas. Partway through the second half of the cycle, there is another valve position change to vent the gas in the first cylinder back into the ambient atmosphere, keeping the concentration of oxygen in the pressure-equalizing reservoir from falling below about 90%. The pressure in the hose delivering oxygen from the equalizing reservoir is kept steady by a pressure-reducing valve.
Older units cycled for a period of about 20 seconds and supplied up to 5 litres per minute of 90+% oxygen. Since about 1999, units capable of supplying up to 10 L/min have been available.
Classic oxygen concentrators use two-bed molecular sieves; newer concentrators use multi-bed molecular sieves. The advantage of the multi-bed technology is the increased availability and redundancy, as the 10 L/min molecular sieves are staggered and multiplied on several platforms. With this, over 960 L/min can be produced. The ramp-up time — the elapsed time until a multi-bed concentrator is producing oxygen at >90% concentration — is often less than 2 minutes, much faster than simple two-bed concentrators. This is a big advantage in mobile emergencies. The option to fill standard oxygen cylinders (e.g., 50 L at 200 bar = 10,000 L each) with high-pressure boosters, to ensure automatic failover to previously filled reserve cylinders and to ensure the oxygen supply chain, e.g., in case of power failure, is given with those systems.
In membrane gas separation , membranes act as a permeable barrier, which different compounds move across at different rates or do not cross at all.
Gas mixtures can be effectively separated by synthetic membranes made from polymers such as polyamide or cellulose acetate , or from ceramic materials. [ 6 ]
While polymeric membranes are economical and technologically useful, they are bound by their performance, known as the Robeson limit (permeability must be sacrificed for selectivity and vice versa). [ 7 ] This limit affects polymeric membrane use for CO 2 separation from flue gas streams, since mass transport becomes limiting and CO 2 separation becomes very expensive due to low permeabilities. Membrane materials have expanded into the realm of silica , zeolites , metal-organic frameworks , and perovskites , due to their strong thermal and chemical resistance as well as high tunability (ability to be modified and functionalized), leading to increased permeability and selectivity. Membranes can be used for separating gas mixtures, where they act as a permeable barrier through which different compounds move across at different rates or don't move at all. The membranes can be nanoporous, polymer, etc., and the gas molecules penetrate according to their size, diffusivity , or solubility.
Gas separation across a membrane is a pressure-driven process, where the driving force is the difference in pressure between inlet of raw material and outlet of product. The membrane used in the process is a generally non-porous layer, so there will not be a severe leakage of gas through the membrane. The performance of the membrane depends on permeability and selectivity. Permeability is affected by the penetrant size. Larger gas molecules have a lower diffusion coefficient. The polymer chain flexibility and free volume in the polymer of the membrane material influence the diffusion coefficient, as the space within the permeable membrane must be large enough for the gas molecules to diffuse across. The solubility is expressed as the ratio of the concentration of the gas in the polymer to the pressure of the gas in contact with it. Permeability is the ability of the membrane to allow the permeating gas to diffuse through the material of the membrane as a consequence of the pressure difference over the membrane, and can be measured in terms of the permeate flow rate, membrane thickness and area, and the pressure difference across the membrane. The selectivity of a membrane is a measure of the ratio of permeability of the relevant gases for the membrane. It can be calculated as the ratio of permeability of two gases in binary separation. [ 3 ]
The membrane gas separation equipment typically pumps gas into the membrane module, and the targeted gases are separated based on difference in diffusivity and solubility. For example, oxygen will be separated from the ambient air and collected at the upstream side and nitrogen at the downstream side. As of 2016, membrane technology was reported as capable of producing 10 to 25 tonnes of 25 to 40% oxygen per day. [ 3 ]
Medical oxygen concentrators are used in hospitals or at home to concentrate oxygen for patients. [ 8 ] PSA generators provide a cost-efficient source of oxygen . They are a safer, [ 9 ] less expensive, [ 10 ] and more convenient alternative to tanks of cryogenic oxygen or pressurised cylinders. They can be used in various industries, including medical, pharmaceutical production, water treatment, and glass manufacture.
PSA generators are particularly useful in remote or inaccessible parts of the world or mobile medical facilities ( military hospitals , disaster facilities). [ 11 ] [ 12 ]
Since the early 2000s, many companies have produced portable oxygen concentrators. [ 13 ] Typically, these devices produce the equivalent of one to five liters per minute of continuous oxygen flow and they use some version of pulse flow or "demand flow" to deliver oxygen only when the patient is inhaling. [ 14 ] They can also provide pulses of oxygen either to provide higher intermittent flows or to reduce power consumption.
Research into oxygen concentration is ongoing, and modern techniques suggest that the amount of adsorbent required by medical oxygen concentrators can be potentially "reduced by a factor of three while offering ~10–20% higher oxygen recovery compared to a typical commercial unit." [ 15 ]
The FAA has approved the use of portable oxygen concentrators on commercial airlines. [ 16 ] However, users of these devices should check in advance as to whether a particular brand or model is permitted on a particular airline. [ 17 ] Unlike in commercial airlines, users of aircraft without cabin pressurization need oxygen concentrators that are able to deliver enough flowrate even at high altitudes.
Usually, "demand," or pulse-flow, oxygen concentrators are not used by patients while they sleep. There have been problems with the oxygen concentrators not being able to detect when the sleeping patient is inhaling. Some larger portable oxygen concentrators are designed to operate in a continuous-flow mode in addition to pulse-flow mode. Continuous-flow mode is considered safe for night use when coupled with a CPAP machine.
Repurposed medical oxygen concentrators or specialized industrial oxygen concentrators can be made to operate small oxyacetylene or other fuel gas cutting, welding, and lampworking torches. [ 18 ]
Oxygen is widely needed for the oxidation of different chemicals for industrial purposes. Previously, these industries purchased oxygen cylinders in large numbers to meet their requirements, but it was very expensive, and oxygen cylinders were not always available in the market.
Oxygen is needed here for the bleaching of paper pulp with the help of the oxidation process to make the paper white.
Moreover, lignin present in the wood is removed by the delignification process, which also needs oxygen.
Huge furnaces are needed to melt the raw materials that combine to form glass. Oxygen flares up the furnace's fire to burn at a higher temperature needed for the production of glass.
Oxygen is needed for the oxidation of different chemicals to form the desired chemical substances. Waste chemical products are burnt down and destroyed in the incinerator with the help of oxygen; thus, the continuous supply of a bulk amount of oxygen is essential, which is possible only by a PSA oxygen generator.
In both clinical and emergency-care situations, oxygen concentrators have the advantage of not being as dangerous as oxygen cylinders , which can, if ruptured or leaking, greatly increase the combustion rate of fire. As such, oxygen concentrators are particularly advantageous in military or disaster situations, where oxygen tanks may be dangerous or unfeasible.
Oxygen concentrators are considered sufficiently foolproof to be supplied to individual patients as a prescription item for use in their homes. Typically they are used as an adjunct to CPAP treatment of severe sleep apnea . There also are other medical uses for oxygen concentrators, including COPD and other respiratory diseases.
People who depend upon oxygen concentrators for home care may have life-threatening emergencies if the electricity fails during a natural disaster . [ 19 ]
Industrial processes may use much higher pressures and flows than medical units. To meet that need, another process, called vacuum swing adsorption (VSA), has been developed by Air Products . This process uses a single low-pressure blower and a valve that reverses the flow through the blower so that the regeneration phase occurs under a vacuum. Generators using this process are being marketed to the aquaculture industry. Industrial oxygen concentrators are often available in a much wider range of capacities than medical concentrators.
Industrial oxygen concentrators are sometimes referred to as oxygen generators within the oxygen and ozone industries to distinguish them from medical oxygen concentrators . The distinction is used in an attempt to clarify that industrial oxygen concentrators are not medical devices approved by the Food and Drug Administration (FDA) and they are not suitable for use as bedside medical concentrators. However, applying the oxygen generator nomenclature can lead to confusion. The term oxygen generator is a misnomer in that the oxygen is not generated as it is with a chemical oxygen generator , but rather it is concentrated from the air.
Non-medical oxygen concentrators can be used as feed gas to a medical oxygen system, such as the oxygen system in a hospital, though governmental approval is required, such as by the FDA, and additional filtering is generally required.
The COVID-19 pandemic increased the demand for oxygen concentrators. During the pandemic open source oxygen concentrators were developed, locally manufactured – with prices below imported products – and used, especially during a COVID-19 pandemic wave in India . [ 20 ] [ 21 ] | https://en.wikipedia.org/wiki/Oxygen_concentrator |
The oxygen cycle refers to the various movements of oxygen through the Earth 's atmosphere ( air ), biosphere ( flora and fauna ), hydrosphere ( water bodies and glaciers ) and the lithosphere (the Earth's crust ). The oxygen cycle demonstrates how free oxygen is made available in each of these regions, as well as how it is used. It is the biogeochemical cycle of oxygen atoms between different oxidation states in ions , oxides and molecules through redox reactions within and between the spheres/reservoirs of the planet Earth. [ 1 ] The word oxygen in the literature typically refers to the most common oxygen allotrope , elemental/diatomic oxygen (O 2 ), as it is a common product or reactant of many biogeochemical redox reactions within the cycle. [ 2 ] Processes within the oxygen cycle are considered to be biological or geological and are evaluated as either a source (O 2 production) or sink (O 2 consumption). [ 1 ] [ 2 ]
Oxygen is one of the most common elements on Earth and represents a large portion of each main reservoir. By far the largest reservoir of Earth's oxygen is within the silicate and oxide minerals of the crust and mantle (99.5% by weight). [ 6 ] The Earth's atmosphere, hydrosphere , and biosphere together hold less than 0.05% of the Earth's total mass of oxygen. Besides O 2 , additional oxygen atoms are present in various forms spread throughout the surface reservoirs in the molecules of biomass , H 2 O , CO 2 , HNO 3 , NO , NO 2 , CO , H 2 O 2 , O 3 , SO 2 , H 2 SO 4 , MgO , CaO , Al 2 O 3 , SiO 2 , and PO 3− 4 . [ 7 ]
While there are many abiotic sources and sinks for O 2 , the presence of the profuse concentration of free oxygen in modern Earth's atmosphere and ocean is attributed to O 2 production in the biological process of oxygenic photosynthesis in conjunction with a biological sink known as the biological pump and a geologic process of carbon burial involving plate tectonics . [ 9 ] [ 10 ] [ 11 ] [ 7 ] Biology is the main driver of O 2 flux on modern Earth, and the evolution of oxygenic photosynthesis by bacteria , which is discussed as part of the Great Oxygenation Event , is thought to be directly responsible for the conditions permitting the development and existence of all complex eukaryotic metabolism . [ 12 ] [ 13 ] [ 14 ]
The main source of atmospheric free oxygen is photosynthesis, which produces sugars and free oxygen from carbon dioxide and water:
Photosynthesizing organisms include the plant life of the land areas, as well as the phytoplankton of the oceans. The tiny marine cyanobacterium Prochlorococcus was discovered in 1986 and accounts for up to half of the photosynthesis of the open oceans. [ 15 ] [ 16 ]
An additional source of atmospheric free oxygen comes from photolysis , whereby high-energy ultraviolet radiation breaks down atmospheric water and nitrous oxide into component atoms. The free hydrogen and nitrogen atoms escape into space, leaving O 2 in the atmosphere:
The main way free oxygen is lost from the atmosphere is via respiration and decay , mechanisms in which animal life and bacteria consume oxygen and release carbon dioxide.
The following tables offer estimates of oxygen cycle reservoir capacities and fluxes.
These numbers are based primarily on estimates from (Walker, J. C. G.): [ 10 ] More recent research indicates that ocean life ( marine primary production ) is actually responsible for more than half the total oxygen production on Earth. [ 17 ] [ 18 ]
The presence of atmospheric oxygen has led to the formation of ozone (O 3 ) and the ozone layer within the stratosphere :
The ozone layer is extremely important to modern life as it absorbs harmful ultraviolet radiation: | https://en.wikipedia.org/wiki/Oxygen_cycle |
An oxygen diffusion-enhancing compound is any substance that increases the availability of oxygen in body tissues by influencing the molecular structure of water in blood plasma and thereby promoting the movement ( diffusion ) of oxygen through plasma. [ 1 ] Oxygen diffusion-enhancing compounds have shown promise in the treatment of conditions associated with hypoxia (a lack of oxygen in tissues) and ischemia (a lack of oxygen in the circulating blood supply). [ 1 ] [ 2 ] Such conditions include hemorrhagic shock , myocardial infarction (heart attack), and stroke . [ 2 ]
One of the first substances that was reported to produce an oxygen diffusion-enhancing effect was crocetin , [ 3 ] a carotenoid that occurs naturally in plants such as crocus sativus , and is related to another carotenoid, saffron . Saffron has been used culturally (e.g., as a dye) and medicinally since ancient times. [ 4 ]
Trans sodium crocetinate (TSC), a synthetic drug containing the carotenoid structure of trans crocetin has been extensively investigated in animal disease models and in human clinical trials. [ 2 ] [ 5 ] [ 6 ] Clinical trials of TSC have focused on testing the compound's effectiveness in sensitizing hypoxic cancer cells to radiation therapy in patients with glioblastoma , an aggressive form of brain cancer. [ 6 ]
TSC, which is being developed by Diffusion Pharmaceuticals , has been shown to enhance the oxygenation of hypoxic tumor tissue [ 7 ] and belongs to a subclass of oxygen diffusion-enhancing compounds known as bipolar trans carotenoid salts. [ 1 ] Diffusion Pharmaceuticals is currently investigating the use of trans sodium crocetinate in the treatment of COVID-19 , acute stroke , and solid cancerous tumors. [ 8 ]
Oxygen diffusion-enhancing compounds are thought to act by exerting hydrophobic forces that interact with water molecules. [ 9 ] These interactions result in greater hydrogen bonding among water molecules, which constitute the majority of the blood plasma medium. [ 9 ] [ 10 ] As hydrogen bonding increases, the overall molecular structure of water in the plasma becomes more lattice-like, a phenomenon known as structure building. [ 11 ] [ 12 ] Structure building reduces resistance to the movement of oxygen through plasma via diffusion. [ 11 ] Since blood plasma offers the major barrier for oxygen to move from the red blood cells and into the tissues, [ 2 ] the more structured character of water imparted by the oxygen diffusion-enhancing compound will enhance movement into tissues. [ 9 ] [ 13 ]
Computer simulations have shown that TSC specifically can increase the transport of oxygen through water by as much as 30 percent. [ 10 ] | https://en.wikipedia.org/wiki/Oxygen_diffusion-enhancing_compound |
In biochemistry , the oxygen effect refers to a tendency for increased radiosensitivity of free living cells and organisms in the presence of oxygen than in anoxic or hypoxic conditions, where the oxygen tension is less than 1% of atmospheric pressure (i.e., <1% of 101.3 kPa, 760 mmHg or 760 torr).
The oxygen effect has particular importance in external beam radiation therapy where the killing of tumour cells with photon and electron beams in well oxygenated regions can be up to three times greater than in a poorly vasculated portion of the tumour.
Besides tumour hypoxia , the oxygen effect is also relevant to hypoxia conditions present in the normal physiology of stem cell niches such as the endosteum adjacent to bone in bone marrow [ 1 ] and the epithelium layer of the intestine . [ 2 ] In addition, there are non-malignant diseases where oxygenated tissues can become hypoxic such as in stenosed coronary arteries associated with cardiovascular disease . [ 3 ]
Holthusen (1921) [ 4 ] first quantified the oxygen effect finding 2.5 to 3.0-fold less hatching eggs of the nematode Ascaris in oxygenated compared to anoxic conditions, which was incorrectly assigned to changes in cell division . However, two years later, Petry (1923) [ 5 ] first attributed oxygen tension as affecting ionizing radiation effects on vegetable seeds. Later, the implications of the effects of oxygen on radiotherapy were discussed by Mottram (1936). [ 6 ]
A key observation limiting hypotheses to explain the biological mechanisms of the oxygen effect is that the gas nitric oxide is a radiosensitizer with similar effects to oxygen observed in tumour cells. [ 7 ] Another important observation is that oxygen must be present at irradiation or within milliseconds afterward for the oxygen effect to take place. [ 8 ]
The best known explanation of the oxygen effect is the oxygen fixation hypothesis developed by Alexander in 1962, [ 9 ] which posited that radiation-induced non-restorable or "fixed" nuclear DNA lesions are lethal to cells in the presence of diatomic oxygen . [ 10 ] [ 11 ] Recent hypotheses include one based on oxygen-enhanced damage from first principles. [ 12 ] Another hypothesis posits that ionizing radiation provokes mitochondria to produce reactive oxygen (and nitrogen species), which are leakage during oxidative phosphorylation that varies with a hyperbolic saturation relationship observed with both the oxygen and nitric oxide effects. [ 13 ]
The oxygen effect is quantified by measuring the radiation sensitivity or Oxygen Enhancement Ratio (OER) of a particular biological effect (e.g., cell death or DNA damage ), [ 14 ] which is the ratio of doses under pure oxygen and anoxic conditions. Consequently, OER varies from unity in anoxia to a maximum value for 100% oxygen of typically up to three for low ionizing-density-radiation ( beta -, gamma -, or x-rays ), or so-called low linear energy transfer (LET) radiations.
Radiosensitivity varies most rapidly for oxygen partial pressures below ~1% atmospheric (Fig. 1). Howard-Flanders and Alper (1957) [ 15 ] developed a formula for the hyperbolic function of OER and its variation with oxygen concentration, or oxygen pressure in air.
Radiobiologists identified additional characteristics of the oxygen effect that influence radiotherapy practices. They found that the maximum OER value diminishes as the ionizing -density of the radiation increases (Fig. 2), from low-LET to high-LET radiations. [ 16 ] The OER is unity irrespective of the oxygen tension for alpha-particles of high-LET around 200 keV/μm. The OER is reduced for low doses as evaluated for cultured mammalian cells exposed to x-rays under aerobic (21% O2, 159 mmHg) and anoxic (nitrogen) conditions. [ 17 ] Typical fractionation treatments are daily 2 Gy exposures, as below this dose the so-called 'shoulder' or repair region of the cell survival curve is encroached upon reducing the OER (Fig. 3). | https://en.wikipedia.org/wiki/Oxygen_effect |
The oxygen enhancement ratio (OER) or oxygen enhancement effect in radiobiology refers to the enhancement of therapeutic or detrimental effect of ionizing radiation due to the presence of oxygen . This so-called oxygen effect [ 1 ] is most notable when cells are exposed to an ionizing radiation dose .
The OER is traditionally defined as the ratio of radiation doses during lack of oxygen compared to no lack of oxygen for the same biological effect. This may give varying numerical values depending on the chosen biological effect. Additionally, OER may be presented in terms of hyperoxic environments and/or with altered oxygen baseline, complicating the significance of this value.
The maximum OER depends mainly on the ionizing density or LET of the radiation. Radiation with higher LET and higher relative biological effectiveness (RBE) have a lower OER in mammalian cell tissues. [ 2 ] The value of the maximum OER varies from about 1–4. The maximum OER ranges from about 2–4 for low-LET radiations such as X-rays, beta particles and gamma rays, whereas the OER is unity for high-LET radiations such as low energy alpha particles.
The effect is used in medical physics to increase the effect of radiation therapy in oncology treatments. Additional oxygen abundance creates additional free radicals and increases the damage to the target tissue.
In solid tumors the inner parts become less oxygenated than normal tissue and up to three times higher dose is needed to achieve the same tumor control probability as in tissue with normal oxygenation.
The best known explanation of the oxygen effect is the oxygen fixation hypothesis which postulates that oxygen permanently fixes radical-induced DNA damage so it becomes permanent. [ 3 ] Recently, it has been posited that the oxygen effect involves radiation exposures of cells causing their mitochondria to produce greater amounts of reactive oxygen species. [ 4 ]
Eric J. Hall and Amato J. Giaccia: Radiobiology for the radiologist, Lippincott Williams & Wilkins, 6th Ed., 2006
This oncology article is a stub . You can help Wikipedia by expanding it . | https://en.wikipedia.org/wiki/Oxygen_enhancement_ratio |
Oxygen evolution is the chemical process of generating diatomic oxygen (O 2 ) by a chemical reaction , usually from water , the most abundant oxide compound in the universe . Oxygen evolution on Earth is effected by biotic oxygenic photosynthesis , photodissociation , hydroelectrolysis , and thermal decomposition of various oxides and oxyacids . When relatively pure oxygen is required industrially, it is isolated by distilling liquefied air. [ 1 ]
Natural oxygen evolution is essential to the biological process of all complex life on Earth, as aerobic respiration has become the most important biochemical process of eukaryotic thermodynamics since eukaryotes evolved through symbiogenesis during the Proterozoic eon , and such consumption can only continue if oxygen is cyclically replenished by photosynthesis. The various oxygenation events during Earth's history had not only influenced changes in Earth's biosphere , but also significantly altered the atmospheric chemistry . The transition of Earth's atmosphere from an anoxic prebiotic reducing atmosphere high in methane and hydrogen sulfide to an oxidative atmosphere of which free nitrogen and oxygen make up 99% of the mole fractions , had led to major climate changes and caused numerous icehouse phenomena and global glaciations .
In industries , oxygen evolution reaction (OER) is a limiting factor in the process of generating molecular oxygen through chemical reactions such as water splitting and electrolysis , and improved OER electrocatalysis is the key to the advancement of a number of renewable energy technologies such as solar fuels , regenerative fuel cells and metal–air batteries .
Photosynthetic oxygen evolution is the fundamental process by which oxygen is generated in the earth's biosphere . The reaction is part of the light-dependent reactions of photosynthesis in cyanobacteria and the chloroplasts of green algae and plants . It utilizes the energy of light to split a water molecule into its protons and electrons for photosynthesis. Free oxygen, generated as a by-product of this reaction, is released into the atmosphere . [ 2 ] [ 3 ]
Water oxidation is catalyzed by a manganese -containing cofactor contained in photosystem II , known as the oxygen-evolving complex (OEC) or the water-splitting complex. Manganese is an important cofactor , and calcium and chloride are also required for the reaction to occur. [ 4 ] The stoichiometry of this reaction is as follows:
The protons are released into the thylakoid lumen , thus contributing to the generation of a proton gradient across the thylakoid membrane. This proton gradient is the driving force for adenosine triphosphate (ATP) synthesis via photophosphorylation and the coupling of the absorption of light energy and the oxidation of water for the creation of chemical energy during photosynthesis. [ 5 ]
It was not until the end of the 18th century that Joseph Priestley accidentally discovered the ability of plants to "restore" air that had been "injured" by the burning of a candle. He followed up on the experiment by showing that air "restored" by vegetation was "not at all inconvenient to a mouse ." He was later awarded a medal for his discoveries that "...no vegetable grows in vain... but cleanses and purifies our atmosphere." Priestley's experiments were further evaluated by Jan Ingenhousz , a Dutch physician, who then showed that the "restoration" of air only worked while in the presence of light and green plant parts. [ 4 ]
Together with hydrogen (H 2 ), oxygen is evolved by the electrolysis of water . The point of water electrolysis is to store energy in the form of hydrogen gas, a clean-burning fuel. The "oxygen evolution reaction (OER) is the major bottleneck [to water electrolysis] due to the sluggish kinetics of this four-electron transfer reaction." [ 6 ] All practical catalysts are heterogeneous .
Electrons (e − ) are transferred from the cathode to protons to form hydrogen gas. The half reaction , balanced with acid, is:
At the positively charged anode, an oxidation reaction occurs, generating oxygen gas and releasing electrons to the anode to complete the circuit:
Combining either half reaction pair yields the same overall decomposition of water into oxygen and hydrogen:
Although some metal oxides eventually release O 2 when heated, these conversions generally require high temperatures. A few compounds release O 2 at mild temperatures. Chemical oxygen generators consist of chemical compounds that release O 2 when stimulated, usually by heat. They are used in submarines and commercial aircraft to provide emergency oxygen. Oxygen is generated by the high-temperature decomposition of sodium chlorate : [ 1 ]
Potassium permanganate also releases oxygen upon heating, but the yield is modest: | https://en.wikipedia.org/wiki/Oxygen_evolution |
The oxygen minimum zone ( OMZ ), sometimes referred to as the shadow zone , is the zone in which oxygen saturation in seawater in the ocean is at its lowest. This zone occurs at depths of about 200 to 1,500 m (700–4,900 ft), depending on local circumstances. OMZs are found worldwide, typically along the western coast of continents, in areas where an interplay of physical and biological processes concurrently lower the oxygen concentration (biological processes) and restrict the water from mixing with surrounding waters (physical processes), creating a "pool" of water where oxygen concentrations fall from the normal range of 4–6 mg/L to below 2 mg/L. [ 1 ]
Surface ocean waters generally have oxygen concentrations close to equilibrium with the Earth's atmosphere . In general, colder waters hold more oxygen than warmer waters. As water moves out of the mixed layer into the thermocline , it is exposed to a rain of organic matter from above. Aerobic bacteria feed on this organic matter; oxygen is used as part of the bacterial metabolic process, lowering its concentration within the water. Therefore, the concentration of oxygen in deep water is dependent on the amount of oxygen it had when it was at the surface, minus depletion by deep sea organisms.
The downward flux of organic matter decreases sharply with depth, with 80–90% being consumed in the top 1,000 m (3,300 ft). The deep ocean thus has higher oxygen because rates of oxygen consumption are low compared with the supply of cold, oxygen-rich deep waters from polar regions. In the surface layers, oxygen is supplied by photosynthesis of phytoplankton. Depths in between, however, have higher rates of oxygen consumption and lower rates of advective supply of oxygen-rich waters. In much of the ocean, mixing processes enable the resupply of oxygen to these waters (see upwelling ).
A distribution of the open-ocean oxygen minimum zones is controlled by the large-scale ocean circulation as well as local physical as well as biological processes. For example, wind blowing parallel to the coast causes Ekman transport that upwells nutrients from deep water. The increased nutrients support phytoplankton blooms, zooplankton grazing, and an overall productive food web at the surface. The byproducts of these blooms and the subsequent grazing sink in the form of particulate and dissolved nutrients (from phytodetritus, dead organisms, fecal pellets, excretions, shed shells, scales, and other parts). This "rain" of organic matter (see the biological pump ) feeds the microbial loop and may lead to bacterial blooms in water below the euphotic zone due to the influx of nutrients. [ 3 ] Since oxygen is not being produced as a byproduct of photosynthesis below the euphotic zone, these microbes use up what oxygen is in the water as they break down the falling organic matter thus creating the lower oxygen conditions. [ 1 ]
Physical processes then constrain the mixing and isolate this low oxygen water from outside water. Vertical mixing is constrained due to the separation from the mixed layer by depth. Horizontal mixing is constrained by bathymetry and boundaries formed by interactions with sub-tropical gyres and other major current systems. [ 4 ] [ 5 ] [ 6 ] Low oxygen water may spread (by advection) from under areas of high productivity up to these physical boundaries to create a stagnant pool of water with no direct connection to the ocean surface even though (as in the Eastern Tropical North Pacific) there may be relatively little organic matter falling from the surface.
In OMZs oxygen concentration drops to levels <10 nM at the base of the oxycline and can remain anoxic for over 700 m depth. [ 7 ] This lack of oxygen can be reinforced or increased due to physical processes changing oxygen supply such as eddy-driven advection, [ 7 ] sluggish ventilation, [ 8 ] increases in ocean stratification , and increases in ocean temperature which reduces oxygen solubility. [ 9 ]
At a microscopic scale the processes causing ocean deoxygenation rely on microbial aerobic respiration. [ 9 ] Aerobic respiration is a metabolic process that microorganisms like bacteria or archaea use to obtain energy by degrading organic matter, consuming oxygen, producing CO 2 and obtaining energy in the form of ATP. [ 9 ] In the ocean surface photosynthetic microorganisms called phytoplankton use solar energy and CO 2 to build organic molecules (organic matter) releasing oxygen in the process. [ 10 ] A large fraction of the organic matter from photosynthesis becomes dissolved organic matter (DOM) that is consumed by bacteria during aerobic respiration in sunlit waters. Another fraction of organic matter sinks to the deep ocean forming aggregates called marine snow. [ 11 ] These sinking aggregates are consumed via degradation of organic matter and respiration at depth. [ 8 ]
At depths in the ocean where no light can reach, aerobic respiration is the dominant process. When the oxygen in a parcel of water is consumed, the oxygen cannot be replaced without the water reaching the surface ocean. When oxygen concentrations drop to below <10 nM, microbial processes that are normally inhibited by oxygen can take place like denitrification and anammox . Both processes extract elemental nitrogen from nitrogen compounds and that elemental nitrogen which does not stay in solution escapes as a gas, resulting in a net loss of nitrogen from the ocean. [ 8 ]
An organism's demand for oxygen is dependent on its metabolic rate . Metabolic rates can be affected by external factors such as the temperature of the water, and internal factors such as the species, life stage, size, and activity level of the organism. The body temperature of ectotherms (such as fishes and invertebrates ) fluctuates with the temperature of the water. As the external temperature increases, ectotherm metabolisms increase as well, increasing their demand for oxygen. [ 12 ] Different species have different basal metabolic rates and therefore different oxygen demands. [ 13 ] [ 14 ]
Life stages of organisms also have different metabolic demands. In general, younger stages tend to grow in size and advance in developmental complexity quickly. As the organism reaches maturity, metabolic demands switch from growth and development to maintenance, which requires far fewer resources. [ 15 ] Smaller organisms have higher metabolisms per unit of mass, so smaller organisms will require more oxygen per unit mass, while larger organisms generally require more total oxygen. [ 16 ] Higher activity levels also require more oxygen.
This is why bioavailability is important in deoxygenated systems: an oxygen quantity which is dangerously low for one species might be more than enough for another species.
Several indices to measure bioavailability have been suggested: Respiration Index, [ 17 ] Oxygen Supply Index, [ 18 ] and the Metabolic Index. [ 19 ] The Respiration Index describes oxygen availability based on the free energy available in the reactants and products of the stoichiometric equation for respiration. [ 17 ] However, organisms have ways of altering their oxygen intake and carbon dioxide release, so the strict stoichiometric equation is not necessarily accurate. [ 20 ] The Oxygen Supply Index accounts for oxygen solubility and partial pressure, along with the Q 10 of the organism, but does not account for behavioral or physiological changes in organisms to compensate for reduced oxygen availability. [ 18 ] The Metabolic Index accounts for the supply of oxygen in terms of solubility, partial pressure, and diffusivity of oxygen in water, and the organism's metabolic rate. [ 19 ] The metabolic index is generally viewed as a closer approximation of oxygen bioavailability than the other indices.
There are two thresholds of oxygen required by organisms:
Since bioavailability is specific to each organism and temperature, calculation of these thresholds is done experimentally by measuring activity and respiration rates under different temperature and oxygen conditions, or by collecting data from separate studies.
Despite the low oxygen conditions, organisms have evolved to live in and around OMZs. For those organisms, like the vampire squid , special adaptations are needed to either make do with lesser amounts of oxygen or to extract oxygen from the water more efficiently. For example, the giant red mysid ( Gnathophausia ingens ) continues to live aerobically (using oxygen) in OMZs. They have highly developed gills with large surface area and thin blood-to-water diffusion distance that enables effective removal of oxygen from the water (up to 90% O 2 removal from inhaled water) and an efficient circulatory system with high capacity and high blood concentration of a protein ( hemocyanin ) that readily binds oxygen. [ 23 ] [ 24 ] [ 25 ]
Another strategy used by some classes of bacteria in the oxygen minimum zones is to use nitrate rather than oxygen, thus drawing down the concentrations of this important nutrient. This process is called denitrification . The oxygen minimum zones thus play an important role in regulating the productivity and ecological community structure of the global ocean. [ 26 ] For example, giant bacterial mats floating in the oxygen minimum zone off the west coast of South America may play a key role in the region's extremely rich fisheries, as bacterial mats the size of Uruguay have been found there. [ 27 ]
Decreased oxygen availability results in decreases in many zooplankton species’ egg production, food intake, respiration, [ 28 ] and metabolic rates. [ 29 ] [ 30 ] [ 31 ] Temperature and salinity in areas of decreased oxygen concentrations also affect oxygen availability. Higher temperatures and salinity lower oxygen solubility decrease the partial pressure of oxygen. This decreased partial pressure increases organisms’ respiration rates, causing the oxygen demand of the organism to increase. [ 28 ] [ 31 ]
In addition to affecting their vital functions, zooplankton alter their distribution in response to hypoxic or anoxic zones. Many species actively avoid low oxygen zones, [ 32 ] [ 33 ] [ 34 ] while others take advantage of their predators’ low tolerance for hypoxia and use these areas as a refuge. [ 32 ] [ 33 ] [ 34 ] Zooplankton that exhibit daily vertical migrations to avoid predation and low oxygen conditions also excrete ammonium near the oxycline and contribute to increased anaerobic ammonium oxidation (anammox, [ 35 ] [ 31 ] which produces N 2 gas. As hypoxic regions expand vertically and horizontally, [ 36 ] [ 37 ] the habitable ranges for phytoplankton, zooplankton, and nekton increasingly overlap, increasing their susceptibility to predation and human exploitation. [ 38 ] [ 29 ] [ 39 ] [ 40 ] [ 33 ]
OMZs have changed over time due to effects from numerous global chemical and biological processes. [ 41 ] To assess these changes, scientists utilize climate models and sediment samples to understand changes to dissolved oxygen in OMZs. [ 42 ] Many recent studies of OMZs have focused on their fluctuations over time and how they may be currently changing as a result of climate change . [ 42 ] [ 43 ]
Some research has aimed to understand how OMZs have changed over geological time scales . [ 43 ] Throughout the history of Earth's oceans, OMZs have fluctuated on long time scales, becoming larger or smaller depending on multiple variables. [ 44 ] The factors that change OMZs are the amount of oceanic primary production resulting in increased respiration at greater depths, changes in the oxygen supply due to poor ventilation, and amount of oxygen supplied through thermohaline circulation . [ 44 ]
While oxygen minimum zones (OMZs) occur naturally, they can be exacerbated by human impacts like climate change and land-based pollution from agriculture and sewage. The prediction of current climate models and climate change scenarios is that substantial warming and loss of oxygen throughout the majority of the upper ocean will occur. [ 45 ] Global warming increases ocean temperatures, especially in shallow coastal areas. When the water temperature increases, its ability to hold oxygen decreases, leading to oxygen concentrations going down in the water. [ 46 ] This compounds the effects of eutrophication in coastal zones described above.
Open ocean areas with no oxygen have grown more than 1.7 million square miles in the last 50 years, and coastal waters have seen a tenfold increase in low-oxygen areas in the same time. [ 47 ]
Measurement of dissolved oxygen in coastal and open ocean waters for the past 50 years has revealed a marked decline in oxygen content. [ 48 ] [ 49 ] [ 50 ] This decline is associated with expanding spatial extent, expanding vertical extent, and prolonged duration of oxygen-poor conditions in all regions of the global oceans. Examinations of the spatial extent of OMZs in the past through paleoceanographical methods clearly shows that the spatial extent of OMZs has expanded through time, and this expansion is coupled to ocean warming and reduced ventilation of thermocline waters. [ 51 ]
Research has attempted to model potential changes to OMZs as a result of rising global temperatures and human impact. This is challenging due to the many factors that could contribute to changes in OMZs. [ 52 ] The factors used for modeling change in OMZs are numerous, and in some cases hard to measure or quantify. [ 53 ] Some of the processes being studied are changes in oxygen gas solubility as a result of rising ocean temperatures, as well as changes in the amount of respiration and photosynthesis occurring around OMZs. [ 54 ] Many studies have concluded that OMZs are expanding in multiple locations, but fluctuations of modern OMZs are still not fully understood. [ 54 ] [ 53 ] [ 55 ] Existing Earth system models project considerable reductions in oxygen and other physical-chemical variables in the ocean due to climate change , with potential ramifications for ecosystems and humans.
The global decrease in oceanic oxygen content is statistically significant and emerging beyond the envelope of natural fluctuations. [ 48 ] This trend of oxygen loss is accelerating, with widespread and obvious losses occurring after the 1980s. [ 56 ] [ 48 ] The rate and total content of oxygen loss varies by region, with the North Pacific emerging as a particular hotspot of deoxygenation due to the increased amount of time since its deep waters were last ventilated (see thermohaline circulation) and related high apparent oxygen utilization (AOU). [ 48 ] [ 49 ] Estimates of total oxygen loss in the global ocean range from 119 to 680 T mol decade −1 since the 1950s. [ 48 ] [ 49 ] These estimates represent 2% of the global ocean oxygen inventory. [ 50 ] | https://en.wikipedia.org/wiki/Oxygen_minimum_zone |
Oxygen monofluoride is an unstable binary inorganic compound radical of fluorine and oxygen with the chemical formula OF. [ 1 ] [ 2 ] [ 3 ] This is the simplest of many oxygen fluorides .
Oxygen- and fluorine-containing radicals like O 2 F and OF occur in the atmosphere. These, along with other halogen radicals, have been implicated in the destruction of ozone in the atmosphere. However, the oxygen monofluoride radicals are assumed to not play as big a role in the ozone depletion because free fluorine atoms in the atmosphere are believed to react with methane to produce hydrofluoric acid which precipitates in rain. [ 6 ] | https://en.wikipedia.org/wiki/Oxygen_monofluoride |
Oxygen plants are industrial systems designed to generate oxygen. They typically use air as a feedstock and separate it from other components of air using pressure swing adsorption or membrane separation techniques. Such plants are distinct from cryogenic separation plants which separate and capture all the components of air.
Oxygen finds broad application in various technological processes and in almost all industry branches. The primary oxygen application is associated with its capability of sustaining burning process, and the powerful oxidant properties.
Due to that, oxygen has become widely used in the metal processing, welding, cutting and brazing processes. In the chemical and petrochemical industries, as well as in the oil and gas sector oxygen is used in commercial volumes as an oxidizer in chemical reactions .
Gas separation by adsorption systems is based on differential rates of adsorption of the components of a gas mixture into a solid adsorbent.
The current means of gaseous oxygen production from air by the use of adsorption technology produce a high fraction of oxygen as their output. The mechanism of operation of a modern oxygen adsorption plant is based on the variation of uptake of a particular gas component by the adsorbent as the temperature and partial pressure of the gas is changed.
The gas adsorption and adsorbent regeneration processes may therefore be regulated by varying of the pressure and temperature parameters.
The oxygen plant flow process is arranged in such a way that highly absorbable gas mixture components are taken in by adsorbent, while low absorbable and non-absorbable components go through the plant. Today, there exist three methods of arranging the adsorption-based air separation process with the use of swing technologies: pressure (PSA), vacuum (VSA) and mixed (VPSA) ones. In the pressure swing adsorption flow processes, oxygen is recovered under above-atmospheric pressure and regeneration is achieved under atmospheric pressure . In vacuum swing adsorption flow processes, oxygen is recovered under atmospheric pressure, and regeneration is achieved under negative pressure . The mixed systems operation combines pressure variations from positive to negative.
The adsorption oxygen plants produce 5 to 5,000 normal cubic meters per hour of oxygen with a purity of 93-95%. These systems, designated for indoor operation, are set to effectively produce gaseous oxygen from atmospheric air.
An unquestionable advantage of adsorption-based oxygen plants is the low cost of oxygen produced in the cases where there are no rigid requirements to the product oxygen purity.
Structurally, the adsorption oxygen plant consists of several adsorbers, the compressor unit, pre-purifier unit, valve system and the plant control system .
A simple adsorber is a column filled with layers of specially selected adsorbents – granular substances preferentially adsorbing highly adsorbable components of a gas mixture.
Where gaseous oxygen purity is required at the level of 90-95% with the capacity of up to 5,000 Nm 3 per hour, adsorption oxygen plants are the optimal choice. This oxygen purity may also be obtained through the use of systems based on the cryogenic technology; however, cryogenic plants are more cumbersome and complex in operation.
Some companies produce high-efficiency systems for oxygen production from atmospheric air with the help of membrane technology .
The basis of gas media separation with the use of membrane systems is the difference in velocity with which various gas mixture components permeate membrane substance. The driving force behind the gas separation process is the difference in partial pressures on different membrane sides.
A modern gas separation membrane used by GRASYS is no longer a flat plate, but is formed by hollow fibers. Membrane consists of a porous polymer fiber with the gas separation layer applied to its external surface. Structurally, a hollow fiber membrane is configured as a cylindrical cartridge representing a spool with specifically reeled polymer fiber.
Due to the membrane material high permeability for oxygen in contrast to nitrogen, the design of membrane oxygen complexes requires a special approach. Basically, there are two membrane-based oxygen production technologies: compressor and vacuum ones.
In the case of compressor technology, air is supplied into the fiber space under excess pressure, oxygen exits the membrane under slight excess pressure, and where necessary, is pressurized by booster compressor to the required pressure level . By the use of vacuum technology , a vacuum pump is used for the achievement of partial pressures difference.
Designed for indoor operation, membrane oxygen plants allow efficient air enrichment with oxygen up to the concentration of 30-45%. The complexes are rated to 5 to 5,000 nm3/hr of oxygenated air. [ 1 ]
In the membrane oxygen plant, gas separation is achieved in the gas separation module composed of hollow-fiber membranes and representing the plant critical and high-technology unit. Apart from the gas separation unit, other important technical components are the booster compressor or vacuum pump, pre-purifier unit, and the plant control system.
The adoption of membrane systems for air enrichment purposes promises multiple oxygen savings where the oxygen concentration of 30-45% is sufficient to cover process needs [ citation needed ] . In addition to customer saving on the product oxygen cost, there is a collateral economic effect based on extremely low operating costs.
With the incorporation of the membrane technology, oxygen plants have outstanding technical characteristics. Membrane oxygen plants are highly reliable due to the absence of moving parts in the gas separation module.
The systems are very simple in operation – control of all operating parameters is carried out automatically [ citation needed ] . Because of the plant's high automation degree, staffed oversight is not required during its operation.
Membrane oxygen plants are finding increasingly broad application in various industries all over the world. With moderate requirements to oxygen purity in product - up to 30-45%, membrane systems generally prove more economically sound than adsorption and cryogenic systems. In addition, membrane plants are much simpler in operation and more reliable [ citation needed ] . | https://en.wikipedia.org/wiki/Oxygen_plant |
In chemistry , the oxygen reduction reaction refers to the reduction half reaction whereby O 2 is reduced to water or hydrogen peroxide. In fuel cells, the reduction to water is preferred because the current is higher. The oxygen reduction reaction is well demonstrated and highly efficient in nature. [ 1 ] [ 2 ]
The stoichiometries of the oxygen reduction reaction, which depends on the medium, are shown: [ 3 ]
4e − pathway in acid medium: O 2 + 4 e − + 4 H + ⟶ 2 H 2 O {\displaystyle {\ce {O2 + 4 e- + 4H+ -> 2 H2O}}}
2e − pathway in acid medium: O 2 + 2 e − + 2 H + ⟶ H 2 O 2 {\displaystyle {\ce {O2 + 2e- + 2H+ -> H2O2}}}
4e − pathway in alkaline medium: O 2 + 4 e − + 2 H 2 O ⟶ 4 OH − {\displaystyle {\ce {O2 + 4e- + 2H2O -> 4 OH-}}}
2e − pathway in alkaline medium: O 2 + 2 e − + H 2 O ⟶ HO 2 − + OH − {\displaystyle {\ce {O2 + 2e- + H2O -> HO2- + OH-}}}
4e- pathway in solid oxide: O 2 + 4 e − ⟶ 2 O 2 − {\displaystyle {\ce {O2 + 4e- -> 2 O^2-}}}
The 4e − pathway reaction is the cathode reaction in fuel cell especially in proton-exchange membrane fuel cells , alkaline fuel cell and solid oxide fuel cell . While the 2e − pathway reaction is often the side reaction of 4e- pathway or can be used in synthesis of H 2 O 2 .
The oxygen reduction reaction is an essential reaction for aerobic organisms. Such organisms are powered by the heat of combustion of fuel (food) by O 2 . Rather than combustion, organisms rely on elaborate sequences of electron-transfer reactions, often coupled to proton transfer. The direct reaction of O 2 with fuel is precluded by the oxygen reduction reaction, which produces water and adenosine triphosphate . Cytochrome c oxidase affects the oxygen reduction reaction by binding O 2 in a heme – Cu complex. In laccase , O 2 is engaged and reduced by a four-copper aggregate. Three Cu centers bind O 2 , and one Cu center functions as an electron donor. [ 1 ]
In fuel cells, platinum is the most common catalyst. Because platinum is expensive, it is dispersed on a carbon support. Certain facets of platinum are more active than others. [ 2 ]
Detailed mechanistic work results from studies on transition metal dioxygen complexes , which represent models for the initial encounter between O 2 and the metal catalyst. Early catalysts for the oxygen reduction reaction were based on cobalt phthalocyanines . [ 4 ] Many related coordination complexes have been tested. [ 5 ] as the oxygen reduction reaction catalyst and different electrocatalysis performance was achieved by these small molecules. These exciting results trigger further research of the non-noble metal contained small molecules used for the oxygen reduction reaction electrocatalyst. [ 6 ] Besides phthalocyanine, porphyrin is also a suitable ligand for metal center to provide N 4 [ definition needed ] part in the M-N 4 site. In biosystems, many oxygen related physical chemical reactions are carried by proteins containing the metal-prophyrin unit such as O 2 delivery, O 2 storage, O 2 reduction and H 2 O 2 oxidation.
Since the oxygen reduction reaction in fuel cells need to be catalyzed heterogeneously, conductive substrates such as carbon materials is always needed in constructing electrocatalysts. To increase the conductivity and enhance the substrate-loading interaction, thermal treatment is usually performed before application. During the treatment, M-N4 active sites turn to aggregate spontaneously due to the high intrinsic energy, which will dramatically decrease the active site density. Therefore, increasing the active site density and creating atomic level dispersed catalyst is a key step to improve the catalyst activity. To solve this problem, we can use some porous substrates to confine the active sites or use some defect or ligands to prevent the migration of the active site. In the mean time, the porous structure or the defect will also be beneficial to the oxygen absorption process. [ 7 ]
Besides active site density, the electron configuration of M center in M-N 4 active site also plays an important role in the activity and stability of an oxygen reduction reaction catalyst. Because the electron configuration of M center can affects the redox potential , which determines the activation energy of the oxygen reduction reaction. To modulate the electron configuration, a simple way is to change the ligands of the metal center. For example, researchers found that whether the N atoms in M-N4 active sites are pyrrolic or pyridinic can affect the performance of the catalyst. [ 8 ] [ 9 ] Besides, some heteroatoms such as S, P other than N can also be used to modulate the electron configuration too, since these atoms have different electronegativity and electron configuration. [ 10 ] | https://en.wikipedia.org/wiki/Oxygen_reduction_reaction |
Oxygen saturation (symbol S O 2 ) is a relative measure of the concentration of oxygen that is dissolved or carried in a given medium as a proportion of the maximal concentration that can be dissolved in that medium at the given temperature. It can be measured with a dissolved oxygen probe such as an oxygen sensor or an optode in liquid media, usually water. [ 1 ] The standard unit of oxygen saturation is percent (%).
Oxygen saturation can be measured regionally and noninvasively. Arterial oxygen saturation (Sa O 2 ) is commonly measured using pulse oximetry . Tissue saturation at peripheral scale can be measured using NIRS . This technique can be applied on both muscle and brain.
In medicine , oxygen saturation refers to oxygenation , or when oxygen molecules ( O 2 ) enter the tissues of the body. In this case blood is oxygenated in the lungs , where oxygen molecules travel from the air into the blood. Oxygen saturation (( O 2 ) sats) measures the percentage of hemoglobin binding sites in the bloodstream occupied by oxygen. Fish, invertebrates, plants, and aerobic bacteria all require oxygen.
In aquatic environments, oxygen saturation is a ratio of the concentration of "dissolved oxygen " (DO, O 2 ), to the maximum amount of oxygen that will dissolve in that water body, at the temperature and pressure which constitute stable equilibrium conditions. Well-aerated water (such as a fast-moving stream) without oxygen producers or consumers is 100% saturated. [ 2 ]
Stagnant water can become somewhat supersaturated with oxygen (i.e., reach more than 100% saturation) either because of the presence of photosynthetic aquatic oxygen producers or because of a slow equilibration after a change of atmospheric conditions. [ 2 ] Stagnant water in the presence of decaying matter will typically have an oxygen concentration much less than 100%, which is due to anaerobic bacteria being much less efficient at breaking down organic material. [ citation needed ] [ 3 ] Similarly as in water, oxygen concentration also plays a key role in the breakdown of organic matter in soils. Higher oxygen saturation allows aerobic bacteria to persist, which breaks down decaying organic material in soils much more efficiently than anaerobic bacteria. [ 4 ] Thus, soils with high oxygen saturation will have less organic matter per volume than those with low oxygen saturation. [ 4 ]
Environmental oxygenation can be important to the sustainability of a particular ecosystem . The US Environmental Protection Agency has published a table of maximum equilibrium dissolved oxygen concentration versus temperature at atmospheric pressure. [ 5 ] The optimal levels in an estuary for dissolved oxygen is higher than six ppm. [ 6 ] Insufficient oxygen ( environmental hypoxia ), often caused by the decomposition of organic matter and nutrient pollution , may occur in bodies of water such as ponds and rivers , tending to suppress the presence of aerobic organisms such as fish . Deoxygenation increases the relative population of anaerobic organisms such as plants and some bacteria , resulting in fish kills and other adverse events. The net effect is to alter the balance of nature by increasing the concentration of anaerobic over aerobic species . | https://en.wikipedia.org/wiki/Oxygen_saturation |
An oxygen tank is an oxygen storage vessel, which is either held under pressure in gas cylinders , referred to in the industry as high pressure oxygen cylinders , or as liquid oxygen in a cryogenic storage tank . [ citation needed ]
Oxygen tanks are used to store gas for:
Breathing oxygen is delivered from the storage tank to users by use of the following methods: oxygen mask , nasal cannula , full face diving mask , diving helmet , demand valve , oxygen rebreather , built in breathing system (BIBS), oxygen tent , and hyperbaric oxygen chamber.
Contrary to popular belief most scuba divers do not carry oxygen tanks. The vast majority of divers breathe air or nitrox stored in a diving cylinder . A small minority breathe trimix , heliox or other exotic gases . Some may carry pure oxygen for accelerated decompression or as supply gas to a rebreather. Some shallow divers, particularly naval combat divers, use oxygen rebreathers, and they use a small oxygen cylinder to provide the gas. [ citation needed ]
Oxygen is rarely held at pressures higher than 20 megapascals (3,000 psi), due to the risks of fire triggered by high temperatures caused by adiabatic heating when the gas changes pressure when moving from one vessel to another. Medical use liquid oxygen airgas tanks are typically 2.4 MPa (350 psi). [ citation needed ]
All equipment coming into contact with high pressure oxygen must be "oxygen clean" and "oxygen compatible", to reduce the risk of fire . [ 3 ] [ 4 ] "Oxygen clean" means the removal of any substance that could act as a source of ignition . "Oxygen compatible" means that internal components must not burn readily or degrade easily in a high pressure oxygen environment.
In some countries there are legal and insurance requirements and restrictions on the use, storage and transport of pure oxygen. [ citation needed ] Oxygen tanks are normally stored in well-ventilated locations, far from potential sources of fire and concentrations of people. [ citation needed ] | https://en.wikipedia.org/wiki/Oxygen_tank |
In the liquid fuel industry, oxygenates are hydrocarbon -derived fuel additives containing at least one oxygen atom [ 1 ] to promote complete combustion . [ 2 ] Absent oxygenates, fuel combustion is usually incomplete , and the exhaust stream pollutes the air with carbon monoxide , soot particles, aromatic and polyaromatic hydrocarbons , and nitrated polyaromatic hydrocarbons. [ 3 ]
The most common oxygenates are either alcohols or ethers , but ketones and aldehydes are also included in this distinction. [ 4 ] Carboxylic acids and esters can be grouped with oxygenates in the simple definition that they contain at least one oxygen atom. [ 4 ] However, they are usually unwanted in oils, and therefore likely fuels, due to their environmental toxicity and tendency to cause catalyst poisoning and corrosion during oil production and refining. [ 5 ]
In the United States , the Environmental Protection Agency (EPA) had authority to mandate that minimum proportions of oxygenates be added to automotive gasoline on regional and seasonal basis from 1992 until 2006 in an attempt to reduce air pollution, in particular ground-level ozone and smog . As of 2023, the EPA continues to require the use of oxygenated gasoline in certain areas during winter to regulate carbon monoxide emissions; however, the programs to fulfill its conditions are implemented by the states. In addition to this North American automakers from 2006 onwards promoted a blend of 85% ethanol and 15% gasoline, marketed as E85 , and their flex-fuel vehicles, e.g. GM 's Live Green, Go Yellow campaign. US Corporate Average Fuel Economy (CAFE) standards give an artificial 54% fuel efficiency bonus to vehicles capable of running on 85% alcohol blends over vehicles not adapted to run on 85% alcohol blends. [ 6 ] There is also alcohols' intrinsically cleaner combustion, however due to its lower energy density it is not capable of producing as much energy per gallon as gasoline. Much gasoline [ citation needed ] sold in the United States is blended with up to 10% of an oxygenating agent. This is known as oxygenated fuel and often (but not entirely correctly, as there are reformulated gasolines without oxygenate) as reformulated gasoline. Methyl tert -butyl ether (MTBE) was the most common fuel additive in the United States, prior to government mandated use of ethanol . Typically, gasoline with added MTBE is called reformulated gasoline, while gasoline with ethanol is called oxygenated gasoline. [ 7 ] | https://en.wikipedia.org/wiki/Oxygenate |
Oxygenated treatment (OT) is a technique used to reduce corrosion in a boiler and its associated feedwater system in flow-through boilers.
With oxygenated treatment, oxygen is injected into the feedwater to keep the oxygen level between 30 and 50 ppb. OT programs are most commonly used in supercritical (i.e. >3250psi) power boilers. The ability to change an existing sub-critical boiler over to an OT program is very limited. "Common injection points are just after the condensate polisher and again at the deaerator outlet." [ 1 ] This forms a thicker protective layer of hematite (Fe 2 O 3 ) on top of the magnetite. This is a denser, flatter film (vs. the undulation scale with OT) so that there is less resistance to water flow compared to AVT. [ 2 ] Also, OT reduces the risk of flow-accelerated corrosion . [ 3 ]
When OT is used, conductivity after cation exchange (CACE) at the economiser inlet must be maintained below 0.15μS/cm [ 4 ] this can be achieved by the use of a full-flow condensate polisher. [ 5 ] | https://en.wikipedia.org/wiki/Oxygenated_treatment |
In chemistry , a sample's oxygen–argon ratio (or oxygen/argon ratio ) is a comparison between the concentrations of oxygen (O 2 ) and the noble gas argon (Ar), either in air or dissolved in a liquid such as seawater. The two gases have very similar physical properties such as solubility and diffusivity , as well as a similar temperature dependence, making them easy to compare. [ 1 ] [ 2 ]
Measurements of primary productivity in the ocean can be made using this ratio. The concentration of oxygen dissolved in seawater varies according to biological processes ( photosynthesis and respiration ) as well as physical processes (air-sea gas exchange, temperature and pressure changes, lateral mixing and vertical diffusion). Argon concentrations, by contrast, vary only by physical processes. [ 3 ]
This technique was first used by Craig and Hayward (1987) when they separated oxygen supersaturations into a biological and a physical component. [ 4 ] This O 2 /Ar supersaturation can be defined as
∆(O 2 /Ar)=(c(O 2 )/c(Ar)) / (c sat (O 2 )/(c sat (Ar))) -1
where (∆O 2 )/Ar is the difference between O 2 production via photosynthesis and removal via respiration, c is the concentration of dissolved gas and c sat is the saturated concentration of the gas in water at a specific temperature, salinity and pressure. [ 3 ]
Oxygen and argon concentrations can be compared using samples from water systems aboard ships using either a membrane inlet mass spectrometer (MIMS) [ 3 ] or an equilibrator inlet mass spectrometer (EIMS). [ 5 ] The measurements can then be used in conjunction with air-sea gas exchange values to calculate biologically induced air-sea O 2 fluxes and net community production.
Because oxygen and argon leak through packaging material at different rates, comparing the ratios inside a package can determine if and how quickly air from outside has leaked in. [ 6 ]
The characteristics of steel, in particular the carbon and chromium content, can be controlled by adjusting the oxygen/argon ratio during the manufacturing process. [ 7 ] The oxygen/argon ratio is also important in the creation of thin films used in the manufacture of lithium-ion batteries. [ 8 ] | https://en.wikipedia.org/wiki/Oxygen–argon_ratio |
The oxygen–hemoglobin dissociation curve , also called the oxyhemoglobin dissociation curve or oxygen dissociation curve ( ODC ), is a curve that plots the proportion of hemoglobin in its saturated (oxygen-laden) form on the vertical axis against the prevailing oxygen tension on the horizontal axis. This curve is an important tool for understanding how our blood carries and releases oxygen. Specifically, the oxyhemoglobin dissociation curve relates oxygen saturation (S O 2 ) and partial pressure of oxygen in the blood (P O 2 ), and is determined by what is called "hemoglobin affinity for oxygen"; that is, how readily hemoglobin acquires and releases oxygen molecules into the fluid that surrounds it.
Hemoglobin (Hb) is the primary vehicle for transporting oxygen in the blood . Each hemoglobin molecule has the capacity to carry four oxygen molecules. These molecules of oxygen bind to the globin chain of the heme prosthetic group . [ 1 ] [ 2 ]
When hemoglobin has no bound oxygen, nor bound carbon dioxide , it has the unbound conformation (shape). The binding of the first oxygen molecule induces change in the shape of the hemoglobin that increases its ability to bind to the other three oxygen molecules.
In the presence of dissolved carbon dioxide, the pH of the blood changes; this causes another change in the shape of hemoglobin, which increases its ability to bind carbon dioxide and decreases its ability to bind oxygen. With the loss of the first oxygen molecule, and the binding of the first carbon dioxide molecule, yet another change in shape occurs, which further decreases the ability to bind oxygen, and increases the ability to bind carbon dioxide. The oxygen bound to the hemoglobin is released into the blood's plasma and absorbed into the tissues , and the carbon dioxide in the tissues is bound to the hemoglobin.
In the lungs the reverse of this process takes place. With the loss of the first carbon dioxide molecule the shape again changes and makes it easier to release the other three carbon dioxides.
Oxygen is also carried dissolved in the blood's plasma , but to a much lesser degree. Hemoglobin is contained in red blood cells . Hemoglobin releases the bound oxygen when carbonic acid is present, as it is in the tissues. In the capillaries , where carbon dioxide is produced, oxygen bound to the hemoglobin is released into the blood's plasma and absorbed into the tissues.
How much of that capacity is filled by oxygen at any time is called the oxygen saturation . Expressed as a percentage, the oxygen saturation is the ratio of the amount of oxygen bound to the hemoglobin, to the oxygen-carrying capacity of the hemoglobin. The oxygen-carrying capacity of hemoglobin is determined by the type of hemoglobin present in the blood. The amount of oxygen bound to the hemoglobin at any time is related, in large part, to the partial pressure of oxygen to which the hemoglobin is exposed. In the lungs, at the alveolar–capillary interface , the partial pressure of oxygen is typically high, and therefore the oxygen binds readily to hemoglobin that is present. As the blood circulates to other body tissue in which the partial pressure of oxygen is less, the hemoglobin releases the oxygen into the tissue because the hemoglobin cannot maintain its full bound capacity of oxygen in the presence of lower oxygen partial pressures.
The curve is usually best described by a sigmoid plot, using a formula of the kind:
A hemoglobin molecule can bind up to four oxygen molecules in a reversible method.
The shape of the curve results from the interaction of bound oxygen molecules with incoming molecules. The binding of the first molecule is difficult. However, this facilitates the binding of the second, third and fourth, this is due to the induced conformational change in the structure of the hemoglobin molecule induced by the binding of an oxygen molecule.
In its simplest form, the oxyhemoglobin dissociation curve describes the relation between the partial pressure of oxygen (x axis) and the oxygen saturation (y axis). Hemoglobin's affinity for oxygen increases as successive molecules of oxygen bind. More molecules bind as the oxygen partial pressure increases until the maximum amount that can be bound is reached. As this limit is approached, very little additional binding occurs and the curve levels out as the hemoglobin becomes saturated with oxygen. Hence the curve has a sigmoidal or S-shape. At pressures above about 60 mmHg, the standard dissociation curve is relatively flat, which means that the oxygen content of the blood does not change significantly even with large increases in the oxygen partial pressure. To get more oxygen to the tissue would require blood transfusions to increase the hemoglobin count (and hence the oxygen-carrying capacity), or supplemental oxygen that would increase the oxygen dissolved in plasma.
Although binding of oxygen to hemoglobin continues to some extent for pressures about 50 mmHg, as oxygen partial pressures decrease in this steep area of the curve, the oxygen is unloaded to peripheral tissue readily as the hemoglobin's affinity diminishes.
The partial pressure of oxygen in the blood at which the hemoglobin is 50% saturated, typically about 26.6 mmHg (3.5 kPa) for a healthy person, is known as the P 50 . The P 50 is a conventional measure of hemoglobin affinity for oxygen. In the presence of disease or other conditions that change the hemoglobin oxygen affinity and, consequently, shift the curve to the right or left, the P 50 changes accordingly. An increased P 50 indicates a rightward shift of the standard curve, which means that a larger partial pressure is necessary to maintain a 50% oxygen saturation. This indicates a decreased affinity. Conversely, a lower P 50 indicates a leftward shift and a higher affinity.
The 'plateau' portion of the oxyhemoglobin dissociation curve is the range that exists at the pulmonary capillaries (minimal reduction of oxygen transported until the p(O 2 ) falls 50 mmHg).
The 'steep' portion of the oxyhemoglobin dissociation curve is the range that exists at the systemic capillaries (a small drop in systemic capillary p(O 2 ) can result in the release of large amounts of oxygen for the metabolically active cells).
To see the relative affinities of each successive oxygen as you remove/add oxygen from/to the hemoglobin from the curve compare the relative increase/decrease in p(O 2 ) needed for the corresponding increase/decrease in s(O 2 ).
The strength with which oxygen binds to hemoglobin is affected by several factors. These factors shift or reshape the oxyhemoglobin dissociation curve. A shift to right indicates that the hemoglobin under study has a decreased affinity for oxygen. This makes it more difficult for hemoglobin to bind to oxygen (requiring a higher partial pressure of oxygen to achieve the same oxygen saturation), but it makes it easier for the hemoglobin to release oxygen bound to it.
The effect of this shift of the curve increases the partial pressure of oxygen in the tissues when it is most needed, such as during exercise, or hemorrhagic shock.
In contrast, the curve is shifted to the left by the opposite of these conditions.
This shift indicates that the hemoglobin under study has an increased affinity for oxygen so that hemoglobin binds oxygen more easily, but unloads it more reluctantly.
Left shift of the curve is a sign of hemoglobin's increased affinity for oxygen (e.g. at the lungs).
Similarly, right shift shows decreased affinity, as would appear with an increase in either body temperature, hydrogen ions, 2,3-bisphosphoglycerate (2,3-BPG) concentration or carbon dioxide concentration.
Note:
The causes of shift to right can be remembered using the mnemonic , " CADET , face Right!" for C O 2 , A cid, 2,3- D PG, [ Note 1 ] E xercise and T emperature. [ 3 ] Factors that move the oxygen dissociation curve to the right are those physiological states where tissues need more oxygen. For example, during exercise, muscles have a higher metabolic rate, and consequently need more oxygen, produce more carbon dioxide and lactic acid, and their temperature rises.
A decrease in pH (increase in H + ion concentration) shifts the standard curve to the right, while an increase shifts it to the left. This occurs because at greater H + ion concentration, various amino acid residues, such as Histidine 146 exist predominantly in their protonated form allowing them to form ion pairs that stabilize deoxyhemoglobin in the T state. [ 4 ] The T state has a lower affinity for oxygen than the R state, so with increased acidity, the hemoglobin binds less O 2 for a given P O2 (and more H + ). This is known as the Bohr effect . [ 5 ] A reduction in the total binding capacity of hemoglobin to oxygen (i.e. shifting the curve down, not just to the right) due to reduced pH is called the root effect . This is seen in bony fish. The binding affinity of hemoglobin to O 2 is greatest under a relatively high pH.
Carbon dioxide affects the curve in two ways. First, CO 2 accumulation causes carbamino compounds to be generated through chemical interactions, which bind to hemoglobin forming carbaminohemoglobin . CO 2 is considered an Allosteric regulation as the inhibition happens not at the binding site of hemoglobin. [ 6 ] Second, it influences intracellular pH due to formation of bicarbonate ion. Formation of carbaminohemoglobin stabilizes T state hemoglobin by formation of ion pairs. [ 4 ] Only about 5–10% of the total CO 2 content of blood is transported as carbamino compounds, whereas (80–90%) is transported as bicarbonate ions and a small amount is dissolved in the plasma. The formation of a bicarbonate ion will release a proton into the plasma, decreasing pH (increased acidity), which also shifts the curve to the right as discussed above; low CO 2 levels in the blood stream results in a high pH, and thus provides more optimal binding conditions for hemoglobin and O 2 . This is a physiologically favored mechanism, since hemoglobin will drop off more oxygen as the concentration of carbon dioxide increases dramatically where tissue respiration is happening rapidly and oxygen is in need. [ 7 ] [ 8 ]
2,3-Bisphosphoglycerate or 2,3-BPG (formerly named 2,3-diphosphoglycerate or 2,3-DPG) is an organophosphate formed in red blood cells during glycolysis and is the conjugate base of 2,3-bisphosphoglyceric acid . The production of 2,3-BPG is likely an important adaptive mechanism, because the production increases for several conditions in the presence of diminished peripheral tissue O 2 availability, such as hypoxemia , chronic lung disease, anemia , and congestive heart failure , among others, which necessitate easier oxygen unloading in the peripheral tissue. High levels of 2,3-BPG shift the curve to the right (as in childhood), while low levels of 2,3-BPG cause a leftward shift, seen in states such as septic shock , and hypophosphataemia . [ 5 ] In the absence of 2,3-BPG, hemoglobin's affinity for oxygen increases. 2,3-BPG acts as a heteroallosteric effector of hemoglobin, lowering hemoglobin's affinity for oxygen by binding preferentially to deoxyhemoglobin. An increased concentration of BPG in red blood cells favours formation of the T (taut or tense), low-affinity state of hemoglobin and so the oxygen-binding curve will shift to the right.
Increase in temperature shifts the oxygen dissociation curve to the right. When temperature is increased keeping the oxygen concentration constant, oxygen saturation decreases as the bond between oxygen and iron gets denatured. Additionally, with increased temperature, the partial pressure of oxygen increases as well. So, one will have a lesser amount of hemoglobin saturated for the same oxygen concentration but at a higher partial pressure of oxygen. Thus, any point in the curve will shift rightwards (due to increased partial pressure of oxygen) and downwards (due to weakened Hb − O 2 {\displaystyle {\ce {Hb-O2}}} bond), hence, the rightward shift of the curve. [ 9 ]
Hemoglobin binds with carbon monoxide 210 times more readily than with oxygen. [ 5 ] Because of this higher affinity of hemoglobin for carbon monoxide than for oxygen, carbon monoxide is a highly successful competitor that will displace oxygen even at minuscule partial pressures. The reaction HbO 2 + CO → HbCO + O 2 almost irreversibly displaces the oxygen molecules forming carboxyhemoglobin ; the binding of the carbon monoxide to the iron centre of hemoglobin is much stronger than that of oxygen, and the binding site remains blocked for the remainder of the life cycle of that affected red blood cell. [ 10 ] With an increased level of carbon monoxide, a person can suffer from severe tissue hypoxia while maintaining a normal pO 2 because carboxyhemoglobin does not carry oxygen to the tissues.
Methemoglobinaemia is a form of abnormal hemoglobin where the iron centre has been oxidised from the ferrous +2 oxidation state (the normal form, which on binding with oxygen changes to the ferric state) to the ferric +3 state. This causes a leftward shift in the oxygen hemoglobin dissociation curve, as any residual heme with oxygenated ferrous iron (+2 state) is unable to unload its bound oxygen into tissues (because 3+ iron impairs hemoglobin's cooperativity), thereby increasing its affinity with oxygen. However, methemoglobin has increased affinity for cyanide , and is therefore useful in the treatment of cyanide poisoning . In cases of accidental ingestion, administration of a nitrite (such as amyl nitrite ) can be used to deliberately oxidise hemoglobin and raise methemoglobin levels, restoring the functioning of cytochrome oxidase . The nitrite also acts as a vasodilator , promoting the cellular supply of oxygen, and the addition of an iron salt provides for competitive binding of the free cyanide as the biochemically inert hexacyanoferrate(III) ion, [Fe(CN) 6 ] 3− . An alternative approach involves administering thiosulfate , thereby converting cyanide to thiocyanate , SCN − , which is excreted via the kidneys. Methemoglobin is also formed in small quantities when the dissociation of oxyhemoglobin results in the formation of methemoglobin and superoxide , O 2 − , instead of the usual products. Superoxide is a free radical and causes biochemical damage, but is neutralised by the action of the enzyme superoxide dismutase .
Myo-inositol trispyrophosphate (ITPP), also known as OXY111A, is an inositol phosphate that causes a rightward shift in the oxygen hemoglobin dissociation curve through allosteric modulation of hemoglobin within red blood cells. It is an experimental drug intended to reduce tissue hypoxia . The effects appear to last roughly as long as the affected red blood cells remain in circulation.
Fetal hemoglobin (HbF) is structurally different from normal adult hemoglobin (HbA), giving HbF a higher affinity for oxygen than HbA. HbF is composed of two alpha and two gamma chains, whereas HbA is composed of two alpha and two beta chains. The fetal dissociation curve is shifted to the left relative to the curve for the normal adult because of these structural differences:
In adult hemoglobin, the binding of 2,3-bisphosphoglycerate (2,3-BPG) primarily occurs with the beta chains, preventing the binding of oxygen with haemoglobin. This binding is crucial for stabilizing the deoxygenated state of hemoglobin, promoting the efficient release of oxygen to body tissues.
In fetal hemoglobin, which possesses a gamma chain instead of a beta chain, the interaction with 2,3-BPG differs because 2,3-BPG does not bind with the gamma chain as it has lower to no affinity with the gamma chain. This distinction contributes to fetal hemoglobin having a higher affinity for oxygen.
Typically, fetal arterial oxygen pressures are lower than adult arterial oxygen pressures. Hence, higher affinity to bind oxygen is required at lower levels of partial pressure in the fetus to allow diffusion of oxygen across the placenta . At the placenta, there is a higher concentration of 2,3-BPG formed, and 2,3-BPG binds readily to beta chains rather than to alpha chains. As a result, 2,3-BPG binds more strongly to adult hemoglobin, causing HbA to release more oxygen for uptake by the fetus, whose HbF is unaffected by the 2,3-BPG. [ 11 ] HbF then delivers that bound oxygen to tissues that have even lower partial pressures where it can be released. | https://en.wikipedia.org/wiki/Oxygen–hemoglobin_dissociation_curve |
Oxyhydrogen is a mixture of hydrogen (H 2 ) and oxygen (O 2 ) gases. This gaseous mixture is used for torches to process refractory materials and was the first [ 1 ] gaseous mixture used for welding . Theoretically, a ratio of 2:1 hydrogen:oxygen is enough to achieve maximum efficiency; in practice a ratio 4:1 or 5:1 is needed to avoid an oxidizing flame . [ 2 ]
This mixture may also be referred to as Knallgas (Scandinavian and German Knallgas ; lit. ' bang-gas ' ), although some authors define knallgas to be a generic term for the mixture of fuel with the precise amount of oxygen required for complete combustion, thus 2:1 oxyhydrogen would be called "hydrogen-knallgas". [ 3 ]
"Brown's gas" and HHO are terms for oxyhydrogen originating in pseudoscience , although x H 2 + y O 2 is preferred due to HHO meaning H 2 O .
Oxyhydrogen will combust when brought to its autoignition temperature . For the stoichiometric mixture in air, at normal atmospheric pressure , autoignition occurs at about 570 °C (1065 °F). [ 4 ] The minimum energy required to ignite such a mixture, at lower temperatures, with a spark is about 20 microjoules . [ 4 ] At standard temperature and pressure , oxyhydrogen can burn when it is between about 4% and 95% hydrogen by volume. [ 5 ] [ 4 ]
When ignited, the gas mixture converts to water vapor and releases energy , which sustains the reaction: 241.8 kJ of energy ( LHV ) for every mole of H 2 burned. The amount of heat energy released is independent of the mode of combustion, but the temperature of the flame varies. [ 6 ] The maximum temperature of about 2,800 °C (5,100 °F) is achieved with an exact stoichiometric mixture, about 700 °C (1,300 °F) hotter than a hydrogen flame in air. [ 7 ] [ 8 ] [ 9 ] When either of the gases are mixed in excess of this ratio, or when mixed with an inert gas like nitrogen, the heat must spread throughout a greater quantity of matter, reducing flame temperature. [ 6 ]
Oxyhydrogen is explosive and can detonate when ignited, releasing a large amount of energy. This is often demonstrated in classroom environments in which teachers fill a balloon with the gas, due to the easy access of hydrogen and oxygen. [ 10 ]
A precisely stoichiometric mixture may be obtained by water electrolysis , which uses an electric current to dissociate the water molecules:
William Nicholson was the first to decompose water in this manner in 1800. In theory, the input energy of a closed system always equals the output energy, as the first law of thermodynamics states. However, in practice no systems are perfectly closed, and the energy required to generate the oxyhydrogen always exceeds the energy released by combusting it, even at maximum practical efficiency, as the second law of thermodynamics implies (see Electrolysis of water#Efficiency ).
Many forms of oxyhydrogen lamps have been described, such as the limelight , which used an oxyhydrogen flame to heat a piece of quicklime to white hot incandescence . [ 11 ] Because of the explosiveness of the oxyhydrogen, limelights have been replaced by electric lighting .
The foundations of the oxy-hydrogen blowpipe were laid down by Carl Wilhelm Scheele and Joseph Priestley around the last quarter of the eighteenth century. The oxy-hydrogen blowpipe itself was developed by the Frenchman Bochard-de-Saron, the English mineralogist Edward Daniel Clarke and the American chemist Robert Hare in the late 18th and early 19th centuries. [ 12 ] It produced a flame hot enough to melt such refractory materials as platinum , porcelain , fire brick , and corundum , and was a valuable tool in several fields of science. [ 13 ] It is used in the Verneuil process to produce synthetic corundum. [ 14 ]
An oxyhydrogen torch (also known as hydrogen torch ) is an oxy-gas torch that burns hydrogen (the fuel ) with oxygen (the oxidizer ). It is used for cutting and welding [ 15 ] metals , glasses , and thermoplastics . [ 11 ]
Due to competition from arc welding and other oxy-fuel torches such as the acetylene-fueled cutting torch, the oxyhydrogen torch is seldom used today, but it remains the preferred cutting tool in some niche applications.
Oxyhydrogen was once used in working platinum , because at the time, only it could burn hot enough to melt the metal 1,768.3 °C (3,214.9 °F). [ 6 ] These techniques have been superseded by the electric arc furnace .
Oxyhydrogen is associated with various exaggerated claims. [ 16 ] [ 17 ] [ 18 ] It is often called "Brown's gas" or "HHO gas", a term popularized by fringe physicist [ 19 ] Ruggero Santilli , who claimed that his HHO gas, produced by a special apparatus, is "a new form of water", with new properties, based on his fringe theory of "magnecules". [ 18 ]
Many other pseudoscientific claims have been made about oxyhydrogen, like an ability to neutralize radioactive waste, help plants to germinate, and more. [ 18 ]
Oxyhydrogen is often mentioned in conjunction with vehicles that claim to use water as a fuel . The most common and decisive counter-argument against producing this gas on board to use as a fuel or fuel additive is that more energy is always needed to split water molecules than is recouped by burning the resulting gas. [ 17 ] [ 20 ] Additionally, the volume of gas that can be produced for on-demand consumption through electrolysis is very small in comparison to the volume consumed by an internal combustion engine. [ 21 ]
An article in Popular Mechanics in 2008 reported that oxyhydrogen does not increase the fuel economy in automobiles . [ 22 ]
"Water-fueled" cars should not be confused with hydrogen-fueled cars , where the hydrogen is produced elsewhere and used as fuel or where it is used as fuel enhancement . | https://en.wikipedia.org/wiki/Oxyhydrogen |
Oxyntomodulin (often abbreviated OXM ) is a naturally occurring 37-amino acid peptide hormone found in the colon , produced by the oxyntic ( fundic ) cells of the oxyntic (fundic) mucosa . It has been found to suppress appetite .
The mechanism of action of oxyntomodulin is not well understood. It is known to bind both the GLP-1 receptor and the glucagon receptor , but it is not known whether the effects of the hormone are mediated through these receptors or through an unidentified receptor.
Oxyntomodulin has been linked to entrainment of the liver's circadian clock . [ 1 ]
Oxyntomodulin has been investigated as a blood-glucose regulation agent in connection with diabetes . [ 2 ]
Oxyntomodulin could be a potential candidate for treating obesity because of its ability to suppress appetite. [ 3 ] In a 4 week study, healthy overweight and obese volunteers were given either saline or oxyntomodulin injections. Their body weight, energy intake, and the levels of adipose hormones were taken prior to the treatment. The volunteers maintained their usual diets and daily activities and self-administered the injections three times daily, 30 minutes before their meals. In the course of 4 weeks, volunteers treated with oxyntomodulin injections had an average weight loss of 2.3±0.4 kg compared to those treated with saline who had an average of 0.5±0.5 kg, indicating oxyntomodulin was successful in weight loss. [ 4 ]
This biochemistry article is a stub . You can help Wikipedia by expanding it . | https://en.wikipedia.org/wiki/Oxyntomodulin |
Oxyphil cells are found in oncocytomas of the kidney , endocrine glands , and salivary glands . [ 1 ]
This article related to pathology is a stub . You can help Wikipedia by expanding it . | https://en.wikipedia.org/wiki/Oxyphil_cell_(pathology) |
In chemistry, oxypnictides are a class of materials composed of oxygen , a pnictogen (group-V, especially phosphorus and arsenic) and one or more other elements. Although this group of compounds has been recognized since 1995, [ 1 ] interest in these compounds increased dramatically after the publication of the superconducting properties of LaOFeP and LaOFeAs which were discovered in 2006 [ 2 ] and 2008. [ 3 ] [ 4 ] In these experiments the oxide was partly replaced by fluoride.
These and related compounds (e.g. the 122 iron arsenides ) form a new group of iron-based superconductors known as iron pnictides or ferropnictides since the oxygen is not essential but the iron seems to be.
Oxypnictides have been patented as magnetic semiconductors in early 2006. [ 5 ]
The different subclasses of oxypnictides are oxynitrides , oxyphosphides , oxyarsenides , oxyantimonides , and oxybismuthides .
Many of the oxypnictides show a layered structure. [ 6 ] For example, LaFePO with layers of La 3+ O 2− and Fe 2+ P 3− . [ 2 ] This structure is similar to that of ZrCuSiAs, which is now the parent structure for most of the oxypnictide. [ 7 ]
The first superconducting iron oxypnictide was discovered in 2006, based on phosphorus. [ 2 ] A drastic increase in the critical temperature was achieved when phosphorus was substituted by arsenic. [ 3 ] This discovery boosted the search for similar compounds, like the search for cuprate -based superconductors after their discovery in 1986.
The superconductivity of the oxypnictides seems to depend on the iron-pnictogen layers.
Some found in 2008 to be high-temperature superconductors (up to 55 K) of composition ReOTmPn, where Re is a rare earth , Tm is a transition metal and Pn is from group V e.g. As. [ 8 ]
Tests in magnetic fields up to 45 teslas [ 15 ] [ 16 ] suggest the upper critical field of LaFeAsO 0.89 F 0.11 may be around 64 T. A different lanthanum -based material tested at 6 K predicts an upper critical field of 122 T in La 0.8 K 0.2 FeAsO 0.8 F 0.2 . [ 10 ]
Because of the brittleness of the oxypnictides, superconducting wires are formed using the powder-in-tube process (using iron tubes). [ 17 ] | https://en.wikipedia.org/wiki/Oxypnictide |
Oxyrrhis marina is a species of heterotrophic dinoflagellate with flagella that is widely distributed in the world's oceans.
This protozoan species has an asymmetrical oval shape to its single-celled body. [ 1 ] It has been likened to a rugby ball . [ 2 ] The cell usually measures between 20 and 30 micrometers, but it is known to reach 60. It has two flagella with a protruding, tentacle-like bulge between them. The flagella are covered in scales. Most individuals have scales on the body surface, as well. The two flagella have separate functions. One undulates in waves and the other is coiled, producing a corkscrew-like propulsion to move the cell. The individual appears colorless, but a concentrated culture of cells may have a pink tinge. [ 1 ]
The species is thought to have a global distribution except for the polar seas , where it is likely absent or rare, though few samples have been taken of these waters. [ 3 ] There are specific records from waters near Europe, North America, Asia, New Zealand , the Canary Islands , [ 4 ] Hawaii , and the Azores . It has been found in isolated inland waters, as well, such as a lake in Ukraine . It is less common in the open waters of the oceans. There is a question as to how it came to inhabit so many islands if it is apparently rare in the open ocean. It may have been slowly dispersed on the currents , carried in mats of algae , or transported by humans when shipping arose. [ 3 ]
It is most common in the intertidal zone and other coastal regions, [ 3 ] where it is a member of the plankton . [ 5 ] Habitat types include tide pools and estuaries . [ 6 ] It was first described from a salt marsh . [ 3 ] It tolerates wide ranges in salinity , temperature, and pH . [ 7 ]
It is heterotrophic , obtaining nutrients externally instead of synthesizing them by an internal process such as photosynthesis . It is an omnivorous grazer , consuming various types of tiny organisms from its environment. It eats phytoplankton such as minute algaes. [ 8 ] It has been observed eating Nannochloris oculata and Micromonas pusilla , other flagellates such as Goniomonas amphinema , Pfiesteria piscicida , and Stoeckeria algicida , and some bacteria . [ 9 ] It often eats the coccolithophore Cricosphaera elongata , and, in experimental situations, readily eats Tetraselmis suecica , Isochrysis galbana , and Rhodomonas sp. Some of these food items are relatively large, as large as the O. marina cell itself. It is selective in its grazing, showing clear preferences for certain food taxa. [ 8 ] It can also pick certain individuals over others, as evidenced by its preference for virus -infected Emiliana huxleyi cells over healthy cells. [ 10 ] It is cannibalistic , as well. It feeds by phagocytosis , totally engulfing its prey. It has been observed spinning one of its flagella in such a way that it creates a current, pulling the item closer so it can seize it. It is also raptorial, approaching and pouncing on the prey item, especially when the item is a protist. [ 9 ] O. marina can sense and respond to certain chemicals that are exuded by algal prey. [ 10 ]
The locomotion of the O. marina cell is helical due to the simultaneous movement of its two flagella. It mostly swims in a straight line, but it makes turns when it detects food. [ 11 ]
In terms of reproduction, O. marina is isogamous , with reproductive cells smaller than the body cell, but very little is known about these. [ 5 ]
This species sometimes forms red tides , [ 3 ] [ 5 ] but will also feed on the raphidophyte , Heterosigma akashiwo , another organism responsible for red tides. [ 12 ] Its blooms when forming red tides are likely stimulated by environmental factors, such as drops in salinity or increases in prey abundance. [ 5 ] O. marina may also affect the environment by producing dimethyl sulfide , which is released when it grazes on some prey types, such as E. huxleyi . [ 10 ]
Predators of O. marina include protozoa such as the ciliate Strombidinopsis jeokjo , copepods such as Acartia tonsa [ 10 ] and rotifers . The mixotrophic flagellate Prymnesium parvum is a prey item for O. marina when the former is nutrient-replete, but can become a predator when it is nutrient-stressed [ 13 ] It has been used as food for fish larvae , including those of black porgy ( Mylio macrocephalus ), lemonpeel angelfish ( Centropyge flavissima ), and grey mullet ( Mugil cephalus ). Bryozoans have been grown on a mixture of the protist and yeast . [ 13 ]
This protist has been studied extensively. It is a model organism for the study of many aspects of protist biology, including feeding behavior, [ 8 ] [ 9 ] physiology , [ 1 ] ecology , [ 5 ] [ 7 ] growth , [ 5 ] trophic position , [ 7 ] evolution , genomics , and biogeography . [ 6 ] Many more studies of its genetics are now underway. [ 7 ] There are some limitations to using the species as a model, in part because dinoflagellates are so diverse . O. marina itself is very diverse, with many varied strains, and their biology is influenced by the environment, so it can be hard to find a representative specimen to use as a model. [ 14 ] In fact, some experts deny that it is a dinoflagellate at all, or at least a "true" dinoflagellate. [ 15 ] In general, it is still very useful for scientific experiments, and researchers recommend it. [ 14 ]
O. marina has genes that have evidently been transferred to it from bacteria. It also has some genes that are related to plastids , indicating that it may have had an ancestor that could perform photosynthesis. Also, it has some genes related to essential amino acid synthesis, something that is uncommon in heterotrophs, as they usually obtain essential amino acids by eating them. [ 7 ]
It is easy to isolate from the environment and easy to grow in the laboratory. Cultures are fed Dunaliella primolecta or any of a number of other readily available protists. Dead E. coli cells can also be used for food. It can also be sustained on a nutritional medium. [ 2 ] Cultures can be maintained for years. [ 6 ]
This protist has been called a morphospecies . As it is now understood, it is composed of a number of isolates , some of which are quite distinct. [ 1 ] [ 3 ] There are 50 to 80 wild isolates. [ 2 ] In the future some of these could be divided into separate taxa, perhaps on the species level. [ 3 ] One of these may become Oxyrrhis maritima . Another called O. tenticulifera may be valid, as well. [ 1 ] | https://en.wikipedia.org/wiki/Oxyrrhis_marina |
Oxyselenides are a group of chemical compounds that contain oxygen and selenium atoms (Figure 1). Oxyselenides can form a wide range of structures in compounds containing various transition metals , and thus can exhibit a wide range of properties. Most importantly, oxyselenides have a wide range of thermal conductivity , which can be controlled with changes in temperature in order to adjust their thermoelectric performance . Current research on oxyselenides indicates their potential for significant application in electronic materials. [ 1 ]
The first oxyselenide to be crystallized was manganese oxyselenide in 1900. [ 2 ] In 1910, oxyselenides containing phosphate were created by treating P 2 Se 5 with metal hydroxides . [ 3 ] Uranium oxyselenide was formed next by treating H 2 Se with uranium dioxides at 1000 °C. [ 4 ] This technique was also utilized in synthesizing oxyselenides of rare-earth elements in the mid-1900s. [ 5 ] Synthesis of oxyselenide compounds currently involves treating oxides with aluminum powder and selenium at high temperatures. [ 6 ]
Recent discoveries in iron oxyarsenides and their superconductivity have highlighted the importance of mixed anion systems. [ 7 ] Mixed copper oxychalcogenides came about when the electronic properties of both chalcogenides and oxides were taken into account. Chemists began pursuing the synthesis of a compound with metallic and charge density wave properties as well as high temperature superconductivity. Upon synthesizing the copper oxyselenide Na 1.9 Cu 2 Se 2 ·Cu 2 O by reacting Na 2 Se 3.6 with Cu 2 O , [ 8 ] they concluded that a new type of oxychalcogenides could be synthesized by reacting metal oxides with polychalcogenide fluxes.
New oxyselenides of the formula Sr 2 AO 2 M 2 Se 2 (A=Co, Mn; M=Cu, Ag) have been synthesized. They crystallize into structures consisting of alternating perovskite -like (metal oxide) and antifluorite (metal selenide) layers (Figure 2). The optical band gap of each oxyselenide is very narrow, indicating semiconductivity . [ 9 ]
Another derivative that reveals oxyselenide properties is β-La 2 O 2 MSe 2 (M= Fe, Mn). This molecule possesses an orthorhombic structure (Figure 3), opening up the possibilities for different packing arrangements of oxyselenides. They are ferromagnetic at low temperatures (~27 K) and show high resistivity at room temperature. The Mn analogue, diluted in NaCl solution, suggests an optical band gap of 1.6 eV at room temperature, making it an insulator . Meanwhile, the band gap for the Fe analogue is approximately 0.7 eV between 150 K and 300 K, making it a semiconductor . [ 7 ] In contrast, cobalt oxyselenide La 2 Co 2 O 3 Se 2 is antiferromagnetically ordered, suggesting that although the different transition metals are responsible for the changes in an oxyselenide's magnetic property, the molecule's overall lattice structure may also influence its conductivity. [ 10 ]
The magnetic and conducting properties of different metal compounds coordinated with oxyselenide are not only affected by the transition metal used, but also by the synthesis conditions. For example, the percentage of aluminium used during the synthesis of Ce 2 O 2 ZnSe 2 as an oxygen retriever affected the band gaps, indicated by the varying product colours. [ 6 ] Various structures allow for many potential configurations. For example, as observed before in La 2 Co 2 O 3 Se 2 , Sr 2 F 2 Mn 2 Se 2 O exhibits a frustrated magnetic correlation in the structure resulting in an antiferromagnetic lattice. [ 11 ]
In 2010, p-type polycrystalline BiCuSeO oxyselenides were reported as possible thermoelectric materials. [ 12 ] The weak bonds between the [Cu 2 Se 2 ] −2 conducting and [Bi 2 O 2 ] +2 insulating layer, as well as the anharmonic crystal lattice structure, may account for the substance's low thermal conductivity and high thermoelectric performance. Recently, BiCuSeO's ZT value, a dimensionless figure-of-merit indicating thermoelectric performance, has been increased from 0.5 to 1.4. Experiment has shown that Ca doping can improve electrical conductivity, thereby increasing the ZT value. [ 1 ] Additionally, replacing 15% of the Bi 3+ ions with group 2 metal ions, Ca 2+ , Sr 2+ , or Ba 2+ (Figure 4), also optimizes the charge carrier concentration. [ 12 ] | https://en.wikipedia.org/wiki/Oxyselenide |
Özgur Baris Akan is a Professor with the Department of Electrical and Electronics Engineering and the Director of Next-generation and Wireless Communications Laboratory (NWCL) at the University of Cambridge and Koç University in Istanbul, Turkey. He was named Fellow of the Institute of Electrical and Electronics Engineers (IEEE) in 2016 [ 1 ] for contributions to wireless sensor networks . Since the same year, he is also a fellow of the Vehicular Technology Society. [ 2 ]
Akan was born in Ankara, Turkey. He attended Ankara Science High School , and after graduation from it, went to study in Bilkent University . After obtaining B.Sc. in electrical and electronics engineering from Bilkent in June 1999, he studied for an M.Sc. degree at Middle East Technical University , where in January 2002 he graduated with it in the same field. Akan received the Ph.D. degree in electrical and computer engineering in 2004, after studying at the Broadband and Wireless Networking Laboratory at Georgia Tech under the supervision of Ian Akyildiz . Following graduation, he joined the Department of Electrical and Electronics Engineering at the Middle East Technical University, serving there until August 2010. From January 2013 to May 2016, Akan served as associate and then as director of Graduate School of Sciences and Engineering. [ 3 ]
This biographical article about a Turkish engineer or inventor is a stub . You can help Wikipedia by expanding it . | https://en.wikipedia.org/wiki/Ozgur_B._Akan |
The term ozogamicin in the names of monoclonal antibodies or antibody-drug conjugates indicates that they are linked to a cytotoxic agent from the class of calicheamicins . [ 2 ] | https://en.wikipedia.org/wiki/Ozogamicin |
Ozone ( / ˈ oʊ z oʊ n / ) (or trioxygen ) is an inorganic molecule with the chemical formula O 3 . It is a pale blue gas with a distinctively pungent smell. It is an allotrope of oxygen that is much less stable than the diatomic allotrope O 2 , breaking down in the lower atmosphere to O 2 ( dioxygen ). Ozone is formed from dioxygen by the action of ultraviolet (UV) light and electrical discharges within the Earth's atmosphere . It is present in very low concentrations throughout the atmosphere, with its highest concentration high in the ozone layer of the stratosphere , which absorbs most of the Sun 's ultraviolet (UV) radiation.
Ozone's odor is reminiscent of chlorine , and detectable by many people at concentrations of as little as 0.1 ppm in air. Ozone's O 3 structure was determined in 1865. The molecule was later proven to have a bent structure and to be weakly diamagnetic . At standard temperature and pressure , ozone is a pale blue gas that condenses at cryogenic temperatures to a dark blue liquid and finally a violet-black solid . Ozone's instability with regard to more common dioxygen is such that both concentrated gas and liquid ozone may decompose explosively at elevated temperatures, physical shock, or fast warming to the boiling point. [ 5 ] [ 6 ] It is therefore used commercially only in low concentrations.
Ozone is a powerful oxidizing agent (far more so than dioxygen) and has many industrial and consumer applications related to oxidation. This same high oxidizing potential, however, causes ozone to damage mucous and respiratory tissues in animals, and also tissues in plants, above concentrations of about 0.1 ppm . While this makes ozone a potent respiratory hazard and pollutant near ground level , a higher concentration in the ozone layer (from two to eight ppm) is beneficial, preventing damaging UV light from reaching the Earth's surface.
The trivial name ozone is the most commonly used and preferred IUPAC name . The systematic names 2λ 4 -trioxidiene [ dubious – discuss ] and catena-trioxygen , valid IUPAC names, are constructed according to the substitutive and additive nomenclatures , respectively. The name ozone derives from ozein (ὄζειν), the Greek neuter present participle for smell, [ 7 ] referring to ozone's distinctive smell.
In appropriate contexts, ozone can be viewed as trioxidane with two hydrogen atoms removed, and as such, trioxidanylidene may be used as a systematic name, according to substitutive nomenclature. By default, these names pay no regard to the radicality of the ozone molecule. In an even more specific context, this can also name the non-radical singlet ground state, whereas the diradical state is named trioxidanediyl .
Trioxidanediyl (or ozonide ) is used, non-systematically, to refer to the substituent group (-OOO-). Care should be taken to avoid confusing the name of the group for the context-specific name for the ozone given above.
In 1785, Dutch chemist Martinus van Marum was conducting experiments involving electrical sparking above water when he noticed an unusual smell, which he attributed to the electrical reactions, failing to realize that he had in fact produced ozone. [ 8 ] [ 9 ]
A half century later, Christian Friedrich Schönbein noticed the same pungent odour and recognized it as the smell often following a bolt of lightning . In 1839, he succeeded in isolating the gaseous chemical and named it "ozone", from the Greek word ozein ( ὄζειν ) meaning "to smell". [ 10 ] [ 11 ] For this reason, Schönbein is generally credited with the discovery of ozone. [ 12 ] [ 13 ] [ 14 ] [ 8 ] He also noted the similarity of ozone smell to the smell of phosphorus, and in 1844 proved that the product of reaction of white phosphorus with air is identical. [ 10 ] A subsequent effort to call ozone "electrified oxygen" he ridiculed by proposing to call the ozone from white phosphorus "phosphorized oxygen". [ 10 ] The chemical formula for ozone, O 3 , was not determined until 1865 by Jacques-Louis Soret [ 15 ] and confirmed by Schönbein in 1867. [ 10 ] [ 16 ]
For much of the second half of the 19th century and well into the 20th, ozone was considered a healthy component of the environment by naturalists and health-seekers. Beaumont, California , had as its official slogan "Beaumont: Zone of Ozone", as evidenced on postcards and Chamber of Commerce letterhead. [ 17 ] Naturalists working outdoors often considered the higher elevations beneficial because of their ozone content which was readily monitored. [ 18 ] "There is quite a different atmosphere [at higher elevation] with enough ozone to sustain the necessary energy [to work]", wrote naturalist Henry Henshaw , working in Hawaii. [ 19 ] Seaside air was considered to be healthy because of its believed ozone content. The smell giving rise to this belief is in fact that of halogenated seaweed metabolites [ 20 ] and dimethyl sulfide . [ 21 ]
Much of ozone's appeal seems to have resulted from its "fresh" smell, which evoked associations with purifying properties. Scientists noted its harmful effects. In 1873 James Dewar and John Gray McKendrick documented that frogs grew sluggish, birds gasped for breath, and rabbits' blood showed decreased levels of oxygen after exposure to "ozonized air", which "exercised a destructive action". [ 22 ] [ 12 ] Schönbein himself reported that chest pains, irritation of the mucous membranes , and difficulty breathing occurred as a result of inhaling ozone, and small mammals died. [ 23 ] In 1911, Leonard Hill and Martin Flack stated in the Proceedings of the Royal Society B that ozone's healthful effects "have, by mere iteration, become part and parcel of common belief; and yet exact physiological evidence in favour of its good effects has been hitherto almost entirely wanting ... The only thoroughly well-ascertained knowledge concerning the physiological effect of ozone, so far attained, is that it causes irritation and œdema of the lungs, and death if inhaled in relatively strong concentration for any time." [ 12 ] [ 24 ]
During World War I , ozone was tested at Queen Alexandra Military Hospital in London as a possible disinfectant for wounds. The gas was applied directly to wounds for as long as 15 minutes. This resulted in damage to both bacterial cells and human tissue. Other sanitizing techniques, such as irrigation with antiseptics , were found preferable. [ 12 ] [ 25 ]
Until the 1920s, it was not certain whether small amounts of oxozone , O 4 , were also present in ozone samples due to the difficulty of applying analytical chemistry techniques to the explosive concentrated chemical. [ 26 ] [ 27 ] In 1923, Georg-Maria Schwab (working for his doctoral thesis under Ernst Hermann Riesenfeld ) was the first to successfully solidify ozone and perform accurate analysis which conclusively refuted the oxozone hypothesis. [ 26 ] [ 27 ] Further hitherto unmeasured physical properties of pure concentrated ozone were determined by the Riesenfeld group in the 1920s. [ 26 ]
Ozone is a colourless or pale blue gas, slightly soluble in water, and much more soluble in inert non-polar solvents such as carbon tetrachloride or fluorocarbons, in which it forms a blue solution. At 161 K (−112 °C; −170 °F), it condenses to form a dark blue liquid . It is dangerous to allow this liquid to warm to its boiling point, because both concentrated gaseous ozone and liquid ozone can detonate. At temperatures below 80 K (−193.2 °C; −315.7 °F), it forms a violet-black solid . [ 28 ]
Ozone has a very specific sharp odour somewhat resembling chlorine bleach . Most people can detect it at the 0.01 μmol/mol level in air. Exposure of 0.1 to 1 μmol/mol produces headaches and burning eyes and irritates the respiratory passages. [ 29 ] Even low concentrations of ozone in air are very destructive to organic materials such as latex, plastics, and animal lung tissue.
The ozone molecule is weakly diamagnetic . [ 30 ]
According to experimental evidence from microwave spectroscopy , ozone is a bent molecule, with C 2v symmetry (similar to the water molecule). [ 31 ] The O–O distances are 127.2 pm (1.272 Å ). The O–O–O angle is 116.78°. [ 32 ] The central atom is sp ² hybridized with one lone pair. Ozone is a polar molecule with a dipole moment of 0.53 D . [ 33 ] The molecule can be represented as a resonance hybrid with two contributing structures, each with a single bond on one side and double bond on the other. The arrangement possesses an overall bond order of 1.5 for both sides. It is isoelectronic with the nitrite anion . Naturally occurring ozone can be composed of substituted isotopes ( 16 O, 17 O, 18 O). A cyclic form has been predicted but not observed.
Ozone is among the most powerful oxidizing agents known, far stronger than O 2 . It is also unstable at high concentrations, decaying into ordinary diatomic oxygen. Its half-life varies with atmospheric conditions such as temperature, humidity, and air movement. Under laboratory conditions, the half-life will average ~1500 minutes (25 hours) in still air at room temperature (24 °C), zero humidity with zero air changes per hour. [ 34 ]
This reaction proceeds more rapidly with increasing temperature. Deflagration of ozone can be triggered by a spark and can occur in ozone concentrations of 10 wt% or higher. [ 35 ]
Ozone can also be produced from oxygen at the anode of an electrochemical cell. This reaction can create smaller quantities of ozone for research purposes. [ 36 ]
This can be observed as an unwanted reaction in a Hoffman apparatus during the electrolysis of water when the voltage is set above the necessary voltage.
Ozone oxidizes most metals (except gold , platinum , and iridium ) into oxides of the metals in their highest oxidation state . For example:
Ozone oxidizes nitric oxide to nitrogen dioxide :
This reaction is accompanied by chemiluminescence .
The NO 2 can be further oxidized to nitrate radical :
The NO 3 formed can react with NO 2 to form dinitrogen pentoxide ( N 2 O 5 ).
Solid nitronium perchlorate can be made from NO 2 , ClO 2 , and O 3 gases:
Ozone does not react with ammonium salts , but it oxidizes ammonia to ammonium nitrate :
Ozone reacts with carbon to form carbon dioxide , even at room temperature:
Ozone oxidizes sulfides to sulfates . For example, lead(II) sulfide is oxidized to lead(II) sulfate :
Sulfuric acid can be produced from ozone, water and either elemental sulfur or sulfur dioxide :
In the gas phase , ozone reacts with hydrogen sulfide to form sulfur dioxide:
In an aqueous solution, however, two competing simultaneous reactions occur, one to produce elemental sulfur, and one to produce sulfuric acid :
Alkenes can be oxidatively cleaved by ozone, in a process called ozonolysis , giving alcohols, aldehydes, ketones, and carboxylic acids, depending on the second step of the workup.
Ozone can also cleave alkynes to form an acid anhydride or diketone product. [ 38 ] If the reaction is performed in the presence of water, the anhydride hydrolyzes to give two carboxylic acids .
Usually ozonolysis is carried out in a solution of dichloromethane , at a temperature of −78 °C. After a sequence of cleavage and rearrangement, an organic ozonide is formed. With reductive workup (e.g. zinc in acetic acid or dimethyl sulfide ), ketones and aldehydes will be formed, with oxidative workup (e.g. aqueous or alcoholic hydrogen peroxide ), carboxylic acids will be formed. [ 39 ]
All three atoms of ozone may also react, as in the reaction of tin(II) chloride with hydrochloric acid and ozone:
Iodine perchlorate can be made by treating iodine dissolved in cold anhydrous perchloric acid with ozone:
Ozone could also react with potassium iodide to give oxygen and iodine gas that can be titrated for quantitative determination: [ 40 ]
Ozone can be used for combustion reactions and combustible gases; ozone provides higher temperatures than burning in dioxygen ( O 2 ). The following is a reaction for the combustion of carbon subnitride which can also cause higher temperatures:
Ozone can react at cryogenic temperatures. At 77 K (−196.2 °C; −321.1 °F), atomic hydrogen reacts with liquid ozone to form a hydrogen superoxide radical , which dimerizes : [ 41 ]
Ozone is a toxic substance, [ 42 ] [ 43 ] commonly found or generated in human environments (aircraft cabins, offices with photocopiers, laser printers, sterilizers, ...).
The catalytic decomposition of ozone is very important to reduce pollution. This type of decomposition is the most widely used, especially with solid catalysts, and it has many advantages such as a higher conversion with a lower temperature. Furthermore, the product and the catalyst can be instantaneously separated, and this way the catalyst can be easily recovered without using any separation operation. The most-used materials in the catalytic decomposition of ozone in the gas phase are manganese dioxide , transition metals such as Mn, Co, Cu, Fe, Ni, or Ag, and noble metals such as Pt, Rh, or Pd.
Free radicals of chlorine (Cl · ), formed by the action of ultraviolet radiation on chlorofluorocarbons (CFCs) and sea salt, are known to catalyze the breakdown of ozone in the atmosphere.
There are two other possibilities for decomposing ozone in the gas phase:
The uncatalyzed process of ozone decomposition in the gas phase is a complex reaction involving two elementary reactions that finally lead to molecular oxygen, [ 45 ] and this means that the reaction order and the rate law cannot be determined by the stoichiometry of the overall reaction.
Overall reaction: 2 O 3 ⟶ 3 O 2 {\displaystyle {\ce {2 O3 -> 3 O2}}}
Rate law (observed): V = K o b s ⋅ [ O 3 ] 2 [ O 2 ] {\displaystyle V={\frac {K_{obs}\cdot [{\ce {O3}}]^{2}}{[{\ce {O2}}]}}}
where K o b s {\displaystyle K_{obs}} is the observed rate constant and V {\displaystyle V} is the reaction rate. From the rate law above it can be determined that the partial order respect to molecular oxygen is −1 and respect to ozone is 2; therefore, the global reaction order is 1.
The first step is a unimolecular reaction wherein one molecule of ozone decomposes into two products (molecular oxygen and oxygen). The oxygen atom from the first step is a reactive intermediate because it participates as a reactant in the second step, which is a bimolecular reaction because there are two different reactants (ozone and oxygen) that give rise to molecular oxygen.
Step 1: Unimolecular reaction O 3 ⟶ O 2 + O {\displaystyle {\ce {O3 -> O2 + O}}}
Step 2: Bimolecular reaction O 3 + O ⟶ 2 O 2 {\displaystyle {\ce {O3 + O -> 2 O2}}}
These two steps have different reaction rates and rate constants. The reaction rate laws for each of these steps are shown below:
The following mechanism allows to explain the rate law of the ozone decomposition observed experimentally, and also it allows to determine the reaction orders with respect to ozone and oxygen, with which the overall reaction order will be determined.
The first step is assumed reversible and faster than the second reaction, which means that the slower rate determining step is the second reaction. This step determines the rate of product formation, and so V = V 2 {\displaystyle V=V_{2}} . However, this equation depends on the concentration of oxygen (intermediate), which does not appear in the observed rate law. Since the first step is a rapid equilibrium, the concentration of the intermediate can be determined as follows:
Then using these equations, the formation rate of molecular oxygen is as shown below:
The mechanism is consistent with the rate law observed experimentally if the rate constant ( K obs ) is given in terms of the individual mechanistic steps' rate constants as follows: [ 46 ]
where K obs = K 2 ⋅ K 1 K − 1 {\displaystyle K_{\text{obs}}={K_{2}\cdot K_{1} \over K_{-1}}}
Reduction of ozone gives the ozonide anion, O − 3 . Derivatives of this anion are explosive and must be stored at cryogenic temperatures. Ozonides for all the alkali metals are known. KO 3 , RbO 3 , and CsO 3 can be prepared from their respective superoxides:
Although KO 3 can be formed as above, it can also be formed from potassium hydroxide and ozone: [ 47 ]
NaO 3 and LiO 3 must be prepared by action of CsO 3 in liquid NH 3 on an ion-exchange resin containing Na + or Li + ions: [ 48 ]
A solution of calcium in ammonia reacts with ozone to give ammonium ozonide and not calcium ozonide: [ 41 ]
Ozone can be used to remove iron and manganese from water , forming a precipitate which can be filtered:
Ozone oxidizes dissolved hydrogen sulfide in water to sulfurous acid :
These three reactions are central in the use of ozone-based well water treatment.
Ozone detoxifies cyanides by converting them to cyanates .
Ozone completely decomposes urea : [ 49 ]
Ozone is a bent triatomic molecule with three vibrational modes: the symmetric stretch (1103.157 cm −1 ), bend (701.42 cm −1 ) and antisymmetric stretch (1042.096 cm −1 ). [ 50 ] The symmetric stretch and bend are weak absorbers, but the antisymmetric stretch is strong and responsible for ozone being an important minor greenhouse gas . This IR band is also used to detect ambient and atmospheric ozone although UV-based measurements are more common. [ 51 ]
The electromagnetic spectrum of ozone is quite complex. An overview can be seen at the MPI Mainz UV/VIS Spectral Atlas of Gaseous Molecules of Atmospheric Interest. [ 52 ]
All of the bands are dissociative, meaning that the molecule falls apart to O + O 2 after absorbing a photon. The most important absorption is the Hartley band, extending from slightly above 300 nm down to slightly above 200 nm. It is this band that is responsible for absorbing UV C in the stratosphere.
On the high wavelength side, the Hartley band transitions to the so-called Huggins band, which falls off rapidly until disappearing by ~360 nm. Above 400 nm, extending well out into the NIR, are the Chappius and Wulf bands. There, unstructured absorption bands are useful for detecting high ambient concentrations of ozone, but are so weak that they do not have much practical effect.
There are additional absorption bands in the far UV, which increase slowly from 200 nm down to reaching a maximum at ~120 nm.
The standard way to express total ozone levels (the amount of ozone in a given vertical column) in the atmosphere is by using Dobson units . Point measurements are reported as mole fractions in nmol/mol (parts per billion, ppb) or as concentrations in μg/m 3 . The study of ozone concentration in the atmosphere started in the 1920s. [ 53 ]
The highest levels of ozone in the atmosphere are in the stratosphere , in a region also known as the ozone layer between about 10 and 50 km above the surface (or between about 6 and 31 miles). However, even in this "layer", the ozone concentrations are only two to eight parts per million, so most of the oxygen there is dioxygen, O 2 , at about 210,000 parts per million by volume. [ 54 ]
Ozone in the stratosphere is mostly produced from short-wave ultraviolet rays between 240 and 160 nm. Oxygen starts to absorb weakly at 240 nm in the Herzberg bands, but most of the oxygen is dissociated by absorption in the strong Schumann–Runge bands between 200 and 160 nm where ozone does not absorb. While shorter wavelength light, extending to even the X-Ray limit, is energetic enough to dissociate molecular oxygen, there is relatively little of it, and, the strong solar emission at Lyman-alpha, 121 nm, falls at a point where molecular oxygen absorption is a minimum. [ 55 ]
The process of ozone creation and destruction is called the Chapman cycle and starts with the photolysis of molecular oxygen
followed by reaction of the oxygen atom with another molecule of oxygen to form ozone.
where "M" denotes the third body that carries off the excess energy of the reaction. The ozone molecule can then absorb a UV-C photon and dissociate
The excess kinetic energy heats the stratosphere when the O atoms and the molecular oxygen fly apart and collide with other molecules. This conversion of UV light into kinetic energy warms the stratosphere. The oxygen atoms produced in the photolysis of ozone then react back with other oxygen molecule as in the previous step to form more ozone. In the clear atmosphere, with only nitrogen and oxygen, ozone can react with the atomic oxygen to form two molecules of O 2 :
An estimate of the rate of this termination step to the cycling of atomic oxygen back to ozone can be found simply by taking the ratios of the concentration of O 2 to O 3 . The termination reaction is catalysed by the presence of certain free radicals, of which the most important are hydroxyl (OH), nitric oxide (NO) and atomic chlorine (Cl) and bromine (Br). In the second half of the 20th century, the amount of ozone in the stratosphere was discovered to be declining , mostly because of increasing concentrations of chlorofluorocarbons (CFC) and similar chlorinated and brominated organic molecules . The concern over the health effects of the decline led to the 1987 Montreal Protocol , the ban on the production of many ozone-depleting chemicals and in the first and second decade of the 21st century the beginning of the recovery of stratospheric ozone concentrations.
Ozone in the ozone layer filters out sunlight wavelengths from about 200 nm UV rays to 315 nm, with ozone peak absorption at about 250 nm. [ 56 ] This ozone UV absorption is important to life, since it extends the absorption of UV by ordinary oxygen and nitrogen in air (which absorb all wavelengths < 200 nm) through the lower UV-C (200–280 nm) and the entire UV-B band (280–315 nm). The small unabsorbed part that remains of UV-B after passage through ozone causes sunburn in humans, and direct DNA damage in living tissues in both plants and animals. Ozone's effect on mid-range UV-B rays is illustrated by its effect on UV-B at 290 nm, which has a radiation intensity 350 million times as powerful at the top of the atmosphere as at the surface. Nevertheless, enough of UV-B radiation at similar frequency reaches the ground to cause some sunburn, and these same wavelengths are also among those responsible for the production of vitamin D in humans.
The ozone layer has little effect on the longer UV wavelengths called UV-A (315–400 nm), but this radiation does not cause sunburn or direct DNA damage. While UV-A probably does cause long-term skin damage in certain humans, it is not as dangerous to plants and to the health of surface-dwelling organisms on Earth in general (see ultraviolet for more information on near ultraviolet).
Ground-level ozone (or tropospheric ozone) is an atmospheric pollutant. [ 57 ] It is not emitted directly by car engines or by industrial operations, but formed by the reaction of sunlight on air containing hydrocarbons and nitrogen oxides that react to form ozone directly at the source of the pollution or many kilometers downwind.
Ozone reacts directly with some hydrocarbons such as aldehydes and thus begins their removal from the air, but the products are themselves key components of smog . Ozone photolysis by UV light leads to production of the hydroxyl radical HO• and this plays a part in the removal of hydrocarbons from the air, but is also the first step in the creation of components of smog such as peroxyacyl nitrates , which can be powerful eye irritants. The atmospheric lifetime of tropospheric ozone is about 22 days; its main removal mechanisms are being deposited to the ground, the above-mentioned reaction giving HO•, and by reactions with OH and the peroxy radical HO 2 •. [ 58 ]
There is evidence of significant reduction in agricultural yields because of increased ground-level ozone and pollution which interferes with photosynthesis and stunts overall growth of some plant species. [ 59 ] [ 60 ] The United States Environmental Protection Agency (EPA) has proposed a secondary regulation to reduce crop damage, in addition to the primary regulation designed for the protection of human health.
Certain examples of cities with elevated ozone readings are Denver, Colorado ; Houston, Texas ; and Mexico City , Mexico . Houston has a reading of around 41 nmol/mol, while Mexico City is far more hazardous, with a reading of about 125 nmol/mol. [ 60 ]
Ground-level ozone, or tropospheric ozone, is the most concerning type of ozone pollution in urban areas and is increasing in general. [ 61 ] Ozone pollution in urban areas affects denser populations, and is worsened by high populations of vehicles, which emit pollutants NO 2 and VOCs , the main contributors to problematic ozone levels. [ 62 ] Ozone pollution in urban areas is especially concerning with increasing temperatures, raising heat-related mortality during heat waves . [ 63 ] During heat waves in urban areas, ground level ozone pollution can be 20% higher than usual. [ 64 ] Ozone pollution in urban areas reaches higher levels of exceedance in the summer and autumn, which may be explained by weather patterns and traffic patterns. [ 62 ] People experiencing poverty are more affected by pollution in general, even though these populations are less likely to be contributing to pollution levels. [ 65 ]
As mentioned above, Denver, Colorado, is one of the many cities in the U.S. that have high amounts of ozone. According to the American Lung Association , the Denver–Aurora area is the 14th most ozone-polluted area in the U.S. [ 66 ] The problem of high ozone levels is not new to this area. In 2004, the EPA allotted the Denver Metro /North Front Range [ b ] as non-attainment areas per 1997's 8-hour ozone standard, [ 67 ] but later deferred this status until 2007. The non-attainment standard indicates that an area does not meet the EPA's air quality standards. The Colorado Ozone Action Plan was created in response, and numerous changes were implemented from this plan. The first major change was that car emission testing was expanded across the state to more counties that did not previously mandate emissions testing, like areas of Larimer and Weld County. There have also been changes made to decrease Nitrogen Oxides (NOx) and Volatile Organic Compound (VOC) emissions, which should help lower ozone levels.
One large contributor to high ozone levels in the area is the oil and natural gas industry situated in the Denver-Julesburg Basin (DJB) which overlaps with a majority of Colorado's metropolitan areas. Ozone is produced naturally in the Earth's stratosphere, but is also produced in the troposphere from human efforts. Briefly mentioned above, NOx and VOCs react with sunlight to create ozone through a process called photochemistry. One hour elevated ozone events (<75 ppb) "occur during June–August indicating that elevated ozone levels are driven by regional photochemistry". [ 68 ] According to an article from the University of Colorado-Boulder, "Oil and natural gas VOC emission have a major role in ozone production and bear the potential to contribute to elevated O 3 levels in the Northern Colorado Front Range (NCFR)". [ 68 ] Using complex analyses to research wind patterns and emissions from large oil and natural gas operations, the authors concluded that "elevated O 3 levels in the NCFR are predominantly correlated with air transport from N– ESE, which are the upwind sectors where the O&NG operations in the Wattenberg Field area of the DJB are located". [ 68 ]
Contained in the Colorado Ozone Action Plan, created in 2008, plans exist to evaluate "emission controls for large industrial sources of NOx" and "statewide control requirements for new oil and gas condensate tanks and pneumatic valves". [ 69 ] In 2011, the Regional Haze Plan was released that included a more specific plan to help decrease NOx emissions. These efforts are increasingly difficult to implement and take many years to come to pass. Of course there are also other reasons that ozone levels remain high. These include: a growing population meaning more car emissions, and the mountains along the NCFR that can trap emissions. If interested, daily air quality readings can be found at the Colorado Department of Public Health and Environment's website. [ 70 ] As noted earlier, Denver continues to experience high levels of ozone to this day. It will take many years and a systems-thinking approach to combat this issue of high ozone levels in the Front Range of Colorado.
Ozone gas attacks any polymer possessing olefinic or double bonds within its chain structure, such as natural rubber , nitrile rubber , and styrene-butadiene rubber. Products made using these polymers are especially susceptible to attack, which causes cracks to grow longer and deeper with time, the rate of crack growth depending on the load carried by the rubber component and the concentration of ozone in the atmosphere. Such materials can be protected by adding antiozonants , such as waxes, which bond to the surface to create a protective film or blend with the material and provide long term protection. Ozone cracking used to be a serious problem in car tires, [ 71 ] for example, but it is not an issue with modern tires. On the other hand, many critical products, like gaskets and O-rings , may be attacked by ozone produced within compressed air systems. Fuel lines made of reinforced rubber are also susceptible to attack, especially within the engine compartment, where some ozone is produced by electrical components. Storing rubber products in close proximity to a DC electric motor can accelerate ozone cracking. The commutator of the motor generates sparks which in turn produce ozone.
Although ozone was present at ground level before the Industrial Revolution , peak concentrations are now far higher than the pre-industrial levels, and even background concentrations well away from sources of pollution are substantially higher. [ 73 ] [ 74 ] Ozone acts as a greenhouse gas , absorbing some of the infrared energy emitted by the earth. Quantifying the greenhouse gas potency of ozone is difficult because it is not present in uniform concentrations across the globe. However, the most widely accepted scientific assessments relating to climate change (e.g. the Intergovernmental Panel on Climate Change Third Assessment Report ) [ 75 ] suggest that the radiative forcing of tropospheric ozone is about 25% that of carbon dioxide .
The annual global warming potential of tropospheric ozone is between 918 and 1022 tons carbon dioxide equivalent /tons tropospheric ozone. This means on a per-molecule basis, ozone in the troposphere has a radiative forcing effect roughly 1,000 times as strong as carbon dioxide . However, tropospheric ozone is a short-lived greenhouse gas, which decays in the atmosphere much more quickly than carbon dioxide . This means that over a 20-year span, the global warming potential of tropospheric ozone is much less, roughly 62 to 69 tons carbon dioxide equivalent / ton tropospheric ozone. [ 76 ]
Because of its short-lived nature, tropospheric ozone does not have strong global effects, but has very strong radiative forcing effects on regional scales. In fact, there are regions of the world where tropospheric ozone has a radiative forcing up to 150% of carbon dioxide . [ 77 ] For example, ozone increase in the troposphere is shown to be responsible for ~30% of upper Southern Ocean interior warming between 1955 and 2000. [ 78 ]
Filters containing an adsorbent or catalyst such as charcoal (carbon) may be used to remove odors and gaseous pollutants such as volatile organic compounds or ozone. [ 79 ]
For the last few decades, scientists studied the effects of acute and chronic ozone exposure on human health. Hundreds of studies suggest that ozone is harmful to people at levels currently found in urban areas. [ 80 ] [ 81 ] Ozone has been shown to affect the respiratory, cardiovascular and central nervous system. Early death and problems in reproductive health and development are also shown to be associated with ozone exposure. [ 82 ]
The American Lung Association has identified five populations who are especially vulnerable to the effects of breathing ozone: [ 83 ]
Additional evidence suggests that women, those with obesity and low-income populations may also face higher risk from ozone, although more research is needed. [ 83 ]
Acute ozone exposure ranges from hours to a few days. Because ozone is a gas, it directly affects the lungs and the entire respiratory system. Inhaled ozone causes inflammation and acute—but reversible—changes in lung function, as well as airway hyperresponsiveness. [ 84 ] These changes lead to shortness of breath, wheezing, and coughing which may exacerbate lung diseases, like asthma or chronic obstructive pulmonary disease (COPD) resulting in the need to receive medical treatment. [ 85 ] [ 86 ] Acute and chronic exposure to ozone has been shown to cause an increased risk of respiratory infections, due to the following mechanism. [ 87 ]
Multiple studies have been conducted to determine the mechanism behind ozone's harmful effects, particularly in the lungs. These studies have shown that exposure to ozone causes changes in the immune response within the lung tissue, resulting in disruption of both the innate and adaptive immune response, as well as altering the protective function of lung epithelial cells. [ 88 ] It is thought that these changes in immune response and the related inflammatory response are factors that likely contribute to the increased risk of lung infections, and worsening or triggering of asthma and reactive airways after exposure to ground-level ozone pollution. [ 88 ] [ 89 ]
The innate (cellular) immune system consists of various chemical signals and cell types that work broadly and against multiple pathogen types, typically bacteria or foreign bodies/substances in the host. [ 89 ] [ 90 ] The cells of the innate system include phagocytes, neutrophils, [ 90 ] both thought to contribute to the mechanism of ozone pathology in the lungs, as the functioning of these cell types have been shown to change after exposure to ozone. [ 89 ] Macrophages, cells that serve the purpose of eliminating pathogens or foreign material through the process of "phagocytosis", [ 90 ] have been shown to change the level of inflammatory signals they release in response to ozone, either up-regulating and resulting in an inflammatory response in the lung, or down-regulating and reducing immune protection. [ 88 ] Neutrophils, another important cell type of the innate immune system that primarily targets bacterial pathogens, [ 90 ] are found to be present in the airways within 6 hours of exposure to high ozone levels. Despite high levels in the lung tissues, however, their ability to clear bacteria appears impaired by exposure to ozone. [ 88 ]
The adaptive immune system is the branch of immunity that provides long-term protection via the development of antibodies targeting specific pathogens and is also impacted by high ozone exposure. [ 89 ] [ 90 ] Lymphocytes, a cellular component of the adaptive immune response, produce an increased amount of inflammatory chemicals called "cytokines" after exposure to ozone, which may contribute to airway hyperreactivity and worsening asthma symptoms. [ 88 ]
The airway epithelial cells also play an important role in protecting individuals from pathogens. In normal tissue, the epithelial layer forms a protective barrier, and also contains specialized ciliary structures that work to clear foreign bodies, mucus and pathogens from the lungs. When exposed to ozone, the cilia become damaged and mucociliary clearance of pathogens is reduced. Furthermore, the epithelial barrier becomes weakened, allowing pathogens to cross the barrier, proliferate and spread into deeper tissues. Together, these changes in the epithelial barrier help make individuals more susceptible to pulmonary infections. [ 88 ]
Inhaling ozone not only affects the immune system and lungs, but it may also affect the heart as well. Ozone causes short-term autonomic imbalance leading to changes in heart rate and reduction in heart rate variability; [ 91 ] and high levels exposure for as little as one-hour results in a supraventricular arrhythmia in the elderly, [ 92 ] both increase the risk of premature death and stroke. Ozone may also lead to vasoconstriction resulting in increased systemic arterial pressure contributing to increased risk of cardiac morbidity and mortality in patients with pre-existing cardiac diseases. [ 93 ] [ 94 ]
Breathing ozone for periods longer than eight hours at a time for weeks, months or years defines chronic exposure. Numerous studies suggest a serious impact on the health of various populations from this exposure.
One study finds significant positive associations between chronic ozone and all-cause, circulatory, and respiratory mortality with 2%, 3%, and 12% increases in risk per 10 ppb [ 95 ] and report an association (95% CI) of annual ozone and all-cause mortality with a hazard ratio of 1.02 (1.01–1.04), and with cardiovascular mortality of 1.03 (1.01–1.05). A similar study finds similar associations with all-cause mortality and even larger effects for cardiovascular mortality. [ 96 ] An increased risk of mortality from respiratory causes is associated with long-term chronic exposure to ozone. [ 97 ]
Chronic ozone has detrimental effects on children, especially those with asthma. The risk for hospitalization in children with asthma increases with chronic exposure to ozone; younger children and those with low-income status are even at greater risk. [ 98 ]
Adults suffering from respiratory diseases (asthma, [ 99 ] COPD, [ 100 ] lung cancer [ 101 ] ) are at a higher risk of mortality and morbidity and critically ill patients have an increased risk of developing acute respiratory distress syndrome with chronic ozone exposure as well. [ 102 ]
Ozone generators sold as air cleaners intentionally produce the gas ozone. [ 43 ] These are often marketed to control indoor air pollution , and use misleading terms to describe ozone. Some examples are describing it as "energized oxygen" or "pure air", suggesting that ozone is a healthy or "better" kind of oxygen. [ 43 ] However, according to the EPA , "There is evidence to show that at concentrations that do not exceed public health standards, ozone is not effective at removing many odor-causing chemicals", and "If used at concentrations that do not exceed public health standards, ozone applied to indoor air does not effectively remove viruses, bacteria, mold, or other biological pollutants.". [ 43 ] Furthermore, another report states that "results of some controlled studies show that concentrations of ozone considerably higher than these [human safety] standards are possible even when a user follows the manufacturer's operating instructions". [ 103 ]
The California Air Resources Board has a page listing air cleaners (many with ionizers ) meeting their indoor ozone limit of 0.050 parts per million. [ 104 ] From that article:
All portable indoor air cleaning devices sold in California must be certified by the California Air Resources Board (CARB). To be certified, air cleaners must be tested for electrical safety and ozone emissions, and meet an ozone emission concentration limit of 0.050 parts per million. For more information about the regulation, visit the air cleaner regulation .
Ozone precursors are a group of pollutants, predominantly those emitted during the combustion of fossil fuels . Ground-level ozone pollution (tropospheric ozone) is produced near the Earth's surface by the action of daylight UV rays on these precursors. The ozone at ground level is primarily from fossil fuel precursors, but methane is a natural precursor, and the very low natural background level of ozone at ground level is considered safe. This section examines the health impacts of fossil fuel burning, which raises ground level ozone far above background levels.
There is a great deal of evidence to show that ground-level ozone can harm lung function and irritate the respiratory system . [ 57 ] [ 106 ] Exposure to ozone (and the pollutants that produce it) is linked to premature death , asthma , bronchitis , heart attack , and other cardiopulmonary problems. [ 107 ] [ 108 ]
Long-term exposure to ozone has been shown to increase risk of death from respiratory illness . [ 43 ] A study of 450,000 people living in U.S. cities saw a significant correlation between ozone levels and respiratory illness over the 18-year follow-up period. The study revealed that people living in cities with high ozone levels, such as Houston or Los Angeles, had an over 30% increased risk of dying from lung disease. [ 109 ] [ 110 ]
Air quality guidelines such as those from the World Health Organization , the U.S. Environmental Protection Agency (EPA), and the European Union are based on detailed studies designed to identify the levels that can cause measurable ill health effects .
According to scientists with the EPA, susceptible people can be adversely affected by ozone levels as low as 40 nmol/mol. [ 108 ] [ 111 ] [ 112 ] In the EU, the current target value for ozone concentrations is 120 μg/m 3 which is about 60 nmol/mol. This target applies to all member states in accordance with Directive 2008/50/EC. [ 113 ] Ozone concentration is measured as a maximum daily mean of 8 hour averages and the target should not be exceeded on more than 25 calendar days per year, starting from January 2010. While the directive requires in the future a strict compliance with 120 μg/m 3 limit (i.e. mean ozone concentration not to be exceeded on any day of the year), there is no date set for this requirement and this is treated as a long-term objective. [ 114 ]
In the US, the Clean Air Act directs the EPA to set National Ambient Air Quality Standards for several pollutants, including ground-level ozone, and counties out of compliance with these standards are required to take steps to reduce their levels. In May 2008, under a court order, the EPA lowered its ozone standard from 80 nmol/mol to 75 nmol/mol. The move proved controversial, since the Agency's own scientists and advisory board had recommended lowering the standard to 60 nmol/mol. [ 108 ] Many public health and environmental groups also supported the 60 nmol/mol standard, [ 115 ] and the World Health Organization recommends 100 μg/m 3 (51 nmol/mol). [ 116 ]
On January 7, 2010, the U.S. Environmental Protection Agency (EPA) announced proposed revisions to the National Ambient Air Quality Standard (NAAQS) for the pollutant ozone, the principal component of smog:
... EPA proposes that the level of the 8-hour primary standard, which was set at 0.075 μmol/mol in the 2008 final rule, should instead be set at a lower level within the range of 0.060 to 0.070 μmol/mol, to provide increased protection for children and other at risk populations against an array of O 3 – related adverse health effects that range from decreased lung function and increased respiratory symptoms to serious indicators of respiratory morbidity including emergency department visits and hospital admissions for respiratory causes, and possibly cardiovascular-related morbidity as well as total non- accidental and cardiopulmonary mortality ... [ 117 ]
On October 26, 2015, the EPA published a final rule with an effective date of December 28, 2015, that revised the 8-hour primary NAAQS from 0.075 ppm to 0.070 ppm. [ 118 ]
The EPA has developed an air quality index (AQI) to help explain air pollution levels to the general public. Under the current standards, eight-hour average ozone mole fractions of 85 to 104 nmol/mol are described as "unhealthy for sensitive groups", 105 nmol/mol to 124 nmol/mol as "unhealthy", and 125 nmol/mol to 404 nmol/mol as "very unhealthy". [ 119 ]
Ozone can also be present in indoor air pollution , partly as a result of electronic equipment such as photocopiers. A connection has also been known to exist between the increased pollen, fungal spores, and ozone caused by thunderstorms and hospital admissions of asthma sufferers. [ 120 ]
In the Victorian era , one British folk myth held that the smell of the sea was caused by ozone. In fact, the characteristic "smell of the sea" is caused by dimethyl sulfide , a chemical generated by phytoplankton . Victorian Britons considered the resulting smell "bracing". [ 121 ]
An investigation to assess the joint mortality effects of ozone and heat during the European heat waves in 2003, concluded that these appear to be additive. [ 122 ]
Ozone, along with reactive forms of oxygen such as superoxide , singlet oxygen , hydrogen peroxide , and hypochlorite ions, is produced by white blood cells and other biological systems (such as the roots of marigolds ) as a means of destroying foreign bodies. Ozone reacts directly with organic double bonds. Also, when ozone breaks down to dioxygen it gives rise to oxygen free radicals , which are highly reactive and capable of damaging many organic molecules . Moreover, it is believed that the powerful oxidizing properties of ozone may be a contributing factor of inflammation . The cause-and-effect relationship of how the ozone is created in the body and what it does is still under consideration and still subject to various interpretations, since other body chemical processes can trigger some of the same reactions. There is evidence linking the antibody-catalyzed water-oxidation pathway of the human immune response to the production of ozone. In this system, ozone is produced by antibody-catalyzed production of trioxidane from water and neutrophil-produced singlet oxygen. [ 123 ]
When inhaled, ozone reacts with compounds lining the lungs to form specific, cholesterol-derived metabolites that are thought to facilitate the build-up and pathogenesis of atherosclerotic plaques (a form of heart disease ). These metabolites have been confirmed as naturally occurring in human atherosclerotic arteries and are categorized into a class of secosterols termed atheronals , generated by ozonolysis of cholesterol's double bond to form a 5,6 secosterol [ 124 ] as well as a secondary condensation product via aldolization. [ 125 ]
Ozone has been implicated to have an adverse effect on plant growth: "... ozone reduced total chlorophylls, carotenoid and carbohydrate concentration, and increased 1-aminocyclopropane-1-carboxylic acid (ACC) content and ethylene production. In treated plants, the ascorbate leaf pool was decreased, while lipid peroxidation and solute leakage were significantly higher than in ozone-free controls. The data indicated that ozone triggered protective mechanisms against oxidative stress in citrus." [ 126 ] Studies that have used pepper plants as a model have shown that ozone decreased fruit yield and changed fruit quality. [ 127 ] [ 128 ] Furthermore, it was also observed a decrease in chlorophylls levels and antioxidant defences on the leaves, as well as increased the reactive oxygen species (ROS) levels and lipid and protein damages. [ 127 ] [ 128 ]
A 2022 study concludes that East Asia loses 63 billion dollars in crops per year due to ozone pollution, a byproduct of fossil fuel combustion. China loses about one-third of its potential wheat production and one-fourth of its rice production. [ 129 ] [ 130 ]
Because of the strongly oxidizing properties of ozone, ozone is a primary irritant, affecting especially the eyes and respiratory systems and can be hazardous at even low concentrations. The Canadian Centre for Occupation Safety and Health reports that:
Even very low concentrations of ozone can be harmful to the upper respiratory tract and the lungs. The severity of injury depends on both the concentration of ozone and the duration of exposure. Severe and permanent lung injury or death could result from even a very short-term exposure to relatively low concentrations." [ 131 ]
To protect workers potentially exposed to ozone, U.S. Occupational Safety and Health Administration has established a permissible exposure limit (PEL) of 0.1 μmol/mol (29 CFR 1910.1000 table Z-1), calculated as an 8-hour time weighted average. Higher concentrations are especially hazardous and NIOSH has established an Immediately Dangerous to Life and Health Limit (IDLH) of 5 μmol/mol. [ 132 ] Work environments where ozone is used or where it is likely to be produced should have adequate ventilation and it is prudent to have a monitor for ozone that will alarm if the concentration exceeds the OSHA PEL. Continuous monitors for ozone are available from several suppliers.
Elevated ozone exposure can occur on passenger aircraft , with levels depending on altitude and atmospheric turbulence. [ 133 ] U.S. Federal Aviation Administration regulations set a limit of 250 nmol/mol with a maximum four-hour average of 100 nmol/mol. [ 134 ] Some planes are equipped with ozone converters in the ventilation system to reduce passenger exposure. [ 133 ]
Ozone generators , or ozonators , [ 135 ] are used to produce ozone for cleaning air or removing smoke odours in unoccupied rooms. These ozone generators can produce over 3 g of ozone per hour. Ozone often forms in nature under conditions where O 2 will not react. [ 29 ] Ozone used in industry is measured in μmol/mol (ppm, parts per million), nmol/mol (ppb, parts per billion), μg/m 3 , mg/h (milligrams per hour) or weight percent. The regime of applied concentrations ranges from 1% to 5% (in air) and from 6% to 14% (in oxygen) for older generation methods. New electrolytic methods can achieve up 20% to 30% dissolved ozone concentrations in output water.
Temperature and humidity play a large role in how much ozone is being produced using traditional generation methods (such as corona discharge and ultraviolet light). Old generation methods will produce less than 50% of nominal capacity if operated with humid ambient air, as opposed to very dry air. New generators, using electrolytic methods, can achieve higher purity and dissolution through using water molecules as the source of ozone production.
This is the most common type of ozone generator for most industrial and personal uses. While variations of the "hot spark" coronal discharge method of ozone production exist, including medical grade and industrial grade ozone generators, these units usually work by means of a corona discharge tube or ozone plate. [ 136 ] [ 137 ] They are typically cost-effective and do not require an oxygen source other than the ambient air to produce ozone concentrations of 3–6%. Fluctuations in ambient air, due to weather or other environmental conditions, cause variability in ozone production. However, they also produce nitrogen oxides as a by-product. Use of an air dryer can reduce or eliminate nitric acid formation by removing water vapor and increase ozone production. At room temperature, nitric acid will form into a vapour that is hazardous if inhaled. Symptoms can include chest pain, shortness of breath, headaches and a dry nose and throat causing a burning sensation. Use of an oxygen concentrator can further increase the ozone production and further reduce the risk of nitric acid formation by removing not only the water vapor, but also the bulk of the nitrogen.
UV ozone generators, or vacuum-ultraviolet (VUV) ozone generators, employ a light source that generates a narrow-band ultraviolet light, a subset of that produced by the Sun. The Sun's UV sustains the ozone layer in the stratosphere of Earth. [ 138 ]
UV ozone generators use ambient air for ozone production, no air prep systems are used (air dryer or oxygen concentrator), therefore these generators tend to be less expensive. However, UV ozone generators usually produce ozone with a concentration of about 0.5% or lower which limits the potential ozone production rate. Another disadvantage of this method is that it requires the ambient air (oxygen) to be exposed to the UV source for a longer amount of time, and any gas that is not exposed to the UV source will not be treated. This makes UV generators impractical for use in situations that deal with rapidly moving air or water streams (in-duct air sterilization , for example). Production of ozone is one of the potential dangers of ultraviolet germicidal irradiation . VUV ozone generators are used in swimming pools and spa applications ranging to millions of gallons of water. VUV ozone generators, unlike corona discharge generators, do not produce harmful nitrogen by-products and also unlike corona discharge systems, VUV ozone generators work extremely well in humid air environments. There is also not normally a need for expensive off-gas mechanisms, and no need for air driers or oxygen concentrators which require extra costs and maintenance.
In the cold plasma method, pure oxygen gas is exposed to a plasma created by DBD . The diatomic oxygen is split into single atoms, which then recombine in triplets to form ozone.
It is common in the industry to mislabel some DBD ozone generators as CD Corona Discharge generators. Typically all solid flat metal electrode ozone generators produce ozone using the dielectric barrier discharge method. Cold plasma machines use pure oxygen as the input source and produce a maximum concentration of about 24% ozone. They produce far greater quantities of ozone in a given time compared to ultraviolet production that has about 2% efficiency. The discharges manifest as filamentary transfer of electrons (micro discharges) in a gap between two electrodes. In order to evenly distribute the micro discharges, a dielectric insulator must be used to separate the metallic electrodes and to prevent arcing.
Electrolytic ozone generation (EOG) splits water molecules into H 2 , O 2 , and O 3 .
In most EOG methods, the hydrogen gas will be removed to leave oxygen and ozone as the only reaction products. Therefore, EOG can achieve higher dissolution in water without other competing gases found in corona discharge method, such as nitrogen gases present in ambient air. This method of generation can achieve concentrations of 20–30% and is independent of air quality because water is used as the source material. Production of ozone electrolytically is typically unfavorable because of the high overpotential required to produce ozone as compared to oxygen. This is why ozone is not produced during typical water electrolysis. However, it is possible to increase the overpotential of oxygen by careful catalyst selection such that ozone is preferentially produced under electrolysis. Catalysts typically chosen for this approach are lead dioxide [ 139 ] or boron-doped diamond. [ 140 ]
The ozone-to-oxygen ratio is improved by increasing current density at the anode, cooling the electrolyte around the anode close to 0 °C, using an acidic electrolyte (such as dilute sulfuric acid) instead of a basic solution, and by applying pulsed current instead of DC. [ 141 ]
Ozone cannot be stored and transported like other industrial gases (because it quickly decays into diatomic oxygen) and must therefore be produced on site. Available ozone generators vary in the arrangement and design of the high-voltage electrodes. At production capacities higher than 20 kg per hour, a gas/water tube heat-exchanger may be utilized as ground electrode and assembled with tubular high-voltage electrodes on the gas-side. The regime of typical gas pressures is around 2 bars (200 kPa ) absolute in oxygen and 3 bars (300 kPa) absolute in air. Several megawatts of electrical power may be installed in large facilities, applied as single phase AC current at 50 to 8000 Hz and peak voltages between 3,000 and 20,000 volts. Applied voltage is usually inversely related to the applied frequency.
The dominating parameter influencing ozone generation efficiency is the gas temperature, which is controlled by cooling water temperature and/or gas velocity. The cooler the water, the better the ozone synthesis. The lower the gas velocity, the higher the concentration (but the lower the net ozone produced). At typical industrial conditions, almost 90% of the effective power is dissipated as heat and needs to be removed by a sufficient cooling water flow.
Because of the high reactivity of ozone, only a few materials may be used like stainless steel (quality 316L), titanium , aluminium (as long as no moisture is present), glass , polytetrafluorethylene , or polyvinylidene fluoride . Viton may be used with the restriction of constant mechanical forces and absence of humidity (humidity limitations apply depending on the formulation). Hypalon may be used with the restriction that no water comes in contact with it, except for normal atmospheric levels. Embrittlement or shrinkage is the common mode of failure of elastomers with exposure to ozone. Ozone cracking is the common mode of failure of elastomer seals like O-rings .
Silicone rubbers are usually adequate for use as gaskets in ozone concentrations below 1 wt%, such as in equipment for accelerated aging of rubber samples.
Ozone may be formed from O 2 by electrical discharges and by action of high energy electromagnetic radiation . Unsuppressed arcing in electrical contacts, motor brushes, or mechanical switches breaks down the chemical bonds of the atmospheric oxygen surrounding the contacts [ O 2 -> 2O]. Free radicals of oxygen in and around the arc recombine to create ozone [ O 3 ]. [ 142 ] Certain electrical equipment generate significant levels of ozone. This is especially true of devices using high voltages , such as ionic air purifiers , laser printers , photocopiers , tasers , and arc welders . Electric motors using brushes can generate ozone from repeated sparking inside the unit. Large motors that use brushes, such as those used by elevators or hydraulic pumps, will generate more ozone than smaller motors.
Ozone is similarly formed in the Catatumbo lightning storms phenomenon on the Catatumbo River in Venezuela , though ozone's instability makes it dubious that it has any effect on the ozonosphere. [ 143 ] It is the world's largest single natural generator of ozone, lending calls for it to be designated a UNESCO World Heritage Site . [ 144 ]
In the laboratory, ozone can be produced by electrolysis using a 9 volt battery , a pencil graphite rod cathode , a platinum wire anode , and a 3 molar sulfuric acid electrolyte . [ 145 ] The half cell reactions taking place are:
where E° represents the standard electrode potential .
In the net reaction, three equivalents of water are converted into one equivalent of ozone and three equivalents of hydrogen . Oxygen formation is a competing reaction.
It can also be generated by a high voltage arc . In its simplest form, high voltage AC, such as the output of a neon-sign transformer is connected to two metal rods with the ends placed sufficiently close to each other to allow an arc. The resulting arc will convert atmospheric oxygen to ozone.
It is often desirable to contain the ozone. This can be done with an apparatus consisting of two concentric glass tubes sealed together at the top with gas ports at the top and bottom of the outer tube. The inner core should have a length of metal foil inserted into it connected to one side of the power source. The other side of the power source should be connected to another piece of foil wrapped around the outer tube. A source of dry O 2 is applied to the bottom port. When high voltage is applied to the foil leads, electricity will discharge between the dry dioxygen in the middle and form O 3 and O 2 which will flow out the top port. This is called a Siemen's ozoniser. The reaction can be summarized as follows: [ 29 ]
The largest use of ozone is in the preparation of pharmaceuticals , synthetic lubricants , and many other commercially useful organic compounds , where it is used to sever carbon -carbon bonds. [ 29 ] It can also be used for bleaching substances and for killing microorganisms in air and water sources. [ 146 ] Many municipal drinking water systems kill bacteria with ozone instead of the more common chlorine . [ 147 ] Ozone has a very high oxidation potential . [ 148 ] Ozone does not form organochlorine compounds, nor does it remain in the water after treatment. Ozone can form the suspected carcinogen bromate in source water with high bromide concentrations. The U.S. Safe Drinking Water Act mandates that these systems introduce an amount of chlorine to maintain a minimum of 0.2 μmol/mol residual free chlorine in the pipes, based on results of regular testing. Where electrical power is abundant, ozone is a cost-effective method of treating water, since it is produced on demand and does not require transportation and storage of hazardous chemicals. Once it has decayed, it leaves no taste or odour in drinking water.
Although low levels of ozone have been advertised to be of some disinfectant use in residential homes, the concentration of ozone in dry air required to have a rapid, substantial effect on airborne pathogens exceeds safe levels recommended by the U.S. Occupational Safety and Health Administration and Environmental Protection Agency . Humidity control can vastly improve both the killing power of the ozone and the rate at which it decays back to oxygen (more humidity allows more effectiveness). Spore forms of most pathogens are very tolerant of atmospheric ozone in concentrations at which asthma patients start to have issues.
In 1908 artificial ozonisation of the Central Line of the London Underground was introduced for aerial disinfection. The process was found to be worthwhile, but was phased out by 1956. However the beneficial effect was maintained by the ozone created incidentally from the electrical discharges of the train motors (see above: Incidental production ). [ 149 ]
Ozone generators were made available to schools and universities in Wales for the Autumn term 2021, to disinfect classrooms after COVID-19 outbreaks. [ 150 ]
Industrially, ozone is used to:
Ozone is a reagent in many organic reactions in the laboratory and in industry. Ozonolysis is the cleavage of an alkene to carbonyl compounds.
Many hospitals around the world use large ozone generators to decontaminate operating rooms between surgeries. The rooms are cleaned and then sealed airtight before being filled with ozone which effectively kills or neutralizes all remaining bacteria. [ 156 ]
Ozone is used as an alternative to chlorine or chlorine dioxide in the bleaching of wood pulp . [ 157 ] It is often used in conjunction with oxygen and hydrogen peroxide to eliminate the need for chlorine-containing compounds in the manufacture of high-quality, white paper . [ 158 ]
Ozone can be used to detoxify cyanide wastes (for example from gold and silver mining ) by oxidizing cyanide to cyanate and eventually to carbon dioxide . [ 159 ]
Since the invention of dielectric barrier discharge (DBD) plasma reactors, it has been employed for water treatment with ozone. [ 160 ] However, with cheaper alternative disinfectants like chlorine, such applications of DBD ozone water decontamination have been limited by high power consumption and bulky equipment. [ 161 ] [ 162 ] Despite this, with research revealing the negative impacts of common disinfectants like chlorine with respect to toxic residuals and ineffectiveness in killing certain micro-organisms, [ 163 ] DBD plasma-based ozone decontamination is of interest in current available technologies. Although ozonation of water with a high concentration of bromide does lead to the formation of undesirable brominated disinfection byproducts, unless drinking water is produced by desalination, ozonation can generally be applied without concern for these byproducts. [ 162 ] [ 164 ] [ 165 ] [ 166 ] Advantages of ozone include high thermodynamic oxidation potential, less sensitivity to organic material and better tolerance for pH variations while retaining the ability to kill bacteria, fungi, viruses, as well as spores and cysts. [ 167 ] [ 168 ] [ 169 ] Although, ozone has been widely accepted in Europe for decades, it is sparingly used for decontamination in the U.S. due to limitations of high-power consumption, bulky installation and stigma attached with ozone toxicity. [ 161 ] [ 170 ] Considering this, recent research efforts have been directed toward the study of effective ozone water treatment systems. [ 171 ] Researchers have looked into lightweight and compact low power surface DBD reactors, [ 172 ] [ 173 ] energy efficient volume DBD reactors [ 174 ] and low power micro-scale DBD reactors. [ 175 ] [ 176 ] Such studies can help pave the path to re-acceptance of DBD plasma-based ozone decontamination of water, especially in the U.S.
Ozone levels which are safe for people are ineffective at killing fungi and bacteria. [ 177 ] Some consumer disinfection and cosmetic products emit ozone at levels harmful to human health. [ 177 ]
Devices generating high levels of ozone, some of which use ionization, are used to sanitize and deodorize uninhabited buildings, rooms, ductwork, woodsheds, boats, and other vehicles.
Ozonated water is used to launder clothes and to sanitize food, drinking water, and surfaces in the home. According to the U.S. Food and Drug Administration (FDA), it is "amending the food additive regulations to provide for the safe use of ozone in gaseous and aqueous phases as an antimicrobial agent on food, including meat and poultry." Studies at California Polytechnic University demonstrated that 0.3 μmol/mol levels of ozone dissolved in filtered tapwater can produce a reduction of more than 99.99% in such food-borne microorganisms as salmonella, E. coli 0157:H7 and Campylobacter . This quantity is 20,000 times the WHO -recommended limits stated above. [ 152 ] [ 178 ]
Ozone can be used to remove pesticide residues from fruits and vegetables . [ 179 ] [ 180 ]
Ozone is used in homes and hot tubs to kill bacteria in the water and to reduce the amount of chlorine or bromine required by reactivating them to their free state. Since ozone does not remain in the water long enough, ozone by itself is ineffective at preventing cross-contamination among bathers and must be used in conjunction with halogens . Gaseous ozone created by ultraviolet light or by corona discharge is injected into the water. [ 181 ]
Ozone is also widely used in the treatment of water in aquariums and fishponds. Its use can minimize bacterial growth, control parasites, eliminate transmission of some diseases, and reduce or eliminate "yellowing" of the water. Ozone must not come in contact with fishes' gill structures. Natural saltwater (with life forms) provides enough "instantaneous demand" that controlled amounts of ozone activate bromide ions to hypobromous acid , and the ozone entirely decays in a few seconds to minutes. If oxygen-fed ozone is used, the water will be higher in dissolved oxygen and fishes' gill structures will atrophy, making them dependent on oxygen-enriched water.
Ozonation – a process of infusing water with ozone – can be used in aquaculture to facilitate organic breakdown. Ozone is also added to recirculating systems to reduce nitrite levels [ 182 ] through conversion into nitrate . If nitrite levels in the water are high, nitrites will also accumulate in the blood and tissues of fish, where it interferes with oxygen transport (it causes oxidation of the heme-group of haemoglobin from ferrous ( Fe 2+ ) to ferric ( Fe 3+ ), making haemoglobin unable to bind O 2 ). [ 183 ] Despite these apparent positive effects, ozone use in recirculation systems has been linked to reducing the level of bioavailable iodine in salt water systems, resulting in iodine deficiency symptoms such as goitre and decreased growth in Senegalese sole ( Solea senegalensis ) larvae. [ 184 ]
Ozonate seawater is used for surface disinfection of haddock and Atlantic halibut eggs against nodavirus. Nodavirus is a lethal and vertically transmitted virus which causes severe mortality in fish. Haddock eggs should not be treated with high ozone level as eggs so treated did not hatch and died after 3–4 days. [ 185 ]
Ozone application on freshly cut pineapple and banana shows increase in flavonoids and total phenol contents when exposure is up to 20 minutes. Decrease in ascorbic acid (one form of vitamin C ) content is observed but the positive effect on total phenol content and flavonoids can overcome the negative effect. [ 186 ] Tomatoes upon treatment with ozone show an increase in β-carotene, lutein and lycopene. [ 187 ] However, ozone application on strawberries in pre-harvest period shows decrease in ascorbic acid content. [ 188 ]
Ozone facilitates the extraction of some heavy metals from soil using EDTA . EDTA forms strong, water-soluble coordination compounds with some heavy metals ( Pb and Zn ) thereby making it possible to dissolve them out from contaminated soil. If contaminated soil is pre-treated with ozone, the extraction efficacy of Pb , Am , and Pu increases by 11.0–28.9%, [ 189 ] 43.5% [ 190 ] and 50.7% [ 190 ] respectively.
Crop pollination is an essential part of an ecosystem. Ozone can have detrimental effects on plant-pollinator interactions. [ 191 ] Pollinators carry pollen from one plant to another. This is an essential cycle inside of an ecosystem. Causing changes in certain atmospheric conditions around pollination sites or with xenobiotics could cause unknown changes to the natural cycles of pollinators and flowering plants. In a study conducted in North-Western Europe, crop pollinators were negatively affected more when ozone levels were higher. [ 192 ]
The use of ozone for the treatment of medical conditions is not supported by high quality evidence, and is generally considered alternative medicine . [ 193 ]
Footnotes
Citations
Nascent oxygen O
Dioxygen ( singlet and triplet ) O 2
Trioxygen ( ozone and cyclic ozone ) O 3
Tetraoxygen O 4
Octaoxygen O 8 | https://en.wikipedia.org/wiki/Ozone |
Cracks can be formed in many different elastomers by ozone attack, and the characteristic form of attack of vulnerable rubbers is known as ozone cracking . The problem was formerly very common, especially in tires , but is now rarely seen in those products owing to preventive measures.
However, it does occur in many other safety-critical items such as fuel lines and rubber seals , such as gaskets and O-rings , where ozone attack is considered unlikely. Only a trace amount of the gas is needed to initiate cracking, and so these items can also succumb to the problem.
Tiny traces of ozone in the air will attack double bonds in rubber chains, with natural rubber , polybutadiene , styrene-butadiene rubber and nitrile rubber being most sensitive to degradation. [ 1 ] Every repeat unit in the first three materials has a double bond , so every unit can be degraded by ozone. Nitrile rubber is a copolymer of butadiene and acrylonitrile units, but the proportion of acrylonitrile is usually lower than butadiene, so attack occurs. Butyl rubber is more resistant but still has a small number of double bonds in its chains, so attack is possible. Exposed surfaces are attacked first, the density of cracks varying with ozone gas concentration. The higher the concentration, the greater the number of cracks formed.
Ozone-resistant elastomers include EPDM , fluoroelastomers like Viton and polychloroprene rubbers like Neoprene . Attack is less likely because double bonds form a very small proportion of the chains, and with the latter, the chlorination reduces the electron density in the double bonds, therefore lowering their propensity to react with ozone. Silicone rubber , Hypalon and polyurethanes are also ozone-resistant.
Ozone cracks form in products under tension, but the critical strain is very small. The cracks are always oriented at right angles to the strain axis, so will form around the circumference in a rubber tube bent over. Such cracks are very dangerous when they occur in fuel pipes because the cracks will grow from the outside exposed surfaces into the bore of the pipe, so fuel leakage and fire may follow.
Seals are also susceptible to attack, such as diaphragm seals in air lines. Such seals are often critical for the operation of pneumatic controls, and if a crack penetrates the seal, all functions of the system can be lost. Nitrile rubber seals are commonly used in pneumatic systems because of its oil resistance. However, if ozone gas is present, cracking will occur in the seals unless preventative measures are taken.
Ozone attack will occur at the most sensitive zones in a seal, especially sharp corners where the strain is greatest when the seal is flexing in use. The corners represent stress concentrations , so the tension is at a maximum when the diaphragm of the seal is bent under air pressure.
The seal shown at left failed from traces of ozone at circa 1 ppm , and once cracking had started, it continued as long as the gas was present. This particular failure led to loss of production on a semi-conductor fabrication line. The problem was solved by adding effective filters in the air line and by modifying the design to eliminate the very sharp corners. An ozone-resistant elastomer such as Viton was also considered as a replacement for the Nitrile rubber . The pictures were taken using ESEM for maximum resolution.
The reaction occurring between double bonds and ozone is known as ozonolysis when one molecule of the gas reacts with the double bond:
The immediate result is formation of an ozonide , which then decomposes rapidly so that the double bond is cleaved. This is the critical step in chain breakage when polymers are attacked. The strength of polymers depends on the chain molecular weight or degree of polymerization , the higher the chain length, the greater the mechanical strength (such as tensile strength ). By cleaving the chain, the molecular weight drops rapidly and there comes a point when it has little strength whatsoever, and a crack forms. Further attack occurs in the freshly exposed crack surfaces and the crack grows steadily until it completes a circuit and the product separates or fails. In the case of a seal or a tube, failure occurs when the wall of the device is penetrated.
The carbonyl end groups which are formed are usually aldehydes or ketones , which can oxidise further to carboxylic acids . The net result is a high concentration of elemental oxygen on the crack surfaces, which can be detected using energy-dispersive X-ray spectroscopy in the environmental SEM, or ESEM . The spectrum at left shows the high oxygen peak compared with a constant sulfur peak. The spectrum at right shows the unaffected elastomer surface spectrum, with a relatively low oxygen peak compared with the sulfur peak.
The problem can be prevented by adding antiozonants to the rubber before vulcanization . Ozone cracks were commonly seen in automobile tire sidewalls, but are now seen rarely thanks to the use of these additives. A common and low cost antiozonant is a wax which bleeds to the surface and forms a protective layer, but other specialist chemicals are also widely used.
On the other hand, the problem does recur in unprotected products such as rubber tubing and seals, where ozone attack is thought to be impossible. Unfortunately, traces of ozone can turn up in the most unexpected situations. Using ozone-resistant rubbers is another way of inhibiting cracking. EPDM rubber and butyl rubber are ozone resistant, for example.
For high value equipment where loss of function can cause serious problems, low cost seals may be replaced at frequent intervals so as to preclude failure.
Ozone gas is produced during electric discharge by sparking or corona discharge for example. Static electricity can build up within machines like compressors with moving parts constructed from insulating materials. If those compressors feed pressurised air into a closed pneumatic system, then all seals in the system may be at risk from ozone cracking.
Ozone is also produced by the action of sunlight on volatile organic compounds or VOCs, such as gasoline vapour present in the air of towns and cities, in a problem known as photochemical smog . The ozone formed can drift many miles before it is destroyed by further reactions. | https://en.wikipedia.org/wiki/Ozone_cracking |
An ozone monitor is electronic equipment that monitors for ozone concentrations in the air. The instrument may be used to monitor ozone values for industrial applications or to determine the amount of ambient ozone at ground level and determine whether these values violate National Ambient Air Quality Standards (NAAQS). Different types of ozone monitoring methods have been used throughout the decades, the two most notable and common methods being the Federal Reference Method and the Federal Equivalent Method.
The Federal Reference Method (FRM) was the original method of measuring ozone concentration in the air, being used throughout the United States around the 1970s and 1980s. It uses what is known as gas-phase ethylene- chemiluminescence or ET-CL. [ 1 ] The ozone content is measured based on the reaction when the air around the monitor reacts with the ethylene reactant gas within the monitor. As of 2015, the EPA added an additional format to the FRM using nitric oxide chemiluminescence or NO-CL. It functions in a very similar manner to that of the ET-CL format except it uses nitric oxide instead of ethylene gas. [ 2 ] The FRM has, for the most part, been phased due water vapor causing skewed results and has been replaced with the Federal Equivalent Method which uses ultraviolet absorption. However, the FRM it still used occasionally as the Federal Equivalent Method can be skewed by concentration of other pollutants in higher quantities such as mercury, sulfur dioxide, carbon dioxide, VOCs , and others. [ 2 ]
The Federal Equivalent Method (FEM) relies on the use of ultraviolet Absorption, more accurately, the ozone molecule absorbs ultraviolet radiation. [ 2 ] Most ozone monitors utilized in regulatory applications use ultraviolet absorption to accurately quantify ozone levels. An ozone monitor of this type operates by pulling an air sample from the atmosphere into the machine with an air pump. [ 3 ] During one cycle, the ozone monitor will take one air sample through the air inlet, and scrub the ozone from the air; for the next cycle, an air sample bypasses the scrubber and the ozone value calculated. The solenoid valve is electronically activated to shift the air flow either through the scrubber or to bypass it on a timed sequence. The difference between the two sampled values determines the actual ozone value at that time. The monitor may also have options to account for air pressure and air temperature to calculate the value of ozone.
The concentration of ozone is determined using the Beer-Lambert Law that basically says that the absorption of light is proportional to the concentration. For ozone, a 254 nanometer wavelength of light created by a mercury lamp is shined through a specific length of tubing with reflective mirrors. A photodiode at the other end of the tube detects the changes of brightness from the light.
The onboard electronics process the values obtained and display the value on the screen and can also output an electrical signal in volts or a 4-20 mA current that can be read by an electronic data logger . Other options for output are RS232 serial port or ethernet or internal data storage on flash memory . | https://en.wikipedia.org/wiki/Ozone_monitor |
The ozone– oxygen cycle is the process by which ozone is continually regenerated in Earth 's stratosphere , converting ultraviolet radiation (UV) into heat . In 1930 Sydney Chapman resolved the chemistry involved. The process is commonly called the Chapman cycle by atmospheric scientists.
Most of the ozone production occurs in the tropical upper stratosphere and mesosphere. The total mass of ozone produced per day over the globe is about 400 million metric tons. The global mass of ozone is relatively constant at about 3 billion metric tons, meaning the Sun produces about 12% of the ozone layer each day. [ 1 ]
The Chapman cycle describes the main reactions that naturally determine, to first approximation, the concentration of ozone in the stratosphere. It includes four processes - and a fifth, less important one - all involving oxygen atoms and molecules, and UV radiation: [ 2 ]
An oxygen molecule is split ( photolyzed ) by higher frequency UV light (top end of UV-B , UV-C and above) into two oxygen atoms (see figure):
Each oxygen atom may then combine with an oxygen molecule to form an ozone molecule:
The ozone molecules formed by the reaction (above) absorb radiation with an appropriate wavelength between UV-C and UV-B . The triatomic ozone molecule becomes diatomic molecular oxygen, plus a free oxygen atom (see figure):
The atomic oxygen produced may react with another oxygen molecule to reform ozone via the ozone creation reaction (reaction 2 above).
These two reactions thus form the ozone–oxygen cycle, wherein the chemical energy released by ozone creation becomes molecular kinetic energy. The net result of the cycle is the conversion of penetrating UV-B light into heat, without any net loss of ozone. While keeping the ozone layer in stable balance, and protecting the lower atmosphere from harmful UV radiation, the cycle also provides one of two major heat sources in the stratosphere (the other being kinetic energy, released when O 2 is photolyzed into individual O atoms).
If an oxygen atom and an ozone molecule meet, they recombine to form two oxygen molecules:
Two oxygen atoms may react to form one oxygen molecule:
Note that reaction 5 is of the least importance in the stratosphere, since, under normal conditions, the concentration of oxygen atoms is much lower than that of diatomic oxygen molecules. This reaction is therefore less common than ozone creation (reaction 2).
The overall amount of ozone in the stratosphere is determined by the balance between production from solar radiation and its removal. The removal rate is slow, since the concentration of free O atoms is very low.
In addition to these five reactions, certain free radicals - the most important being hydroxyl (OH), nitric oxide (NO), and atomic chlorine (Cl) and bromine (Br) - catalyze the recombination reaction , leading to an ozone layer that is thinner than it would be if the catalysts were not present.
Most OH and NO are naturally present in the stratosphere, but human activity - especially emissions of chlorofluorocarbons ( CFCs ) and halons - has greatly increased the concentration of Cl and Br, leading to ozone depletion . Each Cl or Br atom can catalyze tens of thousands of decomposition reactions before it is removed from the stratosphere.
For given relative reactants concentrations, The rates of ozone creation and oxygen recombination (reactions 2 and 5) are proportional to the air density cubed, while the rate of ozone conversion (reaction 4) is proportional to the air density squared, and the photodissociation reactions (reactions 1 and 3) have a linear dependence on air density. Thus, at the upper thermosphere, where air density is very low and photon flux is high, oxygen photodissociation is fast while ozone creation is low, thus its concentration is low. Thus the most important reactions are oxygen photodissociation and oxygen recombination, with most of the oxygen molecules dissociated to oxygen atoms. [ 3 ]
As we go to the lower thermosphere (e.g. 100 km height and below), the photon flux in the <170 nm wavelengths drops sharply due to absorption by oxygen in the oxygen photodissociation reaction (reaction 1). This wavelength regime has the highest cross section for this reaction (10 −17 cm 2 per oxygen molecule), and thus the rate of oxygen photodissociation per oxygen molecule decreases significantly at these altitudes, from more than 10 −7 per second (about once a month) at 100 km to 10 −8 per second (about once every few years) at 80 km . [ 4 ] As a result, the atomic oxygen concentration (both relative and absolute) decreases sharply, and ozone creation (reaction 2) is ongoing, leading to a small but non-negligible ozone presence. [ 5 ]
Note that temperatures also drop as altitude decreases, because lower photon photodissociation rates mean lower heat production per air molecule.
Odd oxygen species (atomic oxygen and ozone) have net creation rate only by oxygen dissociation (reaction 1), and net destruction by either ozone conversion or oxygen recombination (reactions 4 and 5). At steady state these processes are balanced, so the rates of these reactions obey:
At steady state, ozone creation is also balanced with its removal. so:
It thus follows that:
The right-hand side is the total photodissociation rate, of either oxygen or ozone.
Below the thermosphere, the atomic oxygen concentration is very low compared to molecular oxygen. [ 6 ] Therefore, oxygen atoms are much more likely to hit oxygen (diatomic) molecules than to hit other oxygen atoms, making oxygen recombination (reaction 5) far rarer than ozone creation (reaction 2). Following the steady-state relation between the reaction rates, we may therefore approximate: [ 7 ]
In the mesosphere, oxygen photodissociation dominates over ozone photodissociation, so we have approximately: [ 4 ]
Thus, ozone is mainly removed by ozone conversion. Both ozone creation and conversion depend linearly on oxygen atom concentration, but in ozone creation an oxygen atom must encounter an oxygen molecule and another air molecule (typically nitrogen) simultaneously, while in ozone conversion an oxygen atom must only encounter an ozone molecule. Thus, when both reactions are balanced, the ratio between ozone and molecular oxygen concentrations is approximately proportional to air density.
Therefore, the relative ozone concentration is higher at lower altitudes, where air density is higher. This trend continues to some extent lower into the stratosphere, and thus as we go from 60 km to 30 km altitude, both air density and ozone relative concentration increase by ~40-50-fold. [ 8 ] [ 9 ] [ 10 ]
Absorption by oxygen in the mesosphere and thermosphere (in the oxygen photodissociation reaction) reduces photon flux at wavelengths below 200 nanometer, where oxygen photodissociation is dominated by Schumann–Runge bands and continuum , with cross-section of up to 10 −17 cm 2 .
Due to this absorption, photon flux in these wavelengths is so low in the stratosphere, that oxygen photodissociation becomes dominated by the Hertzberg band of the 200-240 nm photon wavelength, even though the cross-section of this process is as low as 10 −24 - 10 −23 cm 2 . The ozone photodissociation rate per ozone molecule has a cross-section 6 orders of magnitude higher in the 220-300 nm wavelength range. With ozone concentrations in the order of 10 −6 -10 −5 relative to molecular oxygen, ozone photodissociation becomes the dominant photodissociation reaction, and most of the stratosphere heat is generated through this procsees, with highest heat generation rate per molecule at the upper limit of the stratosphere ( stratopause ), where ozone concentration is already relatively high while UV flux is still high as well in those wavelengths, before being depleted by this same photodissociation process.
In addition to ozone photodissociation becoming a more dominant removal reaction, catalytic ozone destruction due to free radicals (mainly atomic hydrogen , hydroxyl , nitric oxide , chlorine and bromide ) increases the effective ozone conversion reaction rate. Both processes act to increase ozone removal, leading to a more moderate increase of ozone relative concentration as altitude decreases, even though air density continues to increase. [ 4 ]
Due to both ozone and oxygen growing density as we go to lower altitudes, UV photon flux at wavelengths below 300 nm decreases substantially, and oxygen photodissociation rates fall below 10 −9 per second per molecule at 30 km. [ 4 ] With decreasing oxygen photodissociation rates, odd-oxygen species (atomic oxygen and ozone molecules) are hardly formed de novo (rather than being transmuted to each other by the other reactions), and most atomic oxygen needed for ozone creation is derived almost exclusively from ozone removal by ozone photodissociation. Thus, ozone becomes depleted as we go below 30 km altitude and reaches very low concentrations at the tropopause . [ 8 ]
In the troposphere, ozone formation and destruction are no longer controlled by the ozone-oxygen cycle. Rather, tropospheric ozone chemistry is dominated today by industrial pollutants other gases of volcanic source. [ 4 ] | https://en.wikipedia.org/wiki/Ozone–oxygen_cycle |
Ozonia is a multinational water treatment equipment manufacturer headquartered in Zürich , Switzerland .
It operates under the company, Degrémont , [ 2 ] which is a subsidiary of Suez Environnement , [ 3 ] a French-based utility company which operates largely in the water treatment and waste management sectors.
Ozonia specializes in manufacturing systems that deliver ozone , ultraviolet (UV) and advanced oxidation process (AOP) technologies in the municipal, industrial and leisure markets.
1990 Degrémont and Air Liquide acquired ABB's (formerly BBC's) ozone department. This was the genesis of Ozonia.
1991 The Ozonia brand was officially launched at the International Ozone Association (IOA) Congress in Monaco. As a partnership between Degrémont and Air Liquide, Ozonia offices are opened in Zürich and in Paris.
Also, Ozonia's first big installation (360 kg O 3 /h), a pulp and paper bleaching application, was installed for Union Camp in the US. Ozonia was the first to develop a technology based on an oxygen recovery loop, enabling it to recycle 80% of the process oxygen needs.
1992 Ozonia installed the first project using new ceramic "AT dielectrics ". The Contra Costa Water Treatment Plant in San Francisco (200 kg O 3 /h) illustrated the ability to produce higher concentration ozone while reducing oxygen consumption.
1993 Ozonia North America was founded after Ozonia AG acquired "Griffin". The new American subsidiary manufactured ozone generators compliant with the North American standards.
In the same year, to expand the Ozonia product line with small ozone and UV units, Triogen, based in Glasgow , joined Ozonia and the Degrémont group. In 2019, Triogen was sold to Bio-UV. [ 4 ]
1996 To satisfy the rapidly growing Asian market, Ozonia opened an office in Seongnam City, South Korea, after acquiring a majority stake in "CHAMP Ozone".
1997 Agreements between the Swiss and Russian governments promoted the use of ozone for water treatment throughout Russia. Ozonia Russia was created was created and opened its office in Nizhny Novgorod .
1998 As business continued to grow in Asia, Ozonia opened an office in Tokyo to cater to Japanese customers.
2000 Daegu City in South Korea (350 kg O 3 /h) becomes the first project in the world where ozone is used for the treatment of wastewater . The contract was celebrated at Ozonia's 10th anniversary.
2005 Degrémont bought out Air Liquide's ownership stake and became the sole shareholder of Ozonia. Ozonia expanded its presence to seven countries: Switzerland, France, United States, Great Britain, Russia, South Korea and Japan.
2005 The widespread use of ultraviolet (UV) technology became a focus for the group. Infilco Degrémont and Ozonia form a UV disinfection group to market the Infilco's Aquaray UV product line.
2006 Ozonia unveiled the MODIPAC, a technological breakthrough in ozone power supply. The MODIPAC increased power while reducing footprint and eliminating harmonics. [ 5 ]
2008 Ozonia launched an innovative new dielectric technology: the "Intelligent Gap System (IGS)". The IGS technology drastically improved the efficiency and energy consumption. As a result, Ozonia won the International Water Association's (IWA) Project Innovation Award.
2009 To expand its operations due to the growth in the leisure market, Triogen opened a new facility in Glasgow , Scotland .
2010 Ozonia opened a new "Process Applications" department. The department showcased Ozonia's knowledge in various applications including pulp bleaching , wastewater and aquaculture , with a goal to better educate clients.
Ozonia North America also opened a new facility in Leonia, New Jersey, to address the growing ozone and UV markets in North America.
2012 Ozonia opened a new production center based in Zürich, Dübendorf . The new facility increased production capacity and efficiency, and improved the quality and the ability to engineer project specific equipment.
Micropollutants: The "ARA Neugut" wastewater treatment plant in Zürich-Dübendorf became the first wastewater treatment plant to treat micropollutants in Switzerland. [ 6 ] The project represented a key step for future projects in Switzerland and around the world.
2013 Ozonia opened another new production center in Tianjin - Wuqing , China. The new center is a major milestone that allows Ozonia to design and manufactures ozone generators locally that address the Chinese market.
Ozonia manufactures ozone, ultraviolet (UV) and advanced oxidation process (AOP) technologies and systems for water treatment .
It is a well-known fact that ozone , a very strong oxidant, is used in water treatment disinfection.
It shows an extensive range of applications; drinking water , wastewater , water disinfection , color removal, micropollutants, pulp & paper bleaching, process water, ballast water , cooling towers , beverage and water bottling, aquaculture , aquariums (zoos), cyanide regeneration, food/ produce/ poultry, swimming pools, ultrapure water and many more. It is also used for the deodorization of waste air emitted from water plants and for the reduction of sludge from biological wastewater treatment ( sewage treatment ).
The technique that enables the ozone generators to produce ozone at an industrial scale consists of applying a corona discharge or dielectric barrier discharge to dry gas that contains oxygen. [ 7 ]
Water treatment using ozone is environmentally friendly especially compared to the chlorination disinfection method. This is due to the fact that the residual ozone molecules detach from oxygen molecules after the oxidation process. Thanks to this characteristic of ozone, there is no hazardous by-products produced afterwards.
Ultraviolet(UV) is an electromagnetic radiation found in sunlight.
UV disinfection is environmentally safe and recognized as highly effective on inactivating a wide range of pathogens , including viruses , bacteria and parasites . It is also known for its capability of successfully eliminating hazardous and environmentally unacceptable chemicals such as chlorine and other associated disinfection by-products. [ 8 ]
UV also covers a number of applications: drinking water, food and beverage, aquaculture, power generation, cooling water microelectronics , pharmaceutical, wastewater disinfection, reuse wastewater, combined sewer overflows (CSO) & sanitary sewer overflows (SSO) .
In the UV-C light spectrum (200-280 nm), the wavelength 254 nm has been proven to be the most efficient wavelength to inactivate micro-organisms by damaging the nucleic acids (DNA and RNA), which disrupts the organism's ability to replicate.
Main advantages of UV disinfection are such as short contact times for pathogens to be inactivated and effectiveness on protozoa cysts and other chlorine resistant organisms. UV has another advantage over other disinfectants because no chemicals are added to the water being treated and that no disinfection by-products are formed. These advantages make UV disinfection a top choice for the disinfection of municipal and industrial wastewater and drinking water. Due to UV systems' small footprint, the UV equipment can be easily integrated into most existing treatment plants.
Advanced oxidation processes (AOP) are chemical processes used for treatment of water and wastewater. AOPs further improve chemical oxidation processes by combining three proven treatment technologies – ozone, UV and hydrogen peroxide – to create in-situ highly reactive hydroxyl radicals ( . OH) for elimination of organic pollutants. [ 9 ]
AOPs are aqueous phase oxidation methods consisting of highly reactive species used in the oxidative destruction of target pollutants. AOP creates a more powerful and less selective secondary oxidant, hydroxyl radicals, in the water. This secondary oxidant can cause the oxidation of most organic compounds until they are fully mineralized as carbon dioxide and water. The hydroxyl radical has a much higher oxidation potential than ozone or hydrogen peroxide and usually reacts at least one million times faster, thus leading to a smaller contact time and footprint.
This powerful AOP disinfection method can be applied to micropollutants removal, organic pollutants removal, drinking water treatment, municipal and industrial wastewater treatment , groundwater remediation and many more. Due to industrial and demographic pressures, the use of AOP in water treatment is expanding for a number of critical uses including the oxidation or removal of industrial chemicals, pharmaceuticals, endocrine disrupting compounds (EDC's) , personal care products (PCP's) , pesticides , toxic compounds, pathogens, persistent organic matter , and odor, color and taste.
Ozonia operates under the company, Degrémont, whose mother company is Suez Environnement.
Suez Environnement S.A. ( Euronext : SEV) is a French-based utility company which operates largely in the water treatment and waste management sectors. Formerly an operating division of Suez , the company was spun out as a stand-alone entity as part of the merger to form GDF Suez on 22 July 2008. Suez Environnement shares are listed on the Euronext exchanges in Paris and Brussels. The company has its head office in the 8th arrondissement of Paris .
As of 2012 Suez Environnement employs 79,549 people worldwide with revenues of €15.1 billion.
Degrémont is a company specializing in the production of drinking water , and in the treatment of sewage and sludge . After starting as a family business in France in 1939, it has since become a subsidiary of Suez Environnement .
As of 2012 Degrémont employs 4,600 people in 70 countries and generates annual revenues of €1.4 billion. | https://en.wikipedia.org/wiki/Ozonia |
Ozonide is the polyatomic anion O − 3 . Cyclic organic compounds formed by the addition of ozone ( O 3 ) to an alkene are also called ozonides.
Inorganic ozonides [ 1 ] are dark red salts. The anion has the bent shape of the ozone molecule.
Inorganic ozonides are formed by burning potassium , rubidium , or caesium in ozone, or by treating the alkali metal hydroxide with ozone; this yields potassium ozonide , rubidium ozonide , and caesium ozonide respectively. They are very sensitive explosives that have to be handled at low temperatures in an inert gas atmosphere. Lithium and sodium ozonide are extremely labile and must be prepared by low-temperature ion exchange starting from CsO 3 . Sodium ozonide , NaO 3 , which is prone to decomposition into NaOH and NaO 2 , was previously thought to be impossible to obtain in pure form. [ 2 ] However, with the help of cryptands and methylamine , pure sodium ozonide may be obtained as red crystals isostructural to NaNO 2 . [ 3 ]
Ionic ozonides are being investigated [ citation needed ] as sources of oxygen in chemical oxygen generators . Tetramethylammonium ozonide, which can be made by a metathesis reaction with caesium ozonide in liquid ammonia , is stable up to 348 K (75 °C): [ 4 ]
Alkaline earth metal ozonide compounds have also become known. For instance, magnesium ozonide complexes have been isolated in a low-temperature argon matrix. [ 5 ]
Phosphite ozonides, (RO) 3 PO 3 , are used in the production of singlet oxygen . They are made by ozonizing a phosphite ester in dichloromethane at low temperatures, and decompose to yield singlet oxygen and a phosphate ester : [ 6 ] [ 7 ]
Molozonides are formed by the addition reaction between ozone and alkenes . They are rarely isolated during the course of the ozonolysis reaction sequence. Molozonides are unstable and rapidly convert to the trioxolane ring structure with a five-membered C–O–O–C–O ring. [ 8 ] [ 9 ] They usually appear in the form of foul-smelling oily liquids, and rapidly decompose in the presence of water to carbonyl compounds: aldehydes , ketones , peroxides . | https://en.wikipedia.org/wiki/Ozonide |
The Ozsváth–Schücking metric , or the Ozsváth–Schücking solution , is a vacuum solution of the Einstein field equations . The metric was published by István Ozsváth and Engelbert Schücking in 1962. [ 1 ] It is noteworthy among vacuum solutions for being the first known solution that is stationary , globally defined, and singularity-free but nevertheless not isometric to the Minkowski metric . This stands in contradiction to a claimed strong Mach principle, which would forbid a vacuum solution from being anything but Minkowski without singularities, where the singularities are to be construed as mass as in the Schwarzschild metric . [ 2 ]
With coordinates { x 0 , x 1 , x 2 , x 3 } {\displaystyle \{x^{0},x^{1},x^{2},x^{3}\}} , define the following tetrad :
It is straightforward to verify that e (0) is timelike, e (1) , e (2) , e (3) are spacelike, that they are all orthogonal , and that there are no singularities. The corresponding proper time is
The Riemann tensor has only one algebraically independent, nonzero component
which shows that the spacetime is Ricci flat but not conformally flat . That is sufficient to conclude that it is a vacuum solution distinct from Minkowski spacetime. Under a suitable coordinate transformation, the metric can be rewritten as
and is therefore an example of a pp-wave spacetime . | https://en.wikipedia.org/wiki/Ozsváth–Schücking_metric |
Formaldehyde ( / f ɔːr ˈ m æ l d ɪ h aɪ d / ⓘ for- MAL -di-hide , US also / f ə r -/ ⓘ fər- ) ( systematic name methanal ) is an organic compound with the chemical formula CH 2 O and structure H−CHO , more precisely H 2 C=O . The compound is a pungent, colourless gas that polymerises spontaneously into paraformaldehyde . It is stored as aqueous solutions ( formalin ), which consists mainly of the hydrate CH 2 (OH) 2 . It is the simplest of the aldehydes ( R−CHO ). As a precursor to many other materials and chemical compounds, in 2006 the global production of formaldehyde was estimated at 12 million tons per year. [ 14 ] It is mainly used in the production of industrial resins , e.g., for particle board and coatings .
Formaldehyde also occurs naturally. It is derived from the degradation of serine , dimethylglycine , and lipids . Demethylases act by converting N-methyl groups to formaldehyde. [ 15 ]
Formaldehyde is classified as a group 1 carcinogen [ note 1 ] [ 17 ] and can cause respiratory and skin irritation upon exposure. [ 16 ]
Formaldehyde is more complicated than many simple carbon compounds in that it adopts several diverse forms. These compounds can often be used interchangeably and can be interconverted. [ citation needed ]
A small amount of stabilizer , such as methanol , is usually added to suppress oxidation and polymerization . A typical commercial-grade formalin may contain 10–12% methanol in addition to various metallic impurities.
"Formaldehyde" was first used as a generic trademark in 1893 following a previous trade name, "formalin". [ 18 ]
Molecular formaldehyde contains a central carbon atom with a double bond to the oxygen atom and a single bond to each hydrogen atom . This structure is summarised by the condensed formula H 2 C=O. [ 19 ] The molecule is planar, Y-shaped and its molecular symmetry belongs to the C 2v point group . [ 20 ] The precise molecular geometry of gaseous formaldehyde has been determined by gas electron diffraction [ 19 ] [ 21 ] and microwave spectroscopy . [ 22 ] [ 23 ] The bond lengths are 1.21 Å for the carbon–oxygen bond [ 19 ] [ 21 ] [ 22 ] [ 23 ] [ 24 ] and around 1.11 Å for the carbon–hydrogen bond , [ 19 ] [ 21 ] [ 22 ] [ 23 ] while the H–C–H bond angle is 117°, [ 22 ] [ 23 ] close to the 120° angle found in an ideal trigonal planar molecule . [ 19 ] Some excited electronic states of formaldehyde are pyramidal rather than planar as in the ground state . [ 24 ]
Processes in the upper atmosphere contribute more than 80% of the total formaldehyde in the environment. [ 25 ] Formaldehyde is an intermediate in the oxidation (or combustion ) of methane , as well as of other carbon compounds, e.g. in forest fires , automobile exhaust, and tobacco smoke . When produced in the atmosphere by the action of sunlight and oxygen on atmospheric methane and other hydrocarbons , it becomes part of smog . Formaldehyde has also been detected in outer space.
Formaldehyde and its adducts are ubiquitous in nature. Food may contain formaldehyde at levels 1–100 mg/kg. [ 26 ] Formaldehyde, formed in the metabolism of the amino acids serine and threonine , is found in the bloodstream of humans and other primates at concentrations of approximately 50 micromolar . [ 27 ] Experiments in which animals are exposed to an atmosphere containing isotopically labeled formaldehyde have demonstrated that even in deliberately exposed animals, the majority of formaldehyde-DNA adducts found in non-respiratory tissues are derived from endogenously produced formaldehyde. [ 28 ]
Formaldehyde does not accumulate in the environment, because it is broken down within a few hours by sunlight or by bacteria present in soil or water. Humans metabolize formaldehyde quickly, converting it to formic acid . [ 29 ] [ 30 ] It nonetheless presents significant health concerns , as a contaminant .
Formaldehyde appears to be a useful probe in astrochemistry due to prominence of the 1 10 ←1 11 and 2 11 ←2 12 K -doublet transitions. It was the first polyatomic organic molecule detected in the interstellar medium . [ 31 ] Since its initial detection in 1969, it has been observed in many regions of the galaxy . Because of the widespread interest in interstellar formaldehyde, it has been extensively studied, yielding new extragalactic sources. [ 32 ] A proposed mechanism for the formation is the hydrogenation of CO ice: [ 33 ]
HCN , HNC , H 2 CO, and dust have also been observed inside the comae of comets C/2012 F6 (Lemmon) and C/2012 S1 (ISON) . [ 34 ] [ 35 ]
Formaldehyde was discovered in 1859 by the Russian chemist Aleksandr Butlerov (1828–1886) when he attempted to synthesize methanediol ("methylene glycol") from iodomethane and silver oxalate . [ 36 ] In his paper, Butlerov referred to formaldehyde as "dioxymethylen" (methylene dioxide) because his empirical formula for it was incorrect, as atomic weights were not precisely determined until the Karlsruhe Congress .
The compound was identified as an aldehyde by August Wilhelm von Hofmann , who first announced its production by passing methanol vapor in air over hot platinum wire. [ 37 ] [ 38 ] With modifications, Hofmann's method remains the basis of the present day industrial route.
Solution routes to formaldehyde also entail oxidation of methanol or iodomethane . [ 39 ]
Formaldehyde is produced industrially by the catalytic oxidation of methanol . The most common catalysts are silver metal (i.e. the FASIL process ), iron(III) oxide , [ 40 ] iron molybdenum oxides (e.g. iron(III) molybdate ) with a molybdenum -enriched surface, [ 41 ] or vanadium oxides . In the commonly used formox process , methanol and oxygen react at c. 250–400 °C in presence of iron oxide in combination with molybdenum and/or vanadium to produce formaldehyde according to the chemical equation : [ 42 ]
The silver-based catalyst usually operates at a higher temperature, about 650 °C. Two chemical reactions on it simultaneously produce formaldehyde: that shown above and the dehydrogenation reaction:
In principle, formaldehyde could be generated by oxidation of methane , but this route is not industrially viable because the methanol is more easily oxidized than methane. [ 42 ]
Formaldehyde is produced via several enzyme-catalyzed routes. [ 43 ] Living beings, including humans, produce formaldehyde as part of their metabolism. Formaldehyde is key to several bodily functions (e.g. epigenetics [ 27 ] ), but its amount must also be tightly controlled to avoid self-poisoning. [ 44 ]
Formaldehyde is catabolized by alcohol dehydrogenase ADH5 and aldehyde dehydrogenase ALDH2 . [ 45 ]
Formaldehyde is a building block in the synthesis of many other compounds of specialised and industrial significance. It exhibits most of the chemical properties of other aldehydes but is more reactive. [ 46 ]
Monomeric CH 2 O is a gas and is rarely encountered in the laboratory. Aqueous formaldehyde, unlike some other small aldehydes (which need specific conditions to oligomerize through aldol condensation ) oligomerizes spontaneously at a common state. The trimer 1,3,5-trioxane, (CH 2 O) 3 , is a typical oligomer. Many cyclic oligomers of other sizes have been isolated. Similarly, formaldehyde hydrates to give the geminal diol methanediol , which condenses further to form hydroxy-terminated oligomers HO(CH 2 O) n H. The polymer is called paraformaldehyde . The higher concentration of formaldehyde—the more equilibrium shifts towards polymerization. Diluting with water or increasing the solution temperature, as well as adding alcohols (such as methanol or ethanol) lowers that tendency.
Gaseous formaldehyde polymerizes at active sites on vessel walls, but the mechanism of the reaction is unknown. [ 47 ] Small amounts of hydrogen chloride , boron trifluoride , or stannic chloride present in gaseous formaldehyde provide the catalytic effect and make the polymerization rapid. [ 48 ]
Formaldehyde forms cross-links by first combining with a protein to form methylol , which loses a water molecule to form a Schiff base . [ 49 ] The Schiff base can then react with DNA or protein to create a cross-linked product. [ 49 ] This reaction is the basis for the most common process of chemical fixation .
Formaldehyde is readily oxidized by atmospheric oxygen into formic acid . For this reason, commercial formaldehyde is typically contaminated with formic acid. Formaldehyde can be hydrogenated into methanol .
In the Cannizzaro reaction , formaldehyde and base react to produce formic acid and methanol, a disproportionation reaction .
Formaldehyde reacts with many compounds, resulting in hydroxymethylation :
The resulting hydroxymethyl derivatives typically react further. Thus, amines give hexahydro-1,3,5-triazines :
Similarly, when combined with hydrogen sulfide , it forms trithiane : [ 50 ]
In the presence of acids, it participates in electrophilic aromatic substitution reactions with aromatic compounds resulting in hydroxymethylated derivatives:
When conducted in the presence of hydrogen chloride, the product is the chloromethyl compound, as described in the Blanc chloromethylation . If the arene is electron-rich, as in phenols, elaborate condensations ensue. With 4-substituted phenols one obtains calixarenes . [ 51 ] Phenol results in polymers.
Many amino acids react with formaldehyde. [ 43 ] Cysteine converts to thioproline .
Formaldehyde is a common precursor to more complex compounds and materials. In approximate order of decreasing consumption, products generated from formaldehyde include urea formaldehyde resin , melamine resin , phenol formaldehyde resin , polyoxymethylene plastics , 1,4-butanediol , and methylene diphenyl diisocyanate . [ 42 ] The textile industry uses formaldehyde-based resins as finishers to make fabrics crease-resistant. [ 52 ]
When condensed with phenol , urea , or melamine , formaldehyde produces, respectively, hard thermoset phenol formaldehyde resin, urea formaldehyde resin, and melamine resin. These polymers are permanent adhesives used in plywood and carpeting . They are also foamed to make insulation , or cast into moulded products. Production of formaldehyde resins accounts for more than half of formaldehyde consumption.
Formaldehyde is also a precursor to polyfunctional alcohols such as pentaerythritol , which is used to make paints and explosives . Other formaldehyde derivatives include methylene diphenyl diisocyanate, an important component in polyurethane paints and foams, and hexamine , which is used in phenol-formaldehyde resins as well as the explosive RDX .
Condensation with acetaldehyde affords pentaerythritol , a chemical necessary in synthesizing PETN , a high explosive: [ 53 ]
An aqueous solution of formaldehyde can be useful as a disinfectant as it kills most bacteria and fungi (including their spores). It is used as an additive in vaccine manufacturing to inactivate toxins and pathogens. [ 54 ] Formaldehyde releasers are used as biocides in personal care products such as cosmetics. Although present at levels not normally considered harmful, they are known to cause allergic contact dermatitis in certain sensitised individuals. [ 55 ]
Aquarists use formaldehyde as a treatment for the parasites Ichthyophthirius multifiliis and Cryptocaryon irritans . [ 56 ] Formaldehyde is one of the main disinfectants recommended for destroying anthrax . [ 57 ]
Formaldehyde is also approved for use in the manufacture of animal feeds in the US. It is an antimicrobial agent used to maintain complete animal feeds or feed ingredients Salmonella negative for up to 21 days. [ 58 ]
Formaldehyde preserves or fixes tissue or cells. The process involves cross-linking of primary amino groups . The European Union has banned the use of formaldehyde as a biocide (including embalming ) under the Biocidal Products Directive (98/8/EC) due to its carcinogenic properties. [ 59 ] [ 60 ] Countries with a strong tradition of embalming corpses, such as Ireland and other colder-weather countries, have raised concerns. Despite reports to the contrary, [ 61 ] no decision on the inclusion of formaldehyde on Annex I of the Biocidal Products Directive for product-type 22 (embalming and taxidermist fluids) had been made as of September 2009 [update] . [ 62 ]
Formaldehyde-based crosslinking is exploited in ChIP-on-chip or ChIP-sequencing genomics experiments, where DNA-binding proteins are cross-linked to their cognate binding sites on the chromosome and analyzed to determine what genes are regulated by the proteins. Formaldehyde is also used as a denaturing agent in RNA gel electrophoresis , preventing RNA from forming secondary structures. A solution of 4% formaldehyde fixes pathology tissue specimens at about one mm per hour at room temperature.
Formaldehyde and 18 M (concentrated) sulfuric acid makes Marquis reagent —which can identify alkaloids and other compounds.
In photography, formaldehyde is used in low concentrations for the process C-41 (color negative film) stabilizer in the final wash step, [ 63 ] as well as in the process E-6 pre-bleach step, to make it unnecessary in the final wash. Due to improvements in dye coupler chemistry, more modern (2006 or later) E-6 and C-41 films do not need formaldehyde, as their dyes are already stable.
In view of its widespread use, toxicity, and volatility, formaldehyde poses a significant danger to human health. [ 64 ] [ 65 ] In 2011, the US National Toxicology Program described formaldehyde as "known to be a human carcinogen". [ 66 ] [ 67 ] [ 68 ]
Concerns are associated with chronic (long-term) exposure by inhalation as may happen from thermal or chemical decomposition of formaldehyde-based resins and the production of formaldehyde resulting from the combustion of a variety of organic compounds (for example, exhaust gases). As formaldehyde resins are used in many construction materials , it is one of the more common indoor air pollutants . [ 69 ] [ 70 ] At concentrations above 0.1 ppm in air, formaldehyde can irritate the eyes and mucous membranes . [ 71 ] Formaldehyde inhaled at this concentration may cause headaches, a burning sensation in the throat, and difficulty breathing, and can trigger or aggravate asthma symptoms. [ 72 ] [ 73 ]
The CDC considers formaldehyde as a systemic poison. Formaldehyde poisoning can cause permanent changes in the nervous system 's functions. [ 74 ]
A 1988 Canadian study of houses with urea-formaldehyde foam insulation found that formaldehyde levels as low as 0.046 ppm were positively correlated with eye and nasal irritation. [ 75 ] A 2009 review of studies has shown a strong association between exposure to formaldehyde and the development of childhood asthma . [ 76 ]
A theory was proposed for the carcinogenesis of formaldehyde in 1978. [ 77 ] In 1987 the United States Environmental Protection Agency (EPA) classified it as a probable human carcinogen , and after more studies the WHO International Agency for Research on Cancer (IARC) in 1995 also classified it as a probable human carcinogen . Further information and evaluation of all known data led the IARC to reclassify formaldehyde as a known human carcinogen [ 78 ] associated with nasal sinus cancer and nasopharyngeal cancer . [ 79 ] Studies in 2009 and 2010 have also shown a positive correlation between exposure to formaldehyde and the development of leukemia , particularly myeloid leukemia . [ 80 ] [ 81 ] Nasopharyngeal and sinonasal cancers are relatively rare, with a combined annual incidence in the United States of < 4,000 cases. [ 82 ] [ 83 ] About 30,000 cases of myeloid leukemia occur in the United States each year. [ 84 ] [ 85 ] Some evidence suggests that workplace exposure to formaldehyde contributes to sinonasal cancers. [ 86 ] Professionals exposed to formaldehyde in their occupation, such as funeral industry workers and embalmers , showed an increased risk of leukemia and brain cancer compared with the general population. [ 87 ] Other factors are important in determining individual risk for the development of leukemia or nasopharyngeal cancer. [ 86 ] [ 88 ] [ 89 ] In yeast, formaldehyde is found to perturb pathways for DNA repair and cell cycle. [ 90 ]
In the residential environment, formaldehyde exposure comes from a number of routes; formaldehyde can be emitted by treated wood products, such as plywood or particle board , but it is produced by paints, varnishes , floor finishes, and cigarette smoking as well. [ 91 ] In July 2016, the U.S. EPA released a prepublication version of its final rule on Formaldehyde Emission Standards for Composite Wood Products. [ 92 ] These new rules impact manufacturers, importers, distributors, and retailers of products containing composite wood, including fiberboard, particleboard, and various laminated products, who must comply with more stringent record-keeping and labeling requirements. [ 93 ]
The U.S. EPA allows no more than 0.016 ppm formaldehyde in the air in new buildings constructed for that agency. [ 94 ] [ failed verification ] A U.S. EPA study found a new home measured 0.076 ppm when brand new and 0.045 ppm after 30 days. [ 95 ] The Federal Emergency Management Agency (FEMA) has also announced limits on the formaldehyde levels in trailers purchased by that agency. [ 96 ] The EPA recommends the use of "exterior-grade" pressed-wood products with phenol instead of urea resin to limit formaldehyde exposure, since pressed-wood products containing formaldehyde resins are often a significant source of formaldehyde in homes. [ 79 ]
The eyes are most sensitive to formaldehyde exposure: The lowest level at which many people can begin to smell formaldehyde ranges between 0.05 and 1 ppm. The maximum concentration value at the workplace is 0.3 ppm. [ 97 ] [ need quotation to verify ] In controlled chamber studies, individuals begin to sense eye irritation at about 0.5 ppm; 5 to 20 percent report eye irritation at 0.5 to 1 ppm; and greater certainty for sensory irritation occurred at 1 ppm and above. While some agencies have used a level as low as 0.1 ppm as a threshold for irritation, the expert panel found that a level of 0.3 ppm would protect against nearly all irritation. In fact, the expert panel found that a level of 1.0 ppm would avoid eye irritation—the most sensitive endpoint—in 75–95% of all people exposed. [ 98 ]
Formaldehyde levels in building environments are affected by a number of factors. These include the potency of formaldehyde-emitting products present, the ratio of the surface area of emitting materials to volume of space, environmental factors, product age, interactions with other materials, and ventilation conditions. Formaldehyde emits from a variety of construction materials, furnishings, and consumer products. The three products that emit the highest concentrations are medium density fiberboard , hardwood plywood, and particle board. Environmental factors such as temperature and relative humidity can elevate levels because formaldehyde has a high vapor pressure . Formaldehyde levels from building materials are the highest when a building first opens because materials would have less time to off-gas. Formaldehyde levels decrease over time as the sources suppress.
In operating rooms , formaldehyde is produced as a byproduct of electrosurgery and is present in surgical smoke, exposing surgeons and healthcare workers to potentially unsafe concentrations. [ 99 ]
Formaldehyde levels in air can be sampled and tested in several ways, including impinger, treated sorbent, and passive monitors. [ 100 ] The National Institute for Occupational Safety and Health (NIOSH) has measurement methods numbered 2016, 2541, 3500, and 3800. [ 101 ]
In June 2011, the twelfth edition of the National Toxicology Program (NTP) Report on Carcinogens (RoC) changed the listing status of formaldehyde from "reasonably anticipated to be a human carcinogen" to "known to be a human carcinogen." [ 66 ] [ 67 ] [ 68 ] Concurrently, a National Academy of Sciences (NAS) committee was convened and issued an independent review of the draft U.S. EPA IRIS assessment of formaldehyde, providing a comprehensive health effects assessment and quantitative estimates of human risks of adverse effects. [ 102 ]
For most people, irritation from formaldehyde is temporary and reversible, although formaldehyde can cause allergies and is part of the standard patch test series. In 2005–06, it was the seventh-most-prevalent allergen in patch tests (9.0%). [ 103 ] People with formaldehyde allergy are advised to avoid formaldehyde releasers as well (e.g., Quaternium-15 , imidazolidinyl urea , and diazolidinyl urea ). [ 104 ] People who suffer allergic reactions to formaldehyde tend to display lesions on the skin in the areas that have had direct contact with the substance, such as the neck or thighs (often due to formaldehyde released from permanent press finished clothing) or dermatitis on the face (typically from cosmetics). [ 55 ] Formaldehyde has been banned in cosmetics in both Sweden [ 105 ] and Japan . [ 106 ]
In humans, ingestion of as little as 30 millilitres (1.0 US fl oz) of 37% formaldehyde solution can cause death. Other symptoms associated with ingesting such a solution include gastrointestinal damage (vomiting, abdominal pain), and systematic damage (dizziness). [ 74 ] Testing for formaldehyde is by blood and/or urine by gas chromatography–mass spectrometry . Other methods to detect formaldehyde include infrared detection, gas detector tubes, gas detectors using electrochemical sensors, and high-performance liquid chromatography (HPLC). HPLC is the most sensitive. [ 107 ]
The fifteenth edition (2021) of the U.S. National Toxicology Program Report on Carcinogens notes that currently in the U.S. “The general population can be exposed to formaldehyde primarily from breathing indoor or outdoor air, from tobacco smoke, from use of cosmetic products containing formaldehyde, and, to a more limited extent, from ingestion of food and water.” Affected water includes groundwater, surface water, and bottled water. It also notes that occupational exposure can be significant. [ 108 ]
Formaldehyde in food can be present naturally, added as an inadvertent contaminant, or intentionally added as a preservative, disinfectant, or bacteriostatic agent . Cooking and smoking food can also result in formaldehyde being produced in food. Foods that the U.S. National Toxicology Program has reported to have higher levels compared to other foods are fish, seafood, and smoked ham. It also notes formaldehyde in food generally occurs in a bound form and that formaldehyde is unstable in an aqueous solution . [ 109 ]
Scandals have broken in both the 2005 Indonesia food scare and 2007 Vietnam food scare regarding the addition of formaldehyde to foods to extend shelf life. In 2011, after a four-year absence, Indonesian authorities found foods with formaldehyde being sold in markets in a number of regions across the country. [ 110 ] In August 2011, at least at two Carrefour supermarkets, the Central Jakarta Livestock and Fishery Sub-Department found cendol containing 10 parts per million of formaldehyde. [ 111 ] In 2014, the owner of two noodle factories in Bogor , Indonesia, was arrested for using formaldehyde in noodles. [ 112 ] Foods known to be contaminated included noodles, salted fish, and tofu. Chicken and beer were also rumored to be contaminated. In some places, such as China, manufacturers still use formaldehyde illegally as a preservative in foods, which exposes people to formaldehyde ingestion. [ 113 ]
In 2011 in Nakhon Ratchasima , Thailand, truckloads of rotten chicken were treated with formaldehyde for sale in which "a large network", including 11 slaughterhouses run by a criminal gang, were implicated. [ 114 ] In 2012, 1 billion rupiah (almost US$100,000) of fish imported from Pakistan to Batam , Indonesia, were found laced with formaldehyde. [ 115 ]
Formalin contamination of foods has been reported in Bangladesh , with stores and supermarkets selling fruits, fishes, and vegetables that have been treated with formalin to keep them fresh. [ 116 ] However, in 2015, a Formalin Control Bill was passed in the Parliament of Bangladesh with a provision of life-term imprisonment as the maximum punishment as well as a maximum fine of 2,000,000 BDT but not less than 500,000 BDT for importing, producing, or hoarding formalin without a license. [ 117 ]
In the early 1900s, formaldehyde was frequently added by US milk plants to milk bottles as a method of pasteurization due to the lack of knowledge and concern [ 118 ] regarding formaldehyde's toxicity. [ 119 ] [ 120 ]
Formaldehyde was one of the chemicals used in 19th century industrialised food production that was investigated by Dr. Harvey W. Wiley with his famous 'Poison Squad' as part of the US Department of Agriculture . This led to the 1906 Pure Food and Drug Act , a landmark event in the early history of food regulation in the United States . [ 121 ]
Formaldehyde is banned from use in certain applications (preservatives for liquid-cooling and processing systems, slimicides , metalworking-fluid preservatives, and antifouling products) under the Biocidal Products Directive. [ 122 ] [ 123 ] In the EU, the maximum allowed concentration of formaldehyde in finished products is 0.2%, and any product that exceeds 0.05% has to include a warning that the product contains formaldehyde. [ 55 ]
In the United States, Congress passed a bill July 7, 2010, regarding the use of formaldehyde in hardwood plywood , particle board , and medium density fiberboard . The bill limited the allowable amount of formaldehyde emissions from these wood products to 0.09 ppm, and required companies to meet this standard by January 2013. [ 124 ] The final U.S. EPA rule specified maximum emissions of "0.05 ppm formaldehyde for hardwood plywood, 0.09 ppm formaldehyde for particleboard, 0.11 ppm formaldehyde for medium-density fiberboard, and 0.13 ppm formaldehyde for thin medium-density fiberboard." [ 125 ]
Formaldehyde was declared a toxic substance by the 1999 Canadian Environmental Protection Act . [ 126 ]
The FDA is proposing a ban on hair relaxers with formaldehyde due to cancer concerns. [ 127 ] | https://en.wikipedia.org/wiki/O⚌CH2 |
Dinitrogen tetroxide Dinitrogen trioxide Nitric oxide Nitrous oxide
Nitrogen dioxide is a chemical compound with the formula NO 2 . One of several nitrogen oxides , nitrogen dioxide is a reddish-brown gas. It is a paramagnetic , bent molecule with C 2v point group symmetry . Industrially, NO 2 is an intermediate in the synthesis of nitric acid , millions of tons of which are produced each year, primarily for the production of fertilizers .
Nitrogen dioxide is poisonous and can be fatal if inhaled in large quantities. [ 8 ] Cooking with a gas stove produces nitrogen dioxide which causes poorer indoor air quality . Combustion of gas can lead to increased concentrations of nitrogen dioxide throughout the home environment which is linked to respiratory issues and diseases . [ 9 ] [ 10 ] The LC 50 ( median lethal dose ) for humans has been estimated to be 174 ppm for a 1-hour exposure. [ 11 ] It is also included in the NO x family of atmospheric pollutants .
Nitrogen dioxide is a reddish-brown gas with a pungent, acrid odor above 21.2 °C (70.2 °F; 294.3 K) and becomes a yellowish-brown liquid below 21.2 °C (70.2 °F; 294.3 K). It forms an equilibrium with its dimer , dinitrogen tetroxide ( N 2 O 4 ), and converts almost entirely to N 2 O 4 below −11.2 °C (11.8 °F; 261.9 K). [ 6 ]
The bond length between the nitrogen atom and the oxygen atom is 119.7 pm . This bond length is consistent with a bond order between one and two.
Unlike ozone ( O 3 ) the ground electronic state of nitrogen dioxide is a doublet state , since nitrogen has one unpaired electron, [ 12 ] which decreases the alpha effect compared with nitrite and creates a weak bonding interaction with the oxygen lone pairs. The lone electron in NO 2 also means that this compound is a free radical , so the formula for nitrogen dioxide is often written as • NO 2 .
The reddish-brown color is a consequence of preferential absorption of light in the blue region of the spectrum (400–500 nm), although the absorption extends throughout the visible (at shorter wavelengths) and into the infrared (at longer wavelengths). Absorption of light at wavelengths shorter than about 400 nm results in photolysis (to form NO + O , atomic oxygen); in the atmosphere the addition of the oxygen atom so formed to O 2 results in ozone.
Industrially, nitrogen dioxide is produced and transported as its cryogenic liquid dimer, dinitrogen tetroxide . It is produced industrially by the oxidation of ammonia, the Ostwald Process . This reaction is the first step in the production of nitric acid: [ 13 ]
It can also be produced by the oxidation of nitrosyl chloride :
Instead, most laboratory syntheses stabilize and then heat the nitric acid to accelerate the decomposition. For example, the thermal decomposition of some metal nitrates generates NO 2 : [ 14 ]
Alternatively, dehydration of nitric acid produces nitronium nitrate ...
...which subsequently undergoes thermal decomposition:
NO 2 is generated by the reduction of concentrated nitric acid with a metal (such as copper):
Nitric acid decomposes slowly to nitrogen dioxide by the overall reaction:
The nitrogen dioxide so formed confers the characteristic yellow color often exhibited by this acid. However, the reaction is too slow to be a practical source of NO 2 .
At low temperatures, NO 2 reversibly converts to the colourless gas dinitrogen tetroxide ( N 2 O 4 ):
The exothermic equilibrium has enthalpy change Δ H = −57.23 kJ/mol . [ 15 ]
At 150 °C (302 °F; 423 K), NO 2 decomposes with release of oxygen via an endothermic process ( Δ H = 14 kJ/mol ):
As suggested by the weakness of the N–O bond, NO 2 is a good oxidizer. Consequently, it will combust, sometimes explosively, in the presence of hydrocarbons . [ 16 ]
NO 2 reacts with water to give nitric acid and nitrous acid :
This reaction is one of the steps in the Ostwald process for the industrial production of nitric acid from ammonia. [ 13 ] This reaction is negligibly slow at low concentrations of NO 2 characteristic of the ambient atmosphere, although it does proceed upon NO 2 uptake to surfaces. Such surface reaction is thought to produce gaseous HNO 2 (often written as HONO ) in outdoor and indoor environments. [ 17 ]
NO 2 is used to generate anhydrous metal nitrates from the oxides: [ 15 ]
Alkyl and metal iodides give the corresponding nitrates: [ 12 ]
The reactivity of nitrogen dioxide toward organic compounds has long been known. [ 18 ] For example, it reacts with amides to give N-nitroso derivatives. [ 19 ] It is used for nitrations under anhydrous conditions. [ 20 ]
NO 2 is used as an intermediate in the manufacturing of nitric acid , as a nitrating agent in the manufacturing of chemical explosives , as a polymerization inhibitor for acrylates , as a flour bleaching agent , [ 21 ] : 223 and as a room temperature sterilization agent. [ 22 ] It is also used as an oxidizer in rocket fuel , for example in red fuming nitric acid ; it was used in the Titan rockets , to launch Project Gemini , in the maneuvering thrusters of the Space Shuttle , and in uncrewed space probes sent to various planets. [ 23 ]
Nitrogen dioxide typically arises via the oxidation of nitric oxide by oxygen in air (e.g. as result of corona discharge ): [ 15 ]
NO 2 is introduced into the environment by natural causes, including entry from the stratosphere , bacterial respiration, volcanos, and lightning. These sources make NO 2 a trace gas in the atmosphere of Earth , where it plays a role in absorbing sunlight and regulating the chemistry of the troposphere , especially in determining ozone concentrations. [ 24 ]
Nitrogen dioxide also forms in most combustion processes. At elevated temperatures nitrogen combines with oxygen to form nitrogen dioxide:
For the general public, the most prominent sources of NO 2 are internal combustion engines , as combustion temperatures are high enough to thermally combine some of the nitrogen and oxygen in the air to form NO 2 . [ 8 ] Nitrogen dioxide accounts for a small fraction (generally well under 0.1) of NOx auto emissions. [ 25 ]
Outdoors, NO 2 can be a result of traffic from motor vehicles. [ 26 ] Indoors, exposure arises from cigarette smoke, [ 27 ] and butane and kerosene heaters and stoves. [ 28 ] Indoor exposure levels of NO 2 are, on average, at least three times higher in homes with gas stoves compared to electric stove. [ 29 ] [ 30 ]
Workers in industries where NO 2 is used are also exposed and are at risk for occupational lung diseases , and NIOSH has set exposure limits and safety standards. [ 6 ] Workers in high voltage areas especially those with spark or plasma creation are at risk. [ citation needed ] Agricultural workers can be exposed to NO 2 arising from grain decomposing in silos; chronic exposure can lead to lung damage in a condition called " silo-filler's disease ". [ 31 ] [ 32 ]
NO 2 diffuses into the epithelial lining fluid (ELF) of the respiratory epithelium and dissolves. There, it chemically reacts with antioxidant and lipid molecules in the ELF. The health effects of NO 2 are caused by the reaction products or their metabolites, which are reactive nitrogen species and reactive oxygen species that can drive bronchoconstriction , inflammation, reduced immune response, and may have effects on the heart. [ 33 ]
Acute harm due to NO 2 exposure is rare. 100–200 ppm can cause mild irritation of the nose and throat, 250–500 ppm can cause edema , leading to bronchitis or pneumonia , and levels above 1000 ppm can cause death due to asphyxiation from fluid in the lungs. There are often no symptoms at the time of exposure other than transient cough, fatigue or nausea, but over hours inflammation in the lungs causes edema. [ 34 ] [ 35 ]
For skin or eye exposure, the affected area is flushed with saline. For inhalation, oxygen is administered, bronchodilators may be administered, and if there are signs of methemoglobinemia , a condition that arises when nitrogen-based compounds affect the hemoglobin in red blood cells, methylene blue may be administered. [ 36 ] [ 37 ]
It is classified as an extremely hazardous substance in the United States as defined in Section 302 of the U.S. Emergency Planning and Community Right-to-Know Act (42 U.S.C. 11002), and it is subject to strict reporting requirements by facilities which produce, store, or use it in significant quantities. [ 38 ]
Exposure to low levels of NO 2 over time can cause changes in lung function. [ 39 ] Cooking with a gas stove is associated with poorer indoor air quality . Combustion of gas can lead to increased concentrations of nitrogen dioxide throughout the home environment which is linked to respiratory issues and diseases . [ 9 ] [ 10 ] Children exposed to NO 2 are more likely to be admitted to hospital with asthma . [ 40 ]
In 2019, the Court of Justice of the EU , found that France did not comply with the limit values of the EU air quality standards applicable to the concentrations of nitrogen dioxide (NO 2 ) in 12 air quality zones. [ 41 ]
Interaction of NO 2 and other NO x with water, oxygen and other chemicals in the atmosphere can form acid rain which harms sensitive ecosystems such as lakes and forests. [ 42 ] Elevated levels of NO 2 can also harm vegetation, decreasing growth, and reduce crop yields. [ 43 ] | https://en.wikipedia.org/wiki/O⚍N⚍O |
Dinitrogen tetroxide Dinitrogen trioxide Nitric oxide Nitrous oxide
Nitrogen dioxide is a chemical compound with the formula NO 2 . One of several nitrogen oxides , nitrogen dioxide is a reddish-brown gas. It is a paramagnetic , bent molecule with C 2v point group symmetry . Industrially, NO 2 is an intermediate in the synthesis of nitric acid , millions of tons of which are produced each year, primarily for the production of fertilizers .
Nitrogen dioxide is poisonous and can be fatal if inhaled in large quantities. [ 8 ] Cooking with a gas stove produces nitrogen dioxide which causes poorer indoor air quality . Combustion of gas can lead to increased concentrations of nitrogen dioxide throughout the home environment which is linked to respiratory issues and diseases . [ 9 ] [ 10 ] The LC 50 ( median lethal dose ) for humans has been estimated to be 174 ppm for a 1-hour exposure. [ 11 ] It is also included in the NO x family of atmospheric pollutants .
Nitrogen dioxide is a reddish-brown gas with a pungent, acrid odor above 21.2 °C (70.2 °F; 294.3 K) and becomes a yellowish-brown liquid below 21.2 °C (70.2 °F; 294.3 K). It forms an equilibrium with its dimer , dinitrogen tetroxide ( N 2 O 4 ), and converts almost entirely to N 2 O 4 below −11.2 °C (11.8 °F; 261.9 K). [ 6 ]
The bond length between the nitrogen atom and the oxygen atom is 119.7 pm . This bond length is consistent with a bond order between one and two.
Unlike ozone ( O 3 ) the ground electronic state of nitrogen dioxide is a doublet state , since nitrogen has one unpaired electron, [ 12 ] which decreases the alpha effect compared with nitrite and creates a weak bonding interaction with the oxygen lone pairs. The lone electron in NO 2 also means that this compound is a free radical , so the formula for nitrogen dioxide is often written as • NO 2 .
The reddish-brown color is a consequence of preferential absorption of light in the blue region of the spectrum (400–500 nm), although the absorption extends throughout the visible (at shorter wavelengths) and into the infrared (at longer wavelengths). Absorption of light at wavelengths shorter than about 400 nm results in photolysis (to form NO + O , atomic oxygen); in the atmosphere the addition of the oxygen atom so formed to O 2 results in ozone.
Industrially, nitrogen dioxide is produced and transported as its cryogenic liquid dimer, dinitrogen tetroxide . It is produced industrially by the oxidation of ammonia, the Ostwald Process . This reaction is the first step in the production of nitric acid: [ 13 ]
It can also be produced by the oxidation of nitrosyl chloride :
Instead, most laboratory syntheses stabilize and then heat the nitric acid to accelerate the decomposition. For example, the thermal decomposition of some metal nitrates generates NO 2 : [ 14 ]
Alternatively, dehydration of nitric acid produces nitronium nitrate ...
...which subsequently undergoes thermal decomposition:
NO 2 is generated by the reduction of concentrated nitric acid with a metal (such as copper):
Nitric acid decomposes slowly to nitrogen dioxide by the overall reaction:
The nitrogen dioxide so formed confers the characteristic yellow color often exhibited by this acid. However, the reaction is too slow to be a practical source of NO 2 .
At low temperatures, NO 2 reversibly converts to the colourless gas dinitrogen tetroxide ( N 2 O 4 ):
The exothermic equilibrium has enthalpy change Δ H = −57.23 kJ/mol . [ 15 ]
At 150 °C (302 °F; 423 K), NO 2 decomposes with release of oxygen via an endothermic process ( Δ H = 14 kJ/mol ):
As suggested by the weakness of the N–O bond, NO 2 is a good oxidizer. Consequently, it will combust, sometimes explosively, in the presence of hydrocarbons . [ 16 ]
NO 2 reacts with water to give nitric acid and nitrous acid :
This reaction is one of the steps in the Ostwald process for the industrial production of nitric acid from ammonia. [ 13 ] This reaction is negligibly slow at low concentrations of NO 2 characteristic of the ambient atmosphere, although it does proceed upon NO 2 uptake to surfaces. Such surface reaction is thought to produce gaseous HNO 2 (often written as HONO ) in outdoor and indoor environments. [ 17 ]
NO 2 is used to generate anhydrous metal nitrates from the oxides: [ 15 ]
Alkyl and metal iodides give the corresponding nitrates: [ 12 ]
The reactivity of nitrogen dioxide toward organic compounds has long been known. [ 18 ] For example, it reacts with amides to give N-nitroso derivatives. [ 19 ] It is used for nitrations under anhydrous conditions. [ 20 ]
NO 2 is used as an intermediate in the manufacturing of nitric acid , as a nitrating agent in the manufacturing of chemical explosives , as a polymerization inhibitor for acrylates , as a flour bleaching agent , [ 21 ] : 223 and as a room temperature sterilization agent. [ 22 ] It is also used as an oxidizer in rocket fuel , for example in red fuming nitric acid ; it was used in the Titan rockets , to launch Project Gemini , in the maneuvering thrusters of the Space Shuttle , and in uncrewed space probes sent to various planets. [ 23 ]
Nitrogen dioxide typically arises via the oxidation of nitric oxide by oxygen in air (e.g. as result of corona discharge ): [ 15 ]
NO 2 is introduced into the environment by natural causes, including entry from the stratosphere , bacterial respiration, volcanos, and lightning. These sources make NO 2 a trace gas in the atmosphere of Earth , where it plays a role in absorbing sunlight and regulating the chemistry of the troposphere , especially in determining ozone concentrations. [ 24 ]
Nitrogen dioxide also forms in most combustion processes. At elevated temperatures nitrogen combines with oxygen to form nitrogen dioxide:
For the general public, the most prominent sources of NO 2 are internal combustion engines , as combustion temperatures are high enough to thermally combine some of the nitrogen and oxygen in the air to form NO 2 . [ 8 ] Nitrogen dioxide accounts for a small fraction (generally well under 0.1) of NOx auto emissions. [ 25 ]
Outdoors, NO 2 can be a result of traffic from motor vehicles. [ 26 ] Indoors, exposure arises from cigarette smoke, [ 27 ] and butane and kerosene heaters and stoves. [ 28 ] Indoor exposure levels of NO 2 are, on average, at least three times higher in homes with gas stoves compared to electric stove. [ 29 ] [ 30 ]
Workers in industries where NO 2 is used are also exposed and are at risk for occupational lung diseases , and NIOSH has set exposure limits and safety standards. [ 6 ] Workers in high voltage areas especially those with spark or plasma creation are at risk. [ citation needed ] Agricultural workers can be exposed to NO 2 arising from grain decomposing in silos; chronic exposure can lead to lung damage in a condition called " silo-filler's disease ". [ 31 ] [ 32 ]
NO 2 diffuses into the epithelial lining fluid (ELF) of the respiratory epithelium and dissolves. There, it chemically reacts with antioxidant and lipid molecules in the ELF. The health effects of NO 2 are caused by the reaction products or their metabolites, which are reactive nitrogen species and reactive oxygen species that can drive bronchoconstriction , inflammation, reduced immune response, and may have effects on the heart. [ 33 ]
Acute harm due to NO 2 exposure is rare. 100–200 ppm can cause mild irritation of the nose and throat, 250–500 ppm can cause edema , leading to bronchitis or pneumonia , and levels above 1000 ppm can cause death due to asphyxiation from fluid in the lungs. There are often no symptoms at the time of exposure other than transient cough, fatigue or nausea, but over hours inflammation in the lungs causes edema. [ 34 ] [ 35 ]
For skin or eye exposure, the affected area is flushed with saline. For inhalation, oxygen is administered, bronchodilators may be administered, and if there are signs of methemoglobinemia , a condition that arises when nitrogen-based compounds affect the hemoglobin in red blood cells, methylene blue may be administered. [ 36 ] [ 37 ]
It is classified as an extremely hazardous substance in the United States as defined in Section 302 of the U.S. Emergency Planning and Community Right-to-Know Act (42 U.S.C. 11002), and it is subject to strict reporting requirements by facilities which produce, store, or use it in significant quantities. [ 38 ]
Exposure to low levels of NO 2 over time can cause changes in lung function. [ 39 ] Cooking with a gas stove is associated with poorer indoor air quality . Combustion of gas can lead to increased concentrations of nitrogen dioxide throughout the home environment which is linked to respiratory issues and diseases . [ 9 ] [ 10 ] Children exposed to NO 2 are more likely to be admitted to hospital with asthma . [ 40 ]
In 2019, the Court of Justice of the EU , found that France did not comply with the limit values of the EU air quality standards applicable to the concentrations of nitrogen dioxide (NO 2 ) in 12 air quality zones. [ 41 ]
Interaction of NO 2 and other NO x with water, oxygen and other chemicals in the atmosphere can form acid rain which harms sensitive ecosystems such as lakes and forests. [ 42 ] Elevated levels of NO 2 can also harm vegetation, decreasing growth, and reduce crop yields. [ 43 ] | https://en.wikipedia.org/wiki/O⚍N⚎O |
Ozone ( / ˈ oʊ z oʊ n / ) (or trioxygen ) is an inorganic molecule with the chemical formula O 3 . It is a pale blue gas with a distinctively pungent smell. It is an allotrope of oxygen that is much less stable than the diatomic allotrope O 2 , breaking down in the lower atmosphere to O 2 ( dioxygen ). Ozone is formed from dioxygen by the action of ultraviolet (UV) light and electrical discharges within the Earth's atmosphere . It is present in very low concentrations throughout the atmosphere, with its highest concentration high in the ozone layer of the stratosphere , which absorbs most of the Sun 's ultraviolet (UV) radiation.
Ozone's odor is reminiscent of chlorine , and detectable by many people at concentrations of as little as 0.1 ppm in air. Ozone's O 3 structure was determined in 1865. The molecule was later proven to have a bent structure and to be weakly diamagnetic . At standard temperature and pressure , ozone is a pale blue gas that condenses at cryogenic temperatures to a dark blue liquid and finally a violet-black solid . Ozone's instability with regard to more common dioxygen is such that both concentrated gas and liquid ozone may decompose explosively at elevated temperatures, physical shock, or fast warming to the boiling point. [ 5 ] [ 6 ] It is therefore used commercially only in low concentrations.
Ozone is a powerful oxidizing agent (far more so than dioxygen) and has many industrial and consumer applications related to oxidation. This same high oxidizing potential, however, causes ozone to damage mucous and respiratory tissues in animals, and also tissues in plants, above concentrations of about 0.1 ppm . While this makes ozone a potent respiratory hazard and pollutant near ground level , a higher concentration in the ozone layer (from two to eight ppm) is beneficial, preventing damaging UV light from reaching the Earth's surface.
The trivial name ozone is the most commonly used and preferred IUPAC name . The systematic names 2λ 4 -trioxidiene [ dubious – discuss ] and catena-trioxygen , valid IUPAC names, are constructed according to the substitutive and additive nomenclatures , respectively. The name ozone derives from ozein (ὄζειν), the Greek neuter present participle for smell, [ 7 ] referring to ozone's distinctive smell.
In appropriate contexts, ozone can be viewed as trioxidane with two hydrogen atoms removed, and as such, trioxidanylidene may be used as a systematic name, according to substitutive nomenclature. By default, these names pay no regard to the radicality of the ozone molecule. In an even more specific context, this can also name the non-radical singlet ground state, whereas the diradical state is named trioxidanediyl .
Trioxidanediyl (or ozonide ) is used, non-systematically, to refer to the substituent group (-OOO-). Care should be taken to avoid confusing the name of the group for the context-specific name for the ozone given above.
In 1785, Dutch chemist Martinus van Marum was conducting experiments involving electrical sparking above water when he noticed an unusual smell, which he attributed to the electrical reactions, failing to realize that he had in fact produced ozone. [ 8 ] [ 9 ]
A half century later, Christian Friedrich Schönbein noticed the same pungent odour and recognized it as the smell often following a bolt of lightning . In 1839, he succeeded in isolating the gaseous chemical and named it "ozone", from the Greek word ozein ( ὄζειν ) meaning "to smell". [ 10 ] [ 11 ] For this reason, Schönbein is generally credited with the discovery of ozone. [ 12 ] [ 13 ] [ 14 ] [ 8 ] He also noted the similarity of ozone smell to the smell of phosphorus, and in 1844 proved that the product of reaction of white phosphorus with air is identical. [ 10 ] A subsequent effort to call ozone "electrified oxygen" he ridiculed by proposing to call the ozone from white phosphorus "phosphorized oxygen". [ 10 ] The chemical formula for ozone, O 3 , was not determined until 1865 by Jacques-Louis Soret [ 15 ] and confirmed by Schönbein in 1867. [ 10 ] [ 16 ]
For much of the second half of the 19th century and well into the 20th, ozone was considered a healthy component of the environment by naturalists and health-seekers. Beaumont, California , had as its official slogan "Beaumont: Zone of Ozone", as evidenced on postcards and Chamber of Commerce letterhead. [ 17 ] Naturalists working outdoors often considered the higher elevations beneficial because of their ozone content which was readily monitored. [ 18 ] "There is quite a different atmosphere [at higher elevation] with enough ozone to sustain the necessary energy [to work]", wrote naturalist Henry Henshaw , working in Hawaii. [ 19 ] Seaside air was considered to be healthy because of its believed ozone content. The smell giving rise to this belief is in fact that of halogenated seaweed metabolites [ 20 ] and dimethyl sulfide . [ 21 ]
Much of ozone's appeal seems to have resulted from its "fresh" smell, which evoked associations with purifying properties. Scientists noted its harmful effects. In 1873 James Dewar and John Gray McKendrick documented that frogs grew sluggish, birds gasped for breath, and rabbits' blood showed decreased levels of oxygen after exposure to "ozonized air", which "exercised a destructive action". [ 22 ] [ 12 ] Schönbein himself reported that chest pains, irritation of the mucous membranes , and difficulty breathing occurred as a result of inhaling ozone, and small mammals died. [ 23 ] In 1911, Leonard Hill and Martin Flack stated in the Proceedings of the Royal Society B that ozone's healthful effects "have, by mere iteration, become part and parcel of common belief; and yet exact physiological evidence in favour of its good effects has been hitherto almost entirely wanting ... The only thoroughly well-ascertained knowledge concerning the physiological effect of ozone, so far attained, is that it causes irritation and œdema of the lungs, and death if inhaled in relatively strong concentration for any time." [ 12 ] [ 24 ]
During World War I , ozone was tested at Queen Alexandra Military Hospital in London as a possible disinfectant for wounds. The gas was applied directly to wounds for as long as 15 minutes. This resulted in damage to both bacterial cells and human tissue. Other sanitizing techniques, such as irrigation with antiseptics , were found preferable. [ 12 ] [ 25 ]
Until the 1920s, it was not certain whether small amounts of oxozone , O 4 , were also present in ozone samples due to the difficulty of applying analytical chemistry techniques to the explosive concentrated chemical. [ 26 ] [ 27 ] In 1923, Georg-Maria Schwab (working for his doctoral thesis under Ernst Hermann Riesenfeld ) was the first to successfully solidify ozone and perform accurate analysis which conclusively refuted the oxozone hypothesis. [ 26 ] [ 27 ] Further hitherto unmeasured physical properties of pure concentrated ozone were determined by the Riesenfeld group in the 1920s. [ 26 ]
Ozone is a colourless or pale blue gas, slightly soluble in water, and much more soluble in inert non-polar solvents such as carbon tetrachloride or fluorocarbons, in which it forms a blue solution. At 161 K (−112 °C; −170 °F), it condenses to form a dark blue liquid . It is dangerous to allow this liquid to warm to its boiling point, because both concentrated gaseous ozone and liquid ozone can detonate. At temperatures below 80 K (−193.2 °C; −315.7 °F), it forms a violet-black solid . [ 28 ]
Ozone has a very specific sharp odour somewhat resembling chlorine bleach . Most people can detect it at the 0.01 μmol/mol level in air. Exposure of 0.1 to 1 μmol/mol produces headaches and burning eyes and irritates the respiratory passages. [ 29 ] Even low concentrations of ozone in air are very destructive to organic materials such as latex, plastics, and animal lung tissue.
The ozone molecule is weakly diamagnetic . [ 30 ]
According to experimental evidence from microwave spectroscopy , ozone is a bent molecule, with C 2v symmetry (similar to the water molecule). [ 31 ] The O–O distances are 127.2 pm (1.272 Å ). The O–O–O angle is 116.78°. [ 32 ] The central atom is sp ² hybridized with one lone pair. Ozone is a polar molecule with a dipole moment of 0.53 D . [ 33 ] The molecule can be represented as a resonance hybrid with two contributing structures, each with a single bond on one side and double bond on the other. The arrangement possesses an overall bond order of 1.5 for both sides. It is isoelectronic with the nitrite anion . Naturally occurring ozone can be composed of substituted isotopes ( 16 O, 17 O, 18 O). A cyclic form has been predicted but not observed.
Ozone is among the most powerful oxidizing agents known, far stronger than O 2 . It is also unstable at high concentrations, decaying into ordinary diatomic oxygen. Its half-life varies with atmospheric conditions such as temperature, humidity, and air movement. Under laboratory conditions, the half-life will average ~1500 minutes (25 hours) in still air at room temperature (24 °C), zero humidity with zero air changes per hour. [ 34 ]
This reaction proceeds more rapidly with increasing temperature. Deflagration of ozone can be triggered by a spark and can occur in ozone concentrations of 10 wt% or higher. [ 35 ]
Ozone can also be produced from oxygen at the anode of an electrochemical cell. This reaction can create smaller quantities of ozone for research purposes. [ 36 ]
This can be observed as an unwanted reaction in a Hoffman apparatus during the electrolysis of water when the voltage is set above the necessary voltage.
Ozone oxidizes most metals (except gold , platinum , and iridium ) into oxides of the metals in their highest oxidation state . For example:
Ozone oxidizes nitric oxide to nitrogen dioxide :
This reaction is accompanied by chemiluminescence .
The NO 2 can be further oxidized to nitrate radical :
The NO 3 formed can react with NO 2 to form dinitrogen pentoxide ( N 2 O 5 ).
Solid nitronium perchlorate can be made from NO 2 , ClO 2 , and O 3 gases:
Ozone does not react with ammonium salts , but it oxidizes ammonia to ammonium nitrate :
Ozone reacts with carbon to form carbon dioxide , even at room temperature:
Ozone oxidizes sulfides to sulfates . For example, lead(II) sulfide is oxidized to lead(II) sulfate :
Sulfuric acid can be produced from ozone, water and either elemental sulfur or sulfur dioxide :
In the gas phase , ozone reacts with hydrogen sulfide to form sulfur dioxide:
In an aqueous solution, however, two competing simultaneous reactions occur, one to produce elemental sulfur, and one to produce sulfuric acid :
Alkenes can be oxidatively cleaved by ozone, in a process called ozonolysis , giving alcohols, aldehydes, ketones, and carboxylic acids, depending on the second step of the workup.
Ozone can also cleave alkynes to form an acid anhydride or diketone product. [ 38 ] If the reaction is performed in the presence of water, the anhydride hydrolyzes to give two carboxylic acids .
Usually ozonolysis is carried out in a solution of dichloromethane , at a temperature of −78 °C. After a sequence of cleavage and rearrangement, an organic ozonide is formed. With reductive workup (e.g. zinc in acetic acid or dimethyl sulfide ), ketones and aldehydes will be formed, with oxidative workup (e.g. aqueous or alcoholic hydrogen peroxide ), carboxylic acids will be formed. [ 39 ]
All three atoms of ozone may also react, as in the reaction of tin(II) chloride with hydrochloric acid and ozone:
Iodine perchlorate can be made by treating iodine dissolved in cold anhydrous perchloric acid with ozone:
Ozone could also react with potassium iodide to give oxygen and iodine gas that can be titrated for quantitative determination: [ 40 ]
Ozone can be used for combustion reactions and combustible gases; ozone provides higher temperatures than burning in dioxygen ( O 2 ). The following is a reaction for the combustion of carbon subnitride which can also cause higher temperatures:
Ozone can react at cryogenic temperatures. At 77 K (−196.2 °C; −321.1 °F), atomic hydrogen reacts with liquid ozone to form a hydrogen superoxide radical , which dimerizes : [ 41 ]
Ozone is a toxic substance, [ 42 ] [ 43 ] commonly found or generated in human environments (aircraft cabins, offices with photocopiers, laser printers, sterilizers, ...).
The catalytic decomposition of ozone is very important to reduce pollution. This type of decomposition is the most widely used, especially with solid catalysts, and it has many advantages such as a higher conversion with a lower temperature. Furthermore, the product and the catalyst can be instantaneously separated, and this way the catalyst can be easily recovered without using any separation operation. The most-used materials in the catalytic decomposition of ozone in the gas phase are manganese dioxide , transition metals such as Mn, Co, Cu, Fe, Ni, or Ag, and noble metals such as Pt, Rh, or Pd.
Free radicals of chlorine (Cl · ), formed by the action of ultraviolet radiation on chlorofluorocarbons (CFCs) and sea salt, are known to catalyze the breakdown of ozone in the atmosphere.
There are two other possibilities for decomposing ozone in the gas phase:
The uncatalyzed process of ozone decomposition in the gas phase is a complex reaction involving two elementary reactions that finally lead to molecular oxygen, [ 45 ] and this means that the reaction order and the rate law cannot be determined by the stoichiometry of the overall reaction.
Overall reaction: 2 O 3 ⟶ 3 O 2 {\displaystyle {\ce {2 O3 -> 3 O2}}}
Rate law (observed): V = K o b s ⋅ [ O 3 ] 2 [ O 2 ] {\displaystyle V={\frac {K_{obs}\cdot [{\ce {O3}}]^{2}}{[{\ce {O2}}]}}}
where K o b s {\displaystyle K_{obs}} is the observed rate constant and V {\displaystyle V} is the reaction rate. From the rate law above it can be determined that the partial order respect to molecular oxygen is −1 and respect to ozone is 2; therefore, the global reaction order is 1.
The first step is a unimolecular reaction wherein one molecule of ozone decomposes into two products (molecular oxygen and oxygen). The oxygen atom from the first step is a reactive intermediate because it participates as a reactant in the second step, which is a bimolecular reaction because there are two different reactants (ozone and oxygen) that give rise to molecular oxygen.
Step 1: Unimolecular reaction O 3 ⟶ O 2 + O {\displaystyle {\ce {O3 -> O2 + O}}}
Step 2: Bimolecular reaction O 3 + O ⟶ 2 O 2 {\displaystyle {\ce {O3 + O -> 2 O2}}}
These two steps have different reaction rates and rate constants. The reaction rate laws for each of these steps are shown below:
The following mechanism allows to explain the rate law of the ozone decomposition observed experimentally, and also it allows to determine the reaction orders with respect to ozone and oxygen, with which the overall reaction order will be determined.
The first step is assumed reversible and faster than the second reaction, which means that the slower rate determining step is the second reaction. This step determines the rate of product formation, and so V = V 2 {\displaystyle V=V_{2}} . However, this equation depends on the concentration of oxygen (intermediate), which does not appear in the observed rate law. Since the first step is a rapid equilibrium, the concentration of the intermediate can be determined as follows:
Then using these equations, the formation rate of molecular oxygen is as shown below:
The mechanism is consistent with the rate law observed experimentally if the rate constant ( K obs ) is given in terms of the individual mechanistic steps' rate constants as follows: [ 46 ]
where K obs = K 2 ⋅ K 1 K − 1 {\displaystyle K_{\text{obs}}={K_{2}\cdot K_{1} \over K_{-1}}}
Reduction of ozone gives the ozonide anion, O − 3 . Derivatives of this anion are explosive and must be stored at cryogenic temperatures. Ozonides for all the alkali metals are known. KO 3 , RbO 3 , and CsO 3 can be prepared from their respective superoxides:
Although KO 3 can be formed as above, it can also be formed from potassium hydroxide and ozone: [ 47 ]
NaO 3 and LiO 3 must be prepared by action of CsO 3 in liquid NH 3 on an ion-exchange resin containing Na + or Li + ions: [ 48 ]
A solution of calcium in ammonia reacts with ozone to give ammonium ozonide and not calcium ozonide: [ 41 ]
Ozone can be used to remove iron and manganese from water , forming a precipitate which can be filtered:
Ozone oxidizes dissolved hydrogen sulfide in water to sulfurous acid :
These three reactions are central in the use of ozone-based well water treatment.
Ozone detoxifies cyanides by converting them to cyanates .
Ozone completely decomposes urea : [ 49 ]
Ozone is a bent triatomic molecule with three vibrational modes: the symmetric stretch (1103.157 cm −1 ), bend (701.42 cm −1 ) and antisymmetric stretch (1042.096 cm −1 ). [ 50 ] The symmetric stretch and bend are weak absorbers, but the antisymmetric stretch is strong and responsible for ozone being an important minor greenhouse gas . This IR band is also used to detect ambient and atmospheric ozone although UV-based measurements are more common. [ 51 ]
The electromagnetic spectrum of ozone is quite complex. An overview can be seen at the MPI Mainz UV/VIS Spectral Atlas of Gaseous Molecules of Atmospheric Interest. [ 52 ]
All of the bands are dissociative, meaning that the molecule falls apart to O + O 2 after absorbing a photon. The most important absorption is the Hartley band, extending from slightly above 300 nm down to slightly above 200 nm. It is this band that is responsible for absorbing UV C in the stratosphere.
On the high wavelength side, the Hartley band transitions to the so-called Huggins band, which falls off rapidly until disappearing by ~360 nm. Above 400 nm, extending well out into the NIR, are the Chappius and Wulf bands. There, unstructured absorption bands are useful for detecting high ambient concentrations of ozone, but are so weak that they do not have much practical effect.
There are additional absorption bands in the far UV, which increase slowly from 200 nm down to reaching a maximum at ~120 nm.
The standard way to express total ozone levels (the amount of ozone in a given vertical column) in the atmosphere is by using Dobson units . Point measurements are reported as mole fractions in nmol/mol (parts per billion, ppb) or as concentrations in μg/m 3 . The study of ozone concentration in the atmosphere started in the 1920s. [ 53 ]
The highest levels of ozone in the atmosphere are in the stratosphere , in a region also known as the ozone layer between about 10 and 50 km above the surface (or between about 6 and 31 miles). However, even in this "layer", the ozone concentrations are only two to eight parts per million, so most of the oxygen there is dioxygen, O 2 , at about 210,000 parts per million by volume. [ 54 ]
Ozone in the stratosphere is mostly produced from short-wave ultraviolet rays between 240 and 160 nm. Oxygen starts to absorb weakly at 240 nm in the Herzberg bands, but most of the oxygen is dissociated by absorption in the strong Schumann–Runge bands between 200 and 160 nm where ozone does not absorb. While shorter wavelength light, extending to even the X-Ray limit, is energetic enough to dissociate molecular oxygen, there is relatively little of it, and, the strong solar emission at Lyman-alpha, 121 nm, falls at a point where molecular oxygen absorption is a minimum. [ 55 ]
The process of ozone creation and destruction is called the Chapman cycle and starts with the photolysis of molecular oxygen
followed by reaction of the oxygen atom with another molecule of oxygen to form ozone.
where "M" denotes the third body that carries off the excess energy of the reaction. The ozone molecule can then absorb a UV-C photon and dissociate
The excess kinetic energy heats the stratosphere when the O atoms and the molecular oxygen fly apart and collide with other molecules. This conversion of UV light into kinetic energy warms the stratosphere. The oxygen atoms produced in the photolysis of ozone then react back with other oxygen molecule as in the previous step to form more ozone. In the clear atmosphere, with only nitrogen and oxygen, ozone can react with the atomic oxygen to form two molecules of O 2 :
An estimate of the rate of this termination step to the cycling of atomic oxygen back to ozone can be found simply by taking the ratios of the concentration of O 2 to O 3 . The termination reaction is catalysed by the presence of certain free radicals, of which the most important are hydroxyl (OH), nitric oxide (NO) and atomic chlorine (Cl) and bromine (Br). In the second half of the 20th century, the amount of ozone in the stratosphere was discovered to be declining , mostly because of increasing concentrations of chlorofluorocarbons (CFC) and similar chlorinated and brominated organic molecules . The concern over the health effects of the decline led to the 1987 Montreal Protocol , the ban on the production of many ozone-depleting chemicals and in the first and second decade of the 21st century the beginning of the recovery of stratospheric ozone concentrations.
Ozone in the ozone layer filters out sunlight wavelengths from about 200 nm UV rays to 315 nm, with ozone peak absorption at about 250 nm. [ 56 ] This ozone UV absorption is important to life, since it extends the absorption of UV by ordinary oxygen and nitrogen in air (which absorb all wavelengths < 200 nm) through the lower UV-C (200–280 nm) and the entire UV-B band (280–315 nm). The small unabsorbed part that remains of UV-B after passage through ozone causes sunburn in humans, and direct DNA damage in living tissues in both plants and animals. Ozone's effect on mid-range UV-B rays is illustrated by its effect on UV-B at 290 nm, which has a radiation intensity 350 million times as powerful at the top of the atmosphere as at the surface. Nevertheless, enough of UV-B radiation at similar frequency reaches the ground to cause some sunburn, and these same wavelengths are also among those responsible for the production of vitamin D in humans.
The ozone layer has little effect on the longer UV wavelengths called UV-A (315–400 nm), but this radiation does not cause sunburn or direct DNA damage. While UV-A probably does cause long-term skin damage in certain humans, it is not as dangerous to plants and to the health of surface-dwelling organisms on Earth in general (see ultraviolet for more information on near ultraviolet).
Ground-level ozone (or tropospheric ozone) is an atmospheric pollutant. [ 57 ] It is not emitted directly by car engines or by industrial operations, but formed by the reaction of sunlight on air containing hydrocarbons and nitrogen oxides that react to form ozone directly at the source of the pollution or many kilometers downwind.
Ozone reacts directly with some hydrocarbons such as aldehydes and thus begins their removal from the air, but the products are themselves key components of smog . Ozone photolysis by UV light leads to production of the hydroxyl radical HO• and this plays a part in the removal of hydrocarbons from the air, but is also the first step in the creation of components of smog such as peroxyacyl nitrates , which can be powerful eye irritants. The atmospheric lifetime of tropospheric ozone is about 22 days; its main removal mechanisms are being deposited to the ground, the above-mentioned reaction giving HO•, and by reactions with OH and the peroxy radical HO 2 •. [ 58 ]
There is evidence of significant reduction in agricultural yields because of increased ground-level ozone and pollution which interferes with photosynthesis and stunts overall growth of some plant species. [ 59 ] [ 60 ] The United States Environmental Protection Agency (EPA) has proposed a secondary regulation to reduce crop damage, in addition to the primary regulation designed for the protection of human health.
Certain examples of cities with elevated ozone readings are Denver, Colorado ; Houston, Texas ; and Mexico City , Mexico . Houston has a reading of around 41 nmol/mol, while Mexico City is far more hazardous, with a reading of about 125 nmol/mol. [ 60 ]
Ground-level ozone, or tropospheric ozone, is the most concerning type of ozone pollution in urban areas and is increasing in general. [ 61 ] Ozone pollution in urban areas affects denser populations, and is worsened by high populations of vehicles, which emit pollutants NO 2 and VOCs , the main contributors to problematic ozone levels. [ 62 ] Ozone pollution in urban areas is especially concerning with increasing temperatures, raising heat-related mortality during heat waves . [ 63 ] During heat waves in urban areas, ground level ozone pollution can be 20% higher than usual. [ 64 ] Ozone pollution in urban areas reaches higher levels of exceedance in the summer and autumn, which may be explained by weather patterns and traffic patterns. [ 62 ] People experiencing poverty are more affected by pollution in general, even though these populations are less likely to be contributing to pollution levels. [ 65 ]
As mentioned above, Denver, Colorado, is one of the many cities in the U.S. that have high amounts of ozone. According to the American Lung Association , the Denver–Aurora area is the 14th most ozone-polluted area in the U.S. [ 66 ] The problem of high ozone levels is not new to this area. In 2004, the EPA allotted the Denver Metro /North Front Range [ b ] as non-attainment areas per 1997's 8-hour ozone standard, [ 67 ] but later deferred this status until 2007. The non-attainment standard indicates that an area does not meet the EPA's air quality standards. The Colorado Ozone Action Plan was created in response, and numerous changes were implemented from this plan. The first major change was that car emission testing was expanded across the state to more counties that did not previously mandate emissions testing, like areas of Larimer and Weld County. There have also been changes made to decrease Nitrogen Oxides (NOx) and Volatile Organic Compound (VOC) emissions, which should help lower ozone levels.
One large contributor to high ozone levels in the area is the oil and natural gas industry situated in the Denver-Julesburg Basin (DJB) which overlaps with a majority of Colorado's metropolitan areas. Ozone is produced naturally in the Earth's stratosphere, but is also produced in the troposphere from human efforts. Briefly mentioned above, NOx and VOCs react with sunlight to create ozone through a process called photochemistry. One hour elevated ozone events (<75 ppb) "occur during June–August indicating that elevated ozone levels are driven by regional photochemistry". [ 68 ] According to an article from the University of Colorado-Boulder, "Oil and natural gas VOC emission have a major role in ozone production and bear the potential to contribute to elevated O 3 levels in the Northern Colorado Front Range (NCFR)". [ 68 ] Using complex analyses to research wind patterns and emissions from large oil and natural gas operations, the authors concluded that "elevated O 3 levels in the NCFR are predominantly correlated with air transport from N– ESE, which are the upwind sectors where the O&NG operations in the Wattenberg Field area of the DJB are located". [ 68 ]
Contained in the Colorado Ozone Action Plan, created in 2008, plans exist to evaluate "emission controls for large industrial sources of NOx" and "statewide control requirements for new oil and gas condensate tanks and pneumatic valves". [ 69 ] In 2011, the Regional Haze Plan was released that included a more specific plan to help decrease NOx emissions. These efforts are increasingly difficult to implement and take many years to come to pass. Of course there are also other reasons that ozone levels remain high. These include: a growing population meaning more car emissions, and the mountains along the NCFR that can trap emissions. If interested, daily air quality readings can be found at the Colorado Department of Public Health and Environment's website. [ 70 ] As noted earlier, Denver continues to experience high levels of ozone to this day. It will take many years and a systems-thinking approach to combat this issue of high ozone levels in the Front Range of Colorado.
Ozone gas attacks any polymer possessing olefinic or double bonds within its chain structure, such as natural rubber , nitrile rubber , and styrene-butadiene rubber. Products made using these polymers are especially susceptible to attack, which causes cracks to grow longer and deeper with time, the rate of crack growth depending on the load carried by the rubber component and the concentration of ozone in the atmosphere. Such materials can be protected by adding antiozonants , such as waxes, which bond to the surface to create a protective film or blend with the material and provide long term protection. Ozone cracking used to be a serious problem in car tires, [ 71 ] for example, but it is not an issue with modern tires. On the other hand, many critical products, like gaskets and O-rings , may be attacked by ozone produced within compressed air systems. Fuel lines made of reinforced rubber are also susceptible to attack, especially within the engine compartment, where some ozone is produced by electrical components. Storing rubber products in close proximity to a DC electric motor can accelerate ozone cracking. The commutator of the motor generates sparks which in turn produce ozone.
Although ozone was present at ground level before the Industrial Revolution , peak concentrations are now far higher than the pre-industrial levels, and even background concentrations well away from sources of pollution are substantially higher. [ 73 ] [ 74 ] Ozone acts as a greenhouse gas , absorbing some of the infrared energy emitted by the earth. Quantifying the greenhouse gas potency of ozone is difficult because it is not present in uniform concentrations across the globe. However, the most widely accepted scientific assessments relating to climate change (e.g. the Intergovernmental Panel on Climate Change Third Assessment Report ) [ 75 ] suggest that the radiative forcing of tropospheric ozone is about 25% that of carbon dioxide .
The annual global warming potential of tropospheric ozone is between 918 and 1022 tons carbon dioxide equivalent /tons tropospheric ozone. This means on a per-molecule basis, ozone in the troposphere has a radiative forcing effect roughly 1,000 times as strong as carbon dioxide . However, tropospheric ozone is a short-lived greenhouse gas, which decays in the atmosphere much more quickly than carbon dioxide . This means that over a 20-year span, the global warming potential of tropospheric ozone is much less, roughly 62 to 69 tons carbon dioxide equivalent / ton tropospheric ozone. [ 76 ]
Because of its short-lived nature, tropospheric ozone does not have strong global effects, but has very strong radiative forcing effects on regional scales. In fact, there are regions of the world where tropospheric ozone has a radiative forcing up to 150% of carbon dioxide . [ 77 ] For example, ozone increase in the troposphere is shown to be responsible for ~30% of upper Southern Ocean interior warming between 1955 and 2000. [ 78 ]
Filters containing an adsorbent or catalyst such as charcoal (carbon) may be used to remove odors and gaseous pollutants such as volatile organic compounds or ozone. [ 79 ]
For the last few decades, scientists studied the effects of acute and chronic ozone exposure on human health. Hundreds of studies suggest that ozone is harmful to people at levels currently found in urban areas. [ 80 ] [ 81 ] Ozone has been shown to affect the respiratory, cardiovascular and central nervous system. Early death and problems in reproductive health and development are also shown to be associated with ozone exposure. [ 82 ]
The American Lung Association has identified five populations who are especially vulnerable to the effects of breathing ozone: [ 83 ]
Additional evidence suggests that women, those with obesity and low-income populations may also face higher risk from ozone, although more research is needed. [ 83 ]
Acute ozone exposure ranges from hours to a few days. Because ozone is a gas, it directly affects the lungs and the entire respiratory system. Inhaled ozone causes inflammation and acute—but reversible—changes in lung function, as well as airway hyperresponsiveness. [ 84 ] These changes lead to shortness of breath, wheezing, and coughing which may exacerbate lung diseases, like asthma or chronic obstructive pulmonary disease (COPD) resulting in the need to receive medical treatment. [ 85 ] [ 86 ] Acute and chronic exposure to ozone has been shown to cause an increased risk of respiratory infections, due to the following mechanism. [ 87 ]
Multiple studies have been conducted to determine the mechanism behind ozone's harmful effects, particularly in the lungs. These studies have shown that exposure to ozone causes changes in the immune response within the lung tissue, resulting in disruption of both the innate and adaptive immune response, as well as altering the protective function of lung epithelial cells. [ 88 ] It is thought that these changes in immune response and the related inflammatory response are factors that likely contribute to the increased risk of lung infections, and worsening or triggering of asthma and reactive airways after exposure to ground-level ozone pollution. [ 88 ] [ 89 ]
The innate (cellular) immune system consists of various chemical signals and cell types that work broadly and against multiple pathogen types, typically bacteria or foreign bodies/substances in the host. [ 89 ] [ 90 ] The cells of the innate system include phagocytes, neutrophils, [ 90 ] both thought to contribute to the mechanism of ozone pathology in the lungs, as the functioning of these cell types have been shown to change after exposure to ozone. [ 89 ] Macrophages, cells that serve the purpose of eliminating pathogens or foreign material through the process of "phagocytosis", [ 90 ] have been shown to change the level of inflammatory signals they release in response to ozone, either up-regulating and resulting in an inflammatory response in the lung, or down-regulating and reducing immune protection. [ 88 ] Neutrophils, another important cell type of the innate immune system that primarily targets bacterial pathogens, [ 90 ] are found to be present in the airways within 6 hours of exposure to high ozone levels. Despite high levels in the lung tissues, however, their ability to clear bacteria appears impaired by exposure to ozone. [ 88 ]
The adaptive immune system is the branch of immunity that provides long-term protection via the development of antibodies targeting specific pathogens and is also impacted by high ozone exposure. [ 89 ] [ 90 ] Lymphocytes, a cellular component of the adaptive immune response, produce an increased amount of inflammatory chemicals called "cytokines" after exposure to ozone, which may contribute to airway hyperreactivity and worsening asthma symptoms. [ 88 ]
The airway epithelial cells also play an important role in protecting individuals from pathogens. In normal tissue, the epithelial layer forms a protective barrier, and also contains specialized ciliary structures that work to clear foreign bodies, mucus and pathogens from the lungs. When exposed to ozone, the cilia become damaged and mucociliary clearance of pathogens is reduced. Furthermore, the epithelial barrier becomes weakened, allowing pathogens to cross the barrier, proliferate and spread into deeper tissues. Together, these changes in the epithelial barrier help make individuals more susceptible to pulmonary infections. [ 88 ]
Inhaling ozone not only affects the immune system and lungs, but it may also affect the heart as well. Ozone causes short-term autonomic imbalance leading to changes in heart rate and reduction in heart rate variability; [ 91 ] and high levels exposure for as little as one-hour results in a supraventricular arrhythmia in the elderly, [ 92 ] both increase the risk of premature death and stroke. Ozone may also lead to vasoconstriction resulting in increased systemic arterial pressure contributing to increased risk of cardiac morbidity and mortality in patients with pre-existing cardiac diseases. [ 93 ] [ 94 ]
Breathing ozone for periods longer than eight hours at a time for weeks, months or years defines chronic exposure. Numerous studies suggest a serious impact on the health of various populations from this exposure.
One study finds significant positive associations between chronic ozone and all-cause, circulatory, and respiratory mortality with 2%, 3%, and 12% increases in risk per 10 ppb [ 95 ] and report an association (95% CI) of annual ozone and all-cause mortality with a hazard ratio of 1.02 (1.01–1.04), and with cardiovascular mortality of 1.03 (1.01–1.05). A similar study finds similar associations with all-cause mortality and even larger effects for cardiovascular mortality. [ 96 ] An increased risk of mortality from respiratory causes is associated with long-term chronic exposure to ozone. [ 97 ]
Chronic ozone has detrimental effects on children, especially those with asthma. The risk for hospitalization in children with asthma increases with chronic exposure to ozone; younger children and those with low-income status are even at greater risk. [ 98 ]
Adults suffering from respiratory diseases (asthma, [ 99 ] COPD, [ 100 ] lung cancer [ 101 ] ) are at a higher risk of mortality and morbidity and critically ill patients have an increased risk of developing acute respiratory distress syndrome with chronic ozone exposure as well. [ 102 ]
Ozone generators sold as air cleaners intentionally produce the gas ozone. [ 43 ] These are often marketed to control indoor air pollution , and use misleading terms to describe ozone. Some examples are describing it as "energized oxygen" or "pure air", suggesting that ozone is a healthy or "better" kind of oxygen. [ 43 ] However, according to the EPA , "There is evidence to show that at concentrations that do not exceed public health standards, ozone is not effective at removing many odor-causing chemicals", and "If used at concentrations that do not exceed public health standards, ozone applied to indoor air does not effectively remove viruses, bacteria, mold, or other biological pollutants.". [ 43 ] Furthermore, another report states that "results of some controlled studies show that concentrations of ozone considerably higher than these [human safety] standards are possible even when a user follows the manufacturer's operating instructions". [ 103 ]
The California Air Resources Board has a page listing air cleaners (many with ionizers ) meeting their indoor ozone limit of 0.050 parts per million. [ 104 ] From that article:
All portable indoor air cleaning devices sold in California must be certified by the California Air Resources Board (CARB). To be certified, air cleaners must be tested for electrical safety and ozone emissions, and meet an ozone emission concentration limit of 0.050 parts per million. For more information about the regulation, visit the air cleaner regulation .
Ozone precursors are a group of pollutants, predominantly those emitted during the combustion of fossil fuels . Ground-level ozone pollution (tropospheric ozone) is produced near the Earth's surface by the action of daylight UV rays on these precursors. The ozone at ground level is primarily from fossil fuel precursors, but methane is a natural precursor, and the very low natural background level of ozone at ground level is considered safe. This section examines the health impacts of fossil fuel burning, which raises ground level ozone far above background levels.
There is a great deal of evidence to show that ground-level ozone can harm lung function and irritate the respiratory system . [ 57 ] [ 106 ] Exposure to ozone (and the pollutants that produce it) is linked to premature death , asthma , bronchitis , heart attack , and other cardiopulmonary problems. [ 107 ] [ 108 ]
Long-term exposure to ozone has been shown to increase risk of death from respiratory illness . [ 43 ] A study of 450,000 people living in U.S. cities saw a significant correlation between ozone levels and respiratory illness over the 18-year follow-up period. The study revealed that people living in cities with high ozone levels, such as Houston or Los Angeles, had an over 30% increased risk of dying from lung disease. [ 109 ] [ 110 ]
Air quality guidelines such as those from the World Health Organization , the U.S. Environmental Protection Agency (EPA), and the European Union are based on detailed studies designed to identify the levels that can cause measurable ill health effects .
According to scientists with the EPA, susceptible people can be adversely affected by ozone levels as low as 40 nmol/mol. [ 108 ] [ 111 ] [ 112 ] In the EU, the current target value for ozone concentrations is 120 μg/m 3 which is about 60 nmol/mol. This target applies to all member states in accordance with Directive 2008/50/EC. [ 113 ] Ozone concentration is measured as a maximum daily mean of 8 hour averages and the target should not be exceeded on more than 25 calendar days per year, starting from January 2010. While the directive requires in the future a strict compliance with 120 μg/m 3 limit (i.e. mean ozone concentration not to be exceeded on any day of the year), there is no date set for this requirement and this is treated as a long-term objective. [ 114 ]
In the US, the Clean Air Act directs the EPA to set National Ambient Air Quality Standards for several pollutants, including ground-level ozone, and counties out of compliance with these standards are required to take steps to reduce their levels. In May 2008, under a court order, the EPA lowered its ozone standard from 80 nmol/mol to 75 nmol/mol. The move proved controversial, since the Agency's own scientists and advisory board had recommended lowering the standard to 60 nmol/mol. [ 108 ] Many public health and environmental groups also supported the 60 nmol/mol standard, [ 115 ] and the World Health Organization recommends 100 μg/m 3 (51 nmol/mol). [ 116 ]
On January 7, 2010, the U.S. Environmental Protection Agency (EPA) announced proposed revisions to the National Ambient Air Quality Standard (NAAQS) for the pollutant ozone, the principal component of smog:
... EPA proposes that the level of the 8-hour primary standard, which was set at 0.075 μmol/mol in the 2008 final rule, should instead be set at a lower level within the range of 0.060 to 0.070 μmol/mol, to provide increased protection for children and other at risk populations against an array of O 3 – related adverse health effects that range from decreased lung function and increased respiratory symptoms to serious indicators of respiratory morbidity including emergency department visits and hospital admissions for respiratory causes, and possibly cardiovascular-related morbidity as well as total non- accidental and cardiopulmonary mortality ... [ 117 ]
On October 26, 2015, the EPA published a final rule with an effective date of December 28, 2015, that revised the 8-hour primary NAAQS from 0.075 ppm to 0.070 ppm. [ 118 ]
The EPA has developed an air quality index (AQI) to help explain air pollution levels to the general public. Under the current standards, eight-hour average ozone mole fractions of 85 to 104 nmol/mol are described as "unhealthy for sensitive groups", 105 nmol/mol to 124 nmol/mol as "unhealthy", and 125 nmol/mol to 404 nmol/mol as "very unhealthy". [ 119 ]
Ozone can also be present in indoor air pollution , partly as a result of electronic equipment such as photocopiers. A connection has also been known to exist between the increased pollen, fungal spores, and ozone caused by thunderstorms and hospital admissions of asthma sufferers. [ 120 ]
In the Victorian era , one British folk myth held that the smell of the sea was caused by ozone. In fact, the characteristic "smell of the sea" is caused by dimethyl sulfide , a chemical generated by phytoplankton . Victorian Britons considered the resulting smell "bracing". [ 121 ]
An investigation to assess the joint mortality effects of ozone and heat during the European heat waves in 2003, concluded that these appear to be additive. [ 122 ]
Ozone, along with reactive forms of oxygen such as superoxide , singlet oxygen , hydrogen peroxide , and hypochlorite ions, is produced by white blood cells and other biological systems (such as the roots of marigolds ) as a means of destroying foreign bodies. Ozone reacts directly with organic double bonds. Also, when ozone breaks down to dioxygen it gives rise to oxygen free radicals , which are highly reactive and capable of damaging many organic molecules . Moreover, it is believed that the powerful oxidizing properties of ozone may be a contributing factor of inflammation . The cause-and-effect relationship of how the ozone is created in the body and what it does is still under consideration and still subject to various interpretations, since other body chemical processes can trigger some of the same reactions. There is evidence linking the antibody-catalyzed water-oxidation pathway of the human immune response to the production of ozone. In this system, ozone is produced by antibody-catalyzed production of trioxidane from water and neutrophil-produced singlet oxygen. [ 123 ]
When inhaled, ozone reacts with compounds lining the lungs to form specific, cholesterol-derived metabolites that are thought to facilitate the build-up and pathogenesis of atherosclerotic plaques (a form of heart disease ). These metabolites have been confirmed as naturally occurring in human atherosclerotic arteries and are categorized into a class of secosterols termed atheronals , generated by ozonolysis of cholesterol's double bond to form a 5,6 secosterol [ 124 ] as well as a secondary condensation product via aldolization. [ 125 ]
Ozone has been implicated to have an adverse effect on plant growth: "... ozone reduced total chlorophylls, carotenoid and carbohydrate concentration, and increased 1-aminocyclopropane-1-carboxylic acid (ACC) content and ethylene production. In treated plants, the ascorbate leaf pool was decreased, while lipid peroxidation and solute leakage were significantly higher than in ozone-free controls. The data indicated that ozone triggered protective mechanisms against oxidative stress in citrus." [ 126 ] Studies that have used pepper plants as a model have shown that ozone decreased fruit yield and changed fruit quality. [ 127 ] [ 128 ] Furthermore, it was also observed a decrease in chlorophylls levels and antioxidant defences on the leaves, as well as increased the reactive oxygen species (ROS) levels and lipid and protein damages. [ 127 ] [ 128 ]
A 2022 study concludes that East Asia loses 63 billion dollars in crops per year due to ozone pollution, a byproduct of fossil fuel combustion. China loses about one-third of its potential wheat production and one-fourth of its rice production. [ 129 ] [ 130 ]
Because of the strongly oxidizing properties of ozone, ozone is a primary irritant, affecting especially the eyes and respiratory systems and can be hazardous at even low concentrations. The Canadian Centre for Occupation Safety and Health reports that:
Even very low concentrations of ozone can be harmful to the upper respiratory tract and the lungs. The severity of injury depends on both the concentration of ozone and the duration of exposure. Severe and permanent lung injury or death could result from even a very short-term exposure to relatively low concentrations." [ 131 ]
To protect workers potentially exposed to ozone, U.S. Occupational Safety and Health Administration has established a permissible exposure limit (PEL) of 0.1 μmol/mol (29 CFR 1910.1000 table Z-1), calculated as an 8-hour time weighted average. Higher concentrations are especially hazardous and NIOSH has established an Immediately Dangerous to Life and Health Limit (IDLH) of 5 μmol/mol. [ 132 ] Work environments where ozone is used or where it is likely to be produced should have adequate ventilation and it is prudent to have a monitor for ozone that will alarm if the concentration exceeds the OSHA PEL. Continuous monitors for ozone are available from several suppliers.
Elevated ozone exposure can occur on passenger aircraft , with levels depending on altitude and atmospheric turbulence. [ 133 ] U.S. Federal Aviation Administration regulations set a limit of 250 nmol/mol with a maximum four-hour average of 100 nmol/mol. [ 134 ] Some planes are equipped with ozone converters in the ventilation system to reduce passenger exposure. [ 133 ]
Ozone generators , or ozonators , [ 135 ] are used to produce ozone for cleaning air or removing smoke odours in unoccupied rooms. These ozone generators can produce over 3 g of ozone per hour. Ozone often forms in nature under conditions where O 2 will not react. [ 29 ] Ozone used in industry is measured in μmol/mol (ppm, parts per million), nmol/mol (ppb, parts per billion), μg/m 3 , mg/h (milligrams per hour) or weight percent. The regime of applied concentrations ranges from 1% to 5% (in air) and from 6% to 14% (in oxygen) for older generation methods. New electrolytic methods can achieve up 20% to 30% dissolved ozone concentrations in output water.
Temperature and humidity play a large role in how much ozone is being produced using traditional generation methods (such as corona discharge and ultraviolet light). Old generation methods will produce less than 50% of nominal capacity if operated with humid ambient air, as opposed to very dry air. New generators, using electrolytic methods, can achieve higher purity and dissolution through using water molecules as the source of ozone production.
This is the most common type of ozone generator for most industrial and personal uses. While variations of the "hot spark" coronal discharge method of ozone production exist, including medical grade and industrial grade ozone generators, these units usually work by means of a corona discharge tube or ozone plate. [ 136 ] [ 137 ] They are typically cost-effective and do not require an oxygen source other than the ambient air to produce ozone concentrations of 3–6%. Fluctuations in ambient air, due to weather or other environmental conditions, cause variability in ozone production. However, they also produce nitrogen oxides as a by-product. Use of an air dryer can reduce or eliminate nitric acid formation by removing water vapor and increase ozone production. At room temperature, nitric acid will form into a vapour that is hazardous if inhaled. Symptoms can include chest pain, shortness of breath, headaches and a dry nose and throat causing a burning sensation. Use of an oxygen concentrator can further increase the ozone production and further reduce the risk of nitric acid formation by removing not only the water vapor, but also the bulk of the nitrogen.
UV ozone generators, or vacuum-ultraviolet (VUV) ozone generators, employ a light source that generates a narrow-band ultraviolet light, a subset of that produced by the Sun. The Sun's UV sustains the ozone layer in the stratosphere of Earth. [ 138 ]
UV ozone generators use ambient air for ozone production, no air prep systems are used (air dryer or oxygen concentrator), therefore these generators tend to be less expensive. However, UV ozone generators usually produce ozone with a concentration of about 0.5% or lower which limits the potential ozone production rate. Another disadvantage of this method is that it requires the ambient air (oxygen) to be exposed to the UV source for a longer amount of time, and any gas that is not exposed to the UV source will not be treated. This makes UV generators impractical for use in situations that deal with rapidly moving air or water streams (in-duct air sterilization , for example). Production of ozone is one of the potential dangers of ultraviolet germicidal irradiation . VUV ozone generators are used in swimming pools and spa applications ranging to millions of gallons of water. VUV ozone generators, unlike corona discharge generators, do not produce harmful nitrogen by-products and also unlike corona discharge systems, VUV ozone generators work extremely well in humid air environments. There is also not normally a need for expensive off-gas mechanisms, and no need for air driers or oxygen concentrators which require extra costs and maintenance.
In the cold plasma method, pure oxygen gas is exposed to a plasma created by DBD . The diatomic oxygen is split into single atoms, which then recombine in triplets to form ozone.
It is common in the industry to mislabel some DBD ozone generators as CD Corona Discharge generators. Typically all solid flat metal electrode ozone generators produce ozone using the dielectric barrier discharge method. Cold plasma machines use pure oxygen as the input source and produce a maximum concentration of about 24% ozone. They produce far greater quantities of ozone in a given time compared to ultraviolet production that has about 2% efficiency. The discharges manifest as filamentary transfer of electrons (micro discharges) in a gap between two electrodes. In order to evenly distribute the micro discharges, a dielectric insulator must be used to separate the metallic electrodes and to prevent arcing.
Electrolytic ozone generation (EOG) splits water molecules into H 2 , O 2 , and O 3 .
In most EOG methods, the hydrogen gas will be removed to leave oxygen and ozone as the only reaction products. Therefore, EOG can achieve higher dissolution in water without other competing gases found in corona discharge method, such as nitrogen gases present in ambient air. This method of generation can achieve concentrations of 20–30% and is independent of air quality because water is used as the source material. Production of ozone electrolytically is typically unfavorable because of the high overpotential required to produce ozone as compared to oxygen. This is why ozone is not produced during typical water electrolysis. However, it is possible to increase the overpotential of oxygen by careful catalyst selection such that ozone is preferentially produced under electrolysis. Catalysts typically chosen for this approach are lead dioxide [ 139 ] or boron-doped diamond. [ 140 ]
The ozone-to-oxygen ratio is improved by increasing current density at the anode, cooling the electrolyte around the anode close to 0 °C, using an acidic electrolyte (such as dilute sulfuric acid) instead of a basic solution, and by applying pulsed current instead of DC. [ 141 ]
Ozone cannot be stored and transported like other industrial gases (because it quickly decays into diatomic oxygen) and must therefore be produced on site. Available ozone generators vary in the arrangement and design of the high-voltage electrodes. At production capacities higher than 20 kg per hour, a gas/water tube heat-exchanger may be utilized as ground electrode and assembled with tubular high-voltage electrodes on the gas-side. The regime of typical gas pressures is around 2 bars (200 kPa ) absolute in oxygen and 3 bars (300 kPa) absolute in air. Several megawatts of electrical power may be installed in large facilities, applied as single phase AC current at 50 to 8000 Hz and peak voltages between 3,000 and 20,000 volts. Applied voltage is usually inversely related to the applied frequency.
The dominating parameter influencing ozone generation efficiency is the gas temperature, which is controlled by cooling water temperature and/or gas velocity. The cooler the water, the better the ozone synthesis. The lower the gas velocity, the higher the concentration (but the lower the net ozone produced). At typical industrial conditions, almost 90% of the effective power is dissipated as heat and needs to be removed by a sufficient cooling water flow.
Because of the high reactivity of ozone, only a few materials may be used like stainless steel (quality 316L), titanium , aluminium (as long as no moisture is present), glass , polytetrafluorethylene , or polyvinylidene fluoride . Viton may be used with the restriction of constant mechanical forces and absence of humidity (humidity limitations apply depending on the formulation). Hypalon may be used with the restriction that no water comes in contact with it, except for normal atmospheric levels. Embrittlement or shrinkage is the common mode of failure of elastomers with exposure to ozone. Ozone cracking is the common mode of failure of elastomer seals like O-rings .
Silicone rubbers are usually adequate for use as gaskets in ozone concentrations below 1 wt%, such as in equipment for accelerated aging of rubber samples.
Ozone may be formed from O 2 by electrical discharges and by action of high energy electromagnetic radiation . Unsuppressed arcing in electrical contacts, motor brushes, or mechanical switches breaks down the chemical bonds of the atmospheric oxygen surrounding the contacts [ O 2 -> 2O]. Free radicals of oxygen in and around the arc recombine to create ozone [ O 3 ]. [ 142 ] Certain electrical equipment generate significant levels of ozone. This is especially true of devices using high voltages , such as ionic air purifiers , laser printers , photocopiers , tasers , and arc welders . Electric motors using brushes can generate ozone from repeated sparking inside the unit. Large motors that use brushes, such as those used by elevators or hydraulic pumps, will generate more ozone than smaller motors.
Ozone is similarly formed in the Catatumbo lightning storms phenomenon on the Catatumbo River in Venezuela , though ozone's instability makes it dubious that it has any effect on the ozonosphere. [ 143 ] It is the world's largest single natural generator of ozone, lending calls for it to be designated a UNESCO World Heritage Site . [ 144 ]
In the laboratory, ozone can be produced by electrolysis using a 9 volt battery , a pencil graphite rod cathode , a platinum wire anode , and a 3 molar sulfuric acid electrolyte . [ 145 ] The half cell reactions taking place are:
where E° represents the standard electrode potential .
In the net reaction, three equivalents of water are converted into one equivalent of ozone and three equivalents of hydrogen . Oxygen formation is a competing reaction.
It can also be generated by a high voltage arc . In its simplest form, high voltage AC, such as the output of a neon-sign transformer is connected to two metal rods with the ends placed sufficiently close to each other to allow an arc. The resulting arc will convert atmospheric oxygen to ozone.
It is often desirable to contain the ozone. This can be done with an apparatus consisting of two concentric glass tubes sealed together at the top with gas ports at the top and bottom of the outer tube. The inner core should have a length of metal foil inserted into it connected to one side of the power source. The other side of the power source should be connected to another piece of foil wrapped around the outer tube. A source of dry O 2 is applied to the bottom port. When high voltage is applied to the foil leads, electricity will discharge between the dry dioxygen in the middle and form O 3 and O 2 which will flow out the top port. This is called a Siemen's ozoniser. The reaction can be summarized as follows: [ 29 ]
The largest use of ozone is in the preparation of pharmaceuticals , synthetic lubricants , and many other commercially useful organic compounds , where it is used to sever carbon -carbon bonds. [ 29 ] It can also be used for bleaching substances and for killing microorganisms in air and water sources. [ 146 ] Many municipal drinking water systems kill bacteria with ozone instead of the more common chlorine . [ 147 ] Ozone has a very high oxidation potential . [ 148 ] Ozone does not form organochlorine compounds, nor does it remain in the water after treatment. Ozone can form the suspected carcinogen bromate in source water with high bromide concentrations. The U.S. Safe Drinking Water Act mandates that these systems introduce an amount of chlorine to maintain a minimum of 0.2 μmol/mol residual free chlorine in the pipes, based on results of regular testing. Where electrical power is abundant, ozone is a cost-effective method of treating water, since it is produced on demand and does not require transportation and storage of hazardous chemicals. Once it has decayed, it leaves no taste or odour in drinking water.
Although low levels of ozone have been advertised to be of some disinfectant use in residential homes, the concentration of ozone in dry air required to have a rapid, substantial effect on airborne pathogens exceeds safe levels recommended by the U.S. Occupational Safety and Health Administration and Environmental Protection Agency . Humidity control can vastly improve both the killing power of the ozone and the rate at which it decays back to oxygen (more humidity allows more effectiveness). Spore forms of most pathogens are very tolerant of atmospheric ozone in concentrations at which asthma patients start to have issues.
In 1908 artificial ozonisation of the Central Line of the London Underground was introduced for aerial disinfection. The process was found to be worthwhile, but was phased out by 1956. However the beneficial effect was maintained by the ozone created incidentally from the electrical discharges of the train motors (see above: Incidental production ). [ 149 ]
Ozone generators were made available to schools and universities in Wales for the Autumn term 2021, to disinfect classrooms after COVID-19 outbreaks. [ 150 ]
Industrially, ozone is used to:
Ozone is a reagent in many organic reactions in the laboratory and in industry. Ozonolysis is the cleavage of an alkene to carbonyl compounds.
Many hospitals around the world use large ozone generators to decontaminate operating rooms between surgeries. The rooms are cleaned and then sealed airtight before being filled with ozone which effectively kills or neutralizes all remaining bacteria. [ 156 ]
Ozone is used as an alternative to chlorine or chlorine dioxide in the bleaching of wood pulp . [ 157 ] It is often used in conjunction with oxygen and hydrogen peroxide to eliminate the need for chlorine-containing compounds in the manufacture of high-quality, white paper . [ 158 ]
Ozone can be used to detoxify cyanide wastes (for example from gold and silver mining ) by oxidizing cyanide to cyanate and eventually to carbon dioxide . [ 159 ]
Since the invention of dielectric barrier discharge (DBD) plasma reactors, it has been employed for water treatment with ozone. [ 160 ] However, with cheaper alternative disinfectants like chlorine, such applications of DBD ozone water decontamination have been limited by high power consumption and bulky equipment. [ 161 ] [ 162 ] Despite this, with research revealing the negative impacts of common disinfectants like chlorine with respect to toxic residuals and ineffectiveness in killing certain micro-organisms, [ 163 ] DBD plasma-based ozone decontamination is of interest in current available technologies. Although ozonation of water with a high concentration of bromide does lead to the formation of undesirable brominated disinfection byproducts, unless drinking water is produced by desalination, ozonation can generally be applied without concern for these byproducts. [ 162 ] [ 164 ] [ 165 ] [ 166 ] Advantages of ozone include high thermodynamic oxidation potential, less sensitivity to organic material and better tolerance for pH variations while retaining the ability to kill bacteria, fungi, viruses, as well as spores and cysts. [ 167 ] [ 168 ] [ 169 ] Although, ozone has been widely accepted in Europe for decades, it is sparingly used for decontamination in the U.S. due to limitations of high-power consumption, bulky installation and stigma attached with ozone toxicity. [ 161 ] [ 170 ] Considering this, recent research efforts have been directed toward the study of effective ozone water treatment systems. [ 171 ] Researchers have looked into lightweight and compact low power surface DBD reactors, [ 172 ] [ 173 ] energy efficient volume DBD reactors [ 174 ] and low power micro-scale DBD reactors. [ 175 ] [ 176 ] Such studies can help pave the path to re-acceptance of DBD plasma-based ozone decontamination of water, especially in the U.S.
Ozone levels which are safe for people are ineffective at killing fungi and bacteria. [ 177 ] Some consumer disinfection and cosmetic products emit ozone at levels harmful to human health. [ 177 ]
Devices generating high levels of ozone, some of which use ionization, are used to sanitize and deodorize uninhabited buildings, rooms, ductwork, woodsheds, boats, and other vehicles.
Ozonated water is used to launder clothes and to sanitize food, drinking water, and surfaces in the home. According to the U.S. Food and Drug Administration (FDA), it is "amending the food additive regulations to provide for the safe use of ozone in gaseous and aqueous phases as an antimicrobial agent on food, including meat and poultry." Studies at California Polytechnic University demonstrated that 0.3 μmol/mol levels of ozone dissolved in filtered tapwater can produce a reduction of more than 99.99% in such food-borne microorganisms as salmonella, E. coli 0157:H7 and Campylobacter . This quantity is 20,000 times the WHO -recommended limits stated above. [ 152 ] [ 178 ]
Ozone can be used to remove pesticide residues from fruits and vegetables . [ 179 ] [ 180 ]
Ozone is used in homes and hot tubs to kill bacteria in the water and to reduce the amount of chlorine or bromine required by reactivating them to their free state. Since ozone does not remain in the water long enough, ozone by itself is ineffective at preventing cross-contamination among bathers and must be used in conjunction with halogens . Gaseous ozone created by ultraviolet light or by corona discharge is injected into the water. [ 181 ]
Ozone is also widely used in the treatment of water in aquariums and fishponds. Its use can minimize bacterial growth, control parasites, eliminate transmission of some diseases, and reduce or eliminate "yellowing" of the water. Ozone must not come in contact with fishes' gill structures. Natural saltwater (with life forms) provides enough "instantaneous demand" that controlled amounts of ozone activate bromide ions to hypobromous acid , and the ozone entirely decays in a few seconds to minutes. If oxygen-fed ozone is used, the water will be higher in dissolved oxygen and fishes' gill structures will atrophy, making them dependent on oxygen-enriched water.
Ozonation – a process of infusing water with ozone – can be used in aquaculture to facilitate organic breakdown. Ozone is also added to recirculating systems to reduce nitrite levels [ 182 ] through conversion into nitrate . If nitrite levels in the water are high, nitrites will also accumulate in the blood and tissues of fish, where it interferes with oxygen transport (it causes oxidation of the heme-group of haemoglobin from ferrous ( Fe 2+ ) to ferric ( Fe 3+ ), making haemoglobin unable to bind O 2 ). [ 183 ] Despite these apparent positive effects, ozone use in recirculation systems has been linked to reducing the level of bioavailable iodine in salt water systems, resulting in iodine deficiency symptoms such as goitre and decreased growth in Senegalese sole ( Solea senegalensis ) larvae. [ 184 ]
Ozonate seawater is used for surface disinfection of haddock and Atlantic halibut eggs against nodavirus. Nodavirus is a lethal and vertically transmitted virus which causes severe mortality in fish. Haddock eggs should not be treated with high ozone level as eggs so treated did not hatch and died after 3–4 days. [ 185 ]
Ozone application on freshly cut pineapple and banana shows increase in flavonoids and total phenol contents when exposure is up to 20 minutes. Decrease in ascorbic acid (one form of vitamin C ) content is observed but the positive effect on total phenol content and flavonoids can overcome the negative effect. [ 186 ] Tomatoes upon treatment with ozone show an increase in β-carotene, lutein and lycopene. [ 187 ] However, ozone application on strawberries in pre-harvest period shows decrease in ascorbic acid content. [ 188 ]
Ozone facilitates the extraction of some heavy metals from soil using EDTA . EDTA forms strong, water-soluble coordination compounds with some heavy metals ( Pb and Zn ) thereby making it possible to dissolve them out from contaminated soil. If contaminated soil is pre-treated with ozone, the extraction efficacy of Pb , Am , and Pu increases by 11.0–28.9%, [ 189 ] 43.5% [ 190 ] and 50.7% [ 190 ] respectively.
Crop pollination is an essential part of an ecosystem. Ozone can have detrimental effects on plant-pollinator interactions. [ 191 ] Pollinators carry pollen from one plant to another. This is an essential cycle inside of an ecosystem. Causing changes in certain atmospheric conditions around pollination sites or with xenobiotics could cause unknown changes to the natural cycles of pollinators and flowering plants. In a study conducted in North-Western Europe, crop pollinators were negatively affected more when ozone levels were higher. [ 192 ]
The use of ozone for the treatment of medical conditions is not supported by high quality evidence, and is generally considered alternative medicine . [ 193 ]
Footnotes
Citations
Nascent oxygen O
Dioxygen ( singlet and triplet ) O 2
Trioxygen ( ozone and cyclic ozone ) O 3
Tetraoxygen O 4
Octaoxygen O 8 | https://en.wikipedia.org/wiki/O⚍O⚍O |
Ozone ( / ˈ oʊ z oʊ n / ) (or trioxygen ) is an inorganic molecule with the chemical formula O 3 . It is a pale blue gas with a distinctively pungent smell. It is an allotrope of oxygen that is much less stable than the diatomic allotrope O 2 , breaking down in the lower atmosphere to O 2 ( dioxygen ). Ozone is formed from dioxygen by the action of ultraviolet (UV) light and electrical discharges within the Earth's atmosphere . It is present in very low concentrations throughout the atmosphere, with its highest concentration high in the ozone layer of the stratosphere , which absorbs most of the Sun 's ultraviolet (UV) radiation.
Ozone's odor is reminiscent of chlorine , and detectable by many people at concentrations of as little as 0.1 ppm in air. Ozone's O 3 structure was determined in 1865. The molecule was later proven to have a bent structure and to be weakly diamagnetic . At standard temperature and pressure , ozone is a pale blue gas that condenses at cryogenic temperatures to a dark blue liquid and finally a violet-black solid . Ozone's instability with regard to more common dioxygen is such that both concentrated gas and liquid ozone may decompose explosively at elevated temperatures, physical shock, or fast warming to the boiling point. [ 5 ] [ 6 ] It is therefore used commercially only in low concentrations.
Ozone is a powerful oxidizing agent (far more so than dioxygen) and has many industrial and consumer applications related to oxidation. This same high oxidizing potential, however, causes ozone to damage mucous and respiratory tissues in animals, and also tissues in plants, above concentrations of about 0.1 ppm . While this makes ozone a potent respiratory hazard and pollutant near ground level , a higher concentration in the ozone layer (from two to eight ppm) is beneficial, preventing damaging UV light from reaching the Earth's surface.
The trivial name ozone is the most commonly used and preferred IUPAC name . The systematic names 2λ 4 -trioxidiene [ dubious – discuss ] and catena-trioxygen , valid IUPAC names, are constructed according to the substitutive and additive nomenclatures , respectively. The name ozone derives from ozein (ὄζειν), the Greek neuter present participle for smell, [ 7 ] referring to ozone's distinctive smell.
In appropriate contexts, ozone can be viewed as trioxidane with two hydrogen atoms removed, and as such, trioxidanylidene may be used as a systematic name, according to substitutive nomenclature. By default, these names pay no regard to the radicality of the ozone molecule. In an even more specific context, this can also name the non-radical singlet ground state, whereas the diradical state is named trioxidanediyl .
Trioxidanediyl (or ozonide ) is used, non-systematically, to refer to the substituent group (-OOO-). Care should be taken to avoid confusing the name of the group for the context-specific name for the ozone given above.
In 1785, Dutch chemist Martinus van Marum was conducting experiments involving electrical sparking above water when he noticed an unusual smell, which he attributed to the electrical reactions, failing to realize that he had in fact produced ozone. [ 8 ] [ 9 ]
A half century later, Christian Friedrich Schönbein noticed the same pungent odour and recognized it as the smell often following a bolt of lightning . In 1839, he succeeded in isolating the gaseous chemical and named it "ozone", from the Greek word ozein ( ὄζειν ) meaning "to smell". [ 10 ] [ 11 ] For this reason, Schönbein is generally credited with the discovery of ozone. [ 12 ] [ 13 ] [ 14 ] [ 8 ] He also noted the similarity of ozone smell to the smell of phosphorus, and in 1844 proved that the product of reaction of white phosphorus with air is identical. [ 10 ] A subsequent effort to call ozone "electrified oxygen" he ridiculed by proposing to call the ozone from white phosphorus "phosphorized oxygen". [ 10 ] The chemical formula for ozone, O 3 , was not determined until 1865 by Jacques-Louis Soret [ 15 ] and confirmed by Schönbein in 1867. [ 10 ] [ 16 ]
For much of the second half of the 19th century and well into the 20th, ozone was considered a healthy component of the environment by naturalists and health-seekers. Beaumont, California , had as its official slogan "Beaumont: Zone of Ozone", as evidenced on postcards and Chamber of Commerce letterhead. [ 17 ] Naturalists working outdoors often considered the higher elevations beneficial because of their ozone content which was readily monitored. [ 18 ] "There is quite a different atmosphere [at higher elevation] with enough ozone to sustain the necessary energy [to work]", wrote naturalist Henry Henshaw , working in Hawaii. [ 19 ] Seaside air was considered to be healthy because of its believed ozone content. The smell giving rise to this belief is in fact that of halogenated seaweed metabolites [ 20 ] and dimethyl sulfide . [ 21 ]
Much of ozone's appeal seems to have resulted from its "fresh" smell, which evoked associations with purifying properties. Scientists noted its harmful effects. In 1873 James Dewar and John Gray McKendrick documented that frogs grew sluggish, birds gasped for breath, and rabbits' blood showed decreased levels of oxygen after exposure to "ozonized air", which "exercised a destructive action". [ 22 ] [ 12 ] Schönbein himself reported that chest pains, irritation of the mucous membranes , and difficulty breathing occurred as a result of inhaling ozone, and small mammals died. [ 23 ] In 1911, Leonard Hill and Martin Flack stated in the Proceedings of the Royal Society B that ozone's healthful effects "have, by mere iteration, become part and parcel of common belief; and yet exact physiological evidence in favour of its good effects has been hitherto almost entirely wanting ... The only thoroughly well-ascertained knowledge concerning the physiological effect of ozone, so far attained, is that it causes irritation and œdema of the lungs, and death if inhaled in relatively strong concentration for any time." [ 12 ] [ 24 ]
During World War I , ozone was tested at Queen Alexandra Military Hospital in London as a possible disinfectant for wounds. The gas was applied directly to wounds for as long as 15 minutes. This resulted in damage to both bacterial cells and human tissue. Other sanitizing techniques, such as irrigation with antiseptics , were found preferable. [ 12 ] [ 25 ]
Until the 1920s, it was not certain whether small amounts of oxozone , O 4 , were also present in ozone samples due to the difficulty of applying analytical chemistry techniques to the explosive concentrated chemical. [ 26 ] [ 27 ] In 1923, Georg-Maria Schwab (working for his doctoral thesis under Ernst Hermann Riesenfeld ) was the first to successfully solidify ozone and perform accurate analysis which conclusively refuted the oxozone hypothesis. [ 26 ] [ 27 ] Further hitherto unmeasured physical properties of pure concentrated ozone were determined by the Riesenfeld group in the 1920s. [ 26 ]
Ozone is a colourless or pale blue gas, slightly soluble in water, and much more soluble in inert non-polar solvents such as carbon tetrachloride or fluorocarbons, in which it forms a blue solution. At 161 K (−112 °C; −170 °F), it condenses to form a dark blue liquid . It is dangerous to allow this liquid to warm to its boiling point, because both concentrated gaseous ozone and liquid ozone can detonate. At temperatures below 80 K (−193.2 °C; −315.7 °F), it forms a violet-black solid . [ 28 ]
Ozone has a very specific sharp odour somewhat resembling chlorine bleach . Most people can detect it at the 0.01 μmol/mol level in air. Exposure of 0.1 to 1 μmol/mol produces headaches and burning eyes and irritates the respiratory passages. [ 29 ] Even low concentrations of ozone in air are very destructive to organic materials such as latex, plastics, and animal lung tissue.
The ozone molecule is weakly diamagnetic . [ 30 ]
According to experimental evidence from microwave spectroscopy , ozone is a bent molecule, with C 2v symmetry (similar to the water molecule). [ 31 ] The O–O distances are 127.2 pm (1.272 Å ). The O–O–O angle is 116.78°. [ 32 ] The central atom is sp ² hybridized with one lone pair. Ozone is a polar molecule with a dipole moment of 0.53 D . [ 33 ] The molecule can be represented as a resonance hybrid with two contributing structures, each with a single bond on one side and double bond on the other. The arrangement possesses an overall bond order of 1.5 for both sides. It is isoelectronic with the nitrite anion . Naturally occurring ozone can be composed of substituted isotopes ( 16 O, 17 O, 18 O). A cyclic form has been predicted but not observed.
Ozone is among the most powerful oxidizing agents known, far stronger than O 2 . It is also unstable at high concentrations, decaying into ordinary diatomic oxygen. Its half-life varies with atmospheric conditions such as temperature, humidity, and air movement. Under laboratory conditions, the half-life will average ~1500 minutes (25 hours) in still air at room temperature (24 °C), zero humidity with zero air changes per hour. [ 34 ]
This reaction proceeds more rapidly with increasing temperature. Deflagration of ozone can be triggered by a spark and can occur in ozone concentrations of 10 wt% or higher. [ 35 ]
Ozone can also be produced from oxygen at the anode of an electrochemical cell. This reaction can create smaller quantities of ozone for research purposes. [ 36 ]
This can be observed as an unwanted reaction in a Hoffman apparatus during the electrolysis of water when the voltage is set above the necessary voltage.
Ozone oxidizes most metals (except gold , platinum , and iridium ) into oxides of the metals in their highest oxidation state . For example:
Ozone oxidizes nitric oxide to nitrogen dioxide :
This reaction is accompanied by chemiluminescence .
The NO 2 can be further oxidized to nitrate radical :
The NO 3 formed can react with NO 2 to form dinitrogen pentoxide ( N 2 O 5 ).
Solid nitronium perchlorate can be made from NO 2 , ClO 2 , and O 3 gases:
Ozone does not react with ammonium salts , but it oxidizes ammonia to ammonium nitrate :
Ozone reacts with carbon to form carbon dioxide , even at room temperature:
Ozone oxidizes sulfides to sulfates . For example, lead(II) sulfide is oxidized to lead(II) sulfate :
Sulfuric acid can be produced from ozone, water and either elemental sulfur or sulfur dioxide :
In the gas phase , ozone reacts with hydrogen sulfide to form sulfur dioxide:
In an aqueous solution, however, two competing simultaneous reactions occur, one to produce elemental sulfur, and one to produce sulfuric acid :
Alkenes can be oxidatively cleaved by ozone, in a process called ozonolysis , giving alcohols, aldehydes, ketones, and carboxylic acids, depending on the second step of the workup.
Ozone can also cleave alkynes to form an acid anhydride or diketone product. [ 38 ] If the reaction is performed in the presence of water, the anhydride hydrolyzes to give two carboxylic acids .
Usually ozonolysis is carried out in a solution of dichloromethane , at a temperature of −78 °C. After a sequence of cleavage and rearrangement, an organic ozonide is formed. With reductive workup (e.g. zinc in acetic acid or dimethyl sulfide ), ketones and aldehydes will be formed, with oxidative workup (e.g. aqueous or alcoholic hydrogen peroxide ), carboxylic acids will be formed. [ 39 ]
All three atoms of ozone may also react, as in the reaction of tin(II) chloride with hydrochloric acid and ozone:
Iodine perchlorate can be made by treating iodine dissolved in cold anhydrous perchloric acid with ozone:
Ozone could also react with potassium iodide to give oxygen and iodine gas that can be titrated for quantitative determination: [ 40 ]
Ozone can be used for combustion reactions and combustible gases; ozone provides higher temperatures than burning in dioxygen ( O 2 ). The following is a reaction for the combustion of carbon subnitride which can also cause higher temperatures:
Ozone can react at cryogenic temperatures. At 77 K (−196.2 °C; −321.1 °F), atomic hydrogen reacts with liquid ozone to form a hydrogen superoxide radical , which dimerizes : [ 41 ]
Ozone is a toxic substance, [ 42 ] [ 43 ] commonly found or generated in human environments (aircraft cabins, offices with photocopiers, laser printers, sterilizers, ...).
The catalytic decomposition of ozone is very important to reduce pollution. This type of decomposition is the most widely used, especially with solid catalysts, and it has many advantages such as a higher conversion with a lower temperature. Furthermore, the product and the catalyst can be instantaneously separated, and this way the catalyst can be easily recovered without using any separation operation. The most-used materials in the catalytic decomposition of ozone in the gas phase are manganese dioxide , transition metals such as Mn, Co, Cu, Fe, Ni, or Ag, and noble metals such as Pt, Rh, or Pd.
Free radicals of chlorine (Cl · ), formed by the action of ultraviolet radiation on chlorofluorocarbons (CFCs) and sea salt, are known to catalyze the breakdown of ozone in the atmosphere.
There are two other possibilities for decomposing ozone in the gas phase:
The uncatalyzed process of ozone decomposition in the gas phase is a complex reaction involving two elementary reactions that finally lead to molecular oxygen, [ 45 ] and this means that the reaction order and the rate law cannot be determined by the stoichiometry of the overall reaction.
Overall reaction: 2 O 3 ⟶ 3 O 2 {\displaystyle {\ce {2 O3 -> 3 O2}}}
Rate law (observed): V = K o b s ⋅ [ O 3 ] 2 [ O 2 ] {\displaystyle V={\frac {K_{obs}\cdot [{\ce {O3}}]^{2}}{[{\ce {O2}}]}}}
where K o b s {\displaystyle K_{obs}} is the observed rate constant and V {\displaystyle V} is the reaction rate. From the rate law above it can be determined that the partial order respect to molecular oxygen is −1 and respect to ozone is 2; therefore, the global reaction order is 1.
The first step is a unimolecular reaction wherein one molecule of ozone decomposes into two products (molecular oxygen and oxygen). The oxygen atom from the first step is a reactive intermediate because it participates as a reactant in the second step, which is a bimolecular reaction because there are two different reactants (ozone and oxygen) that give rise to molecular oxygen.
Step 1: Unimolecular reaction O 3 ⟶ O 2 + O {\displaystyle {\ce {O3 -> O2 + O}}}
Step 2: Bimolecular reaction O 3 + O ⟶ 2 O 2 {\displaystyle {\ce {O3 + O -> 2 O2}}}
These two steps have different reaction rates and rate constants. The reaction rate laws for each of these steps are shown below:
The following mechanism allows to explain the rate law of the ozone decomposition observed experimentally, and also it allows to determine the reaction orders with respect to ozone and oxygen, with which the overall reaction order will be determined.
The first step is assumed reversible and faster than the second reaction, which means that the slower rate determining step is the second reaction. This step determines the rate of product formation, and so V = V 2 {\displaystyle V=V_{2}} . However, this equation depends on the concentration of oxygen (intermediate), which does not appear in the observed rate law. Since the first step is a rapid equilibrium, the concentration of the intermediate can be determined as follows:
Then using these equations, the formation rate of molecular oxygen is as shown below:
The mechanism is consistent with the rate law observed experimentally if the rate constant ( K obs ) is given in terms of the individual mechanistic steps' rate constants as follows: [ 46 ]
where K obs = K 2 ⋅ K 1 K − 1 {\displaystyle K_{\text{obs}}={K_{2}\cdot K_{1} \over K_{-1}}}
Reduction of ozone gives the ozonide anion, O − 3 . Derivatives of this anion are explosive and must be stored at cryogenic temperatures. Ozonides for all the alkali metals are known. KO 3 , RbO 3 , and CsO 3 can be prepared from their respective superoxides:
Although KO 3 can be formed as above, it can also be formed from potassium hydroxide and ozone: [ 47 ]
NaO 3 and LiO 3 must be prepared by action of CsO 3 in liquid NH 3 on an ion-exchange resin containing Na + or Li + ions: [ 48 ]
A solution of calcium in ammonia reacts with ozone to give ammonium ozonide and not calcium ozonide: [ 41 ]
Ozone can be used to remove iron and manganese from water , forming a precipitate which can be filtered:
Ozone oxidizes dissolved hydrogen sulfide in water to sulfurous acid :
These three reactions are central in the use of ozone-based well water treatment.
Ozone detoxifies cyanides by converting them to cyanates .
Ozone completely decomposes urea : [ 49 ]
Ozone is a bent triatomic molecule with three vibrational modes: the symmetric stretch (1103.157 cm −1 ), bend (701.42 cm −1 ) and antisymmetric stretch (1042.096 cm −1 ). [ 50 ] The symmetric stretch and bend are weak absorbers, but the antisymmetric stretch is strong and responsible for ozone being an important minor greenhouse gas . This IR band is also used to detect ambient and atmospheric ozone although UV-based measurements are more common. [ 51 ]
The electromagnetic spectrum of ozone is quite complex. An overview can be seen at the MPI Mainz UV/VIS Spectral Atlas of Gaseous Molecules of Atmospheric Interest. [ 52 ]
All of the bands are dissociative, meaning that the molecule falls apart to O + O 2 after absorbing a photon. The most important absorption is the Hartley band, extending from slightly above 300 nm down to slightly above 200 nm. It is this band that is responsible for absorbing UV C in the stratosphere.
On the high wavelength side, the Hartley band transitions to the so-called Huggins band, which falls off rapidly until disappearing by ~360 nm. Above 400 nm, extending well out into the NIR, are the Chappius and Wulf bands. There, unstructured absorption bands are useful for detecting high ambient concentrations of ozone, but are so weak that they do not have much practical effect.
There are additional absorption bands in the far UV, which increase slowly from 200 nm down to reaching a maximum at ~120 nm.
The standard way to express total ozone levels (the amount of ozone in a given vertical column) in the atmosphere is by using Dobson units . Point measurements are reported as mole fractions in nmol/mol (parts per billion, ppb) or as concentrations in μg/m 3 . The study of ozone concentration in the atmosphere started in the 1920s. [ 53 ]
The highest levels of ozone in the atmosphere are in the stratosphere , in a region also known as the ozone layer between about 10 and 50 km above the surface (or between about 6 and 31 miles). However, even in this "layer", the ozone concentrations are only two to eight parts per million, so most of the oxygen there is dioxygen, O 2 , at about 210,000 parts per million by volume. [ 54 ]
Ozone in the stratosphere is mostly produced from short-wave ultraviolet rays between 240 and 160 nm. Oxygen starts to absorb weakly at 240 nm in the Herzberg bands, but most of the oxygen is dissociated by absorption in the strong Schumann–Runge bands between 200 and 160 nm where ozone does not absorb. While shorter wavelength light, extending to even the X-Ray limit, is energetic enough to dissociate molecular oxygen, there is relatively little of it, and, the strong solar emission at Lyman-alpha, 121 nm, falls at a point where molecular oxygen absorption is a minimum. [ 55 ]
The process of ozone creation and destruction is called the Chapman cycle and starts with the photolysis of molecular oxygen
followed by reaction of the oxygen atom with another molecule of oxygen to form ozone.
where "M" denotes the third body that carries off the excess energy of the reaction. The ozone molecule can then absorb a UV-C photon and dissociate
The excess kinetic energy heats the stratosphere when the O atoms and the molecular oxygen fly apart and collide with other molecules. This conversion of UV light into kinetic energy warms the stratosphere. The oxygen atoms produced in the photolysis of ozone then react back with other oxygen molecule as in the previous step to form more ozone. In the clear atmosphere, with only nitrogen and oxygen, ozone can react with the atomic oxygen to form two molecules of O 2 :
An estimate of the rate of this termination step to the cycling of atomic oxygen back to ozone can be found simply by taking the ratios of the concentration of O 2 to O 3 . The termination reaction is catalysed by the presence of certain free radicals, of which the most important are hydroxyl (OH), nitric oxide (NO) and atomic chlorine (Cl) and bromine (Br). In the second half of the 20th century, the amount of ozone in the stratosphere was discovered to be declining , mostly because of increasing concentrations of chlorofluorocarbons (CFC) and similar chlorinated and brominated organic molecules . The concern over the health effects of the decline led to the 1987 Montreal Protocol , the ban on the production of many ozone-depleting chemicals and in the first and second decade of the 21st century the beginning of the recovery of stratospheric ozone concentrations.
Ozone in the ozone layer filters out sunlight wavelengths from about 200 nm UV rays to 315 nm, with ozone peak absorption at about 250 nm. [ 56 ] This ozone UV absorption is important to life, since it extends the absorption of UV by ordinary oxygen and nitrogen in air (which absorb all wavelengths < 200 nm) through the lower UV-C (200–280 nm) and the entire UV-B band (280–315 nm). The small unabsorbed part that remains of UV-B after passage through ozone causes sunburn in humans, and direct DNA damage in living tissues in both plants and animals. Ozone's effect on mid-range UV-B rays is illustrated by its effect on UV-B at 290 nm, which has a radiation intensity 350 million times as powerful at the top of the atmosphere as at the surface. Nevertheless, enough of UV-B radiation at similar frequency reaches the ground to cause some sunburn, and these same wavelengths are also among those responsible for the production of vitamin D in humans.
The ozone layer has little effect on the longer UV wavelengths called UV-A (315–400 nm), but this radiation does not cause sunburn or direct DNA damage. While UV-A probably does cause long-term skin damage in certain humans, it is not as dangerous to plants and to the health of surface-dwelling organisms on Earth in general (see ultraviolet for more information on near ultraviolet).
Ground-level ozone (or tropospheric ozone) is an atmospheric pollutant. [ 57 ] It is not emitted directly by car engines or by industrial operations, but formed by the reaction of sunlight on air containing hydrocarbons and nitrogen oxides that react to form ozone directly at the source of the pollution or many kilometers downwind.
Ozone reacts directly with some hydrocarbons such as aldehydes and thus begins their removal from the air, but the products are themselves key components of smog . Ozone photolysis by UV light leads to production of the hydroxyl radical HO• and this plays a part in the removal of hydrocarbons from the air, but is also the first step in the creation of components of smog such as peroxyacyl nitrates , which can be powerful eye irritants. The atmospheric lifetime of tropospheric ozone is about 22 days; its main removal mechanisms are being deposited to the ground, the above-mentioned reaction giving HO•, and by reactions with OH and the peroxy radical HO 2 •. [ 58 ]
There is evidence of significant reduction in agricultural yields because of increased ground-level ozone and pollution which interferes with photosynthesis and stunts overall growth of some plant species. [ 59 ] [ 60 ] The United States Environmental Protection Agency (EPA) has proposed a secondary regulation to reduce crop damage, in addition to the primary regulation designed for the protection of human health.
Certain examples of cities with elevated ozone readings are Denver, Colorado ; Houston, Texas ; and Mexico City , Mexico . Houston has a reading of around 41 nmol/mol, while Mexico City is far more hazardous, with a reading of about 125 nmol/mol. [ 60 ]
Ground-level ozone, or tropospheric ozone, is the most concerning type of ozone pollution in urban areas and is increasing in general. [ 61 ] Ozone pollution in urban areas affects denser populations, and is worsened by high populations of vehicles, which emit pollutants NO 2 and VOCs , the main contributors to problematic ozone levels. [ 62 ] Ozone pollution in urban areas is especially concerning with increasing temperatures, raising heat-related mortality during heat waves . [ 63 ] During heat waves in urban areas, ground level ozone pollution can be 20% higher than usual. [ 64 ] Ozone pollution in urban areas reaches higher levels of exceedance in the summer and autumn, which may be explained by weather patterns and traffic patterns. [ 62 ] People experiencing poverty are more affected by pollution in general, even though these populations are less likely to be contributing to pollution levels. [ 65 ]
As mentioned above, Denver, Colorado, is one of the many cities in the U.S. that have high amounts of ozone. According to the American Lung Association , the Denver–Aurora area is the 14th most ozone-polluted area in the U.S. [ 66 ] The problem of high ozone levels is not new to this area. In 2004, the EPA allotted the Denver Metro /North Front Range [ b ] as non-attainment areas per 1997's 8-hour ozone standard, [ 67 ] but later deferred this status until 2007. The non-attainment standard indicates that an area does not meet the EPA's air quality standards. The Colorado Ozone Action Plan was created in response, and numerous changes were implemented from this plan. The first major change was that car emission testing was expanded across the state to more counties that did not previously mandate emissions testing, like areas of Larimer and Weld County. There have also been changes made to decrease Nitrogen Oxides (NOx) and Volatile Organic Compound (VOC) emissions, which should help lower ozone levels.
One large contributor to high ozone levels in the area is the oil and natural gas industry situated in the Denver-Julesburg Basin (DJB) which overlaps with a majority of Colorado's metropolitan areas. Ozone is produced naturally in the Earth's stratosphere, but is also produced in the troposphere from human efforts. Briefly mentioned above, NOx and VOCs react with sunlight to create ozone through a process called photochemistry. One hour elevated ozone events (<75 ppb) "occur during June–August indicating that elevated ozone levels are driven by regional photochemistry". [ 68 ] According to an article from the University of Colorado-Boulder, "Oil and natural gas VOC emission have a major role in ozone production and bear the potential to contribute to elevated O 3 levels in the Northern Colorado Front Range (NCFR)". [ 68 ] Using complex analyses to research wind patterns and emissions from large oil and natural gas operations, the authors concluded that "elevated O 3 levels in the NCFR are predominantly correlated with air transport from N– ESE, which are the upwind sectors where the O&NG operations in the Wattenberg Field area of the DJB are located". [ 68 ]
Contained in the Colorado Ozone Action Plan, created in 2008, plans exist to evaluate "emission controls for large industrial sources of NOx" and "statewide control requirements for new oil and gas condensate tanks and pneumatic valves". [ 69 ] In 2011, the Regional Haze Plan was released that included a more specific plan to help decrease NOx emissions. These efforts are increasingly difficult to implement and take many years to come to pass. Of course there are also other reasons that ozone levels remain high. These include: a growing population meaning more car emissions, and the mountains along the NCFR that can trap emissions. If interested, daily air quality readings can be found at the Colorado Department of Public Health and Environment's website. [ 70 ] As noted earlier, Denver continues to experience high levels of ozone to this day. It will take many years and a systems-thinking approach to combat this issue of high ozone levels in the Front Range of Colorado.
Ozone gas attacks any polymer possessing olefinic or double bonds within its chain structure, such as natural rubber , nitrile rubber , and styrene-butadiene rubber. Products made using these polymers are especially susceptible to attack, which causes cracks to grow longer and deeper with time, the rate of crack growth depending on the load carried by the rubber component and the concentration of ozone in the atmosphere. Such materials can be protected by adding antiozonants , such as waxes, which bond to the surface to create a protective film or blend with the material and provide long term protection. Ozone cracking used to be a serious problem in car tires, [ 71 ] for example, but it is not an issue with modern tires. On the other hand, many critical products, like gaskets and O-rings , may be attacked by ozone produced within compressed air systems. Fuel lines made of reinforced rubber are also susceptible to attack, especially within the engine compartment, where some ozone is produced by electrical components. Storing rubber products in close proximity to a DC electric motor can accelerate ozone cracking. The commutator of the motor generates sparks which in turn produce ozone.
Although ozone was present at ground level before the Industrial Revolution , peak concentrations are now far higher than the pre-industrial levels, and even background concentrations well away from sources of pollution are substantially higher. [ 73 ] [ 74 ] Ozone acts as a greenhouse gas , absorbing some of the infrared energy emitted by the earth. Quantifying the greenhouse gas potency of ozone is difficult because it is not present in uniform concentrations across the globe. However, the most widely accepted scientific assessments relating to climate change (e.g. the Intergovernmental Panel on Climate Change Third Assessment Report ) [ 75 ] suggest that the radiative forcing of tropospheric ozone is about 25% that of carbon dioxide .
The annual global warming potential of tropospheric ozone is between 918 and 1022 tons carbon dioxide equivalent /tons tropospheric ozone. This means on a per-molecule basis, ozone in the troposphere has a radiative forcing effect roughly 1,000 times as strong as carbon dioxide . However, tropospheric ozone is a short-lived greenhouse gas, which decays in the atmosphere much more quickly than carbon dioxide . This means that over a 20-year span, the global warming potential of tropospheric ozone is much less, roughly 62 to 69 tons carbon dioxide equivalent / ton tropospheric ozone. [ 76 ]
Because of its short-lived nature, tropospheric ozone does not have strong global effects, but has very strong radiative forcing effects on regional scales. In fact, there are regions of the world where tropospheric ozone has a radiative forcing up to 150% of carbon dioxide . [ 77 ] For example, ozone increase in the troposphere is shown to be responsible for ~30% of upper Southern Ocean interior warming between 1955 and 2000. [ 78 ]
Filters containing an adsorbent or catalyst such as charcoal (carbon) may be used to remove odors and gaseous pollutants such as volatile organic compounds or ozone. [ 79 ]
For the last few decades, scientists studied the effects of acute and chronic ozone exposure on human health. Hundreds of studies suggest that ozone is harmful to people at levels currently found in urban areas. [ 80 ] [ 81 ] Ozone has been shown to affect the respiratory, cardiovascular and central nervous system. Early death and problems in reproductive health and development are also shown to be associated with ozone exposure. [ 82 ]
The American Lung Association has identified five populations who are especially vulnerable to the effects of breathing ozone: [ 83 ]
Additional evidence suggests that women, those with obesity and low-income populations may also face higher risk from ozone, although more research is needed. [ 83 ]
Acute ozone exposure ranges from hours to a few days. Because ozone is a gas, it directly affects the lungs and the entire respiratory system. Inhaled ozone causes inflammation and acute—but reversible—changes in lung function, as well as airway hyperresponsiveness. [ 84 ] These changes lead to shortness of breath, wheezing, and coughing which may exacerbate lung diseases, like asthma or chronic obstructive pulmonary disease (COPD) resulting in the need to receive medical treatment. [ 85 ] [ 86 ] Acute and chronic exposure to ozone has been shown to cause an increased risk of respiratory infections, due to the following mechanism. [ 87 ]
Multiple studies have been conducted to determine the mechanism behind ozone's harmful effects, particularly in the lungs. These studies have shown that exposure to ozone causes changes in the immune response within the lung tissue, resulting in disruption of both the innate and adaptive immune response, as well as altering the protective function of lung epithelial cells. [ 88 ] It is thought that these changes in immune response and the related inflammatory response are factors that likely contribute to the increased risk of lung infections, and worsening or triggering of asthma and reactive airways after exposure to ground-level ozone pollution. [ 88 ] [ 89 ]
The innate (cellular) immune system consists of various chemical signals and cell types that work broadly and against multiple pathogen types, typically bacteria or foreign bodies/substances in the host. [ 89 ] [ 90 ] The cells of the innate system include phagocytes, neutrophils, [ 90 ] both thought to contribute to the mechanism of ozone pathology in the lungs, as the functioning of these cell types have been shown to change after exposure to ozone. [ 89 ] Macrophages, cells that serve the purpose of eliminating pathogens or foreign material through the process of "phagocytosis", [ 90 ] have been shown to change the level of inflammatory signals they release in response to ozone, either up-regulating and resulting in an inflammatory response in the lung, or down-regulating and reducing immune protection. [ 88 ] Neutrophils, another important cell type of the innate immune system that primarily targets bacterial pathogens, [ 90 ] are found to be present in the airways within 6 hours of exposure to high ozone levels. Despite high levels in the lung tissues, however, their ability to clear bacteria appears impaired by exposure to ozone. [ 88 ]
The adaptive immune system is the branch of immunity that provides long-term protection via the development of antibodies targeting specific pathogens and is also impacted by high ozone exposure. [ 89 ] [ 90 ] Lymphocytes, a cellular component of the adaptive immune response, produce an increased amount of inflammatory chemicals called "cytokines" after exposure to ozone, which may contribute to airway hyperreactivity and worsening asthma symptoms. [ 88 ]
The airway epithelial cells also play an important role in protecting individuals from pathogens. In normal tissue, the epithelial layer forms a protective barrier, and also contains specialized ciliary structures that work to clear foreign bodies, mucus and pathogens from the lungs. When exposed to ozone, the cilia become damaged and mucociliary clearance of pathogens is reduced. Furthermore, the epithelial barrier becomes weakened, allowing pathogens to cross the barrier, proliferate and spread into deeper tissues. Together, these changes in the epithelial barrier help make individuals more susceptible to pulmonary infections. [ 88 ]
Inhaling ozone not only affects the immune system and lungs, but it may also affect the heart as well. Ozone causes short-term autonomic imbalance leading to changes in heart rate and reduction in heart rate variability; [ 91 ] and high levels exposure for as little as one-hour results in a supraventricular arrhythmia in the elderly, [ 92 ] both increase the risk of premature death and stroke. Ozone may also lead to vasoconstriction resulting in increased systemic arterial pressure contributing to increased risk of cardiac morbidity and mortality in patients with pre-existing cardiac diseases. [ 93 ] [ 94 ]
Breathing ozone for periods longer than eight hours at a time for weeks, months or years defines chronic exposure. Numerous studies suggest a serious impact on the health of various populations from this exposure.
One study finds significant positive associations between chronic ozone and all-cause, circulatory, and respiratory mortality with 2%, 3%, and 12% increases in risk per 10 ppb [ 95 ] and report an association (95% CI) of annual ozone and all-cause mortality with a hazard ratio of 1.02 (1.01–1.04), and with cardiovascular mortality of 1.03 (1.01–1.05). A similar study finds similar associations with all-cause mortality and even larger effects for cardiovascular mortality. [ 96 ] An increased risk of mortality from respiratory causes is associated with long-term chronic exposure to ozone. [ 97 ]
Chronic ozone has detrimental effects on children, especially those with asthma. The risk for hospitalization in children with asthma increases with chronic exposure to ozone; younger children and those with low-income status are even at greater risk. [ 98 ]
Adults suffering from respiratory diseases (asthma, [ 99 ] COPD, [ 100 ] lung cancer [ 101 ] ) are at a higher risk of mortality and morbidity and critically ill patients have an increased risk of developing acute respiratory distress syndrome with chronic ozone exposure as well. [ 102 ]
Ozone generators sold as air cleaners intentionally produce the gas ozone. [ 43 ] These are often marketed to control indoor air pollution , and use misleading terms to describe ozone. Some examples are describing it as "energized oxygen" or "pure air", suggesting that ozone is a healthy or "better" kind of oxygen. [ 43 ] However, according to the EPA , "There is evidence to show that at concentrations that do not exceed public health standards, ozone is not effective at removing many odor-causing chemicals", and "If used at concentrations that do not exceed public health standards, ozone applied to indoor air does not effectively remove viruses, bacteria, mold, or other biological pollutants.". [ 43 ] Furthermore, another report states that "results of some controlled studies show that concentrations of ozone considerably higher than these [human safety] standards are possible even when a user follows the manufacturer's operating instructions". [ 103 ]
The California Air Resources Board has a page listing air cleaners (many with ionizers ) meeting their indoor ozone limit of 0.050 parts per million. [ 104 ] From that article:
All portable indoor air cleaning devices sold in California must be certified by the California Air Resources Board (CARB). To be certified, air cleaners must be tested for electrical safety and ozone emissions, and meet an ozone emission concentration limit of 0.050 parts per million. For more information about the regulation, visit the air cleaner regulation .
Ozone precursors are a group of pollutants, predominantly those emitted during the combustion of fossil fuels . Ground-level ozone pollution (tropospheric ozone) is produced near the Earth's surface by the action of daylight UV rays on these precursors. The ozone at ground level is primarily from fossil fuel precursors, but methane is a natural precursor, and the very low natural background level of ozone at ground level is considered safe. This section examines the health impacts of fossil fuel burning, which raises ground level ozone far above background levels.
There is a great deal of evidence to show that ground-level ozone can harm lung function and irritate the respiratory system . [ 57 ] [ 106 ] Exposure to ozone (and the pollutants that produce it) is linked to premature death , asthma , bronchitis , heart attack , and other cardiopulmonary problems. [ 107 ] [ 108 ]
Long-term exposure to ozone has been shown to increase risk of death from respiratory illness . [ 43 ] A study of 450,000 people living in U.S. cities saw a significant correlation between ozone levels and respiratory illness over the 18-year follow-up period. The study revealed that people living in cities with high ozone levels, such as Houston or Los Angeles, had an over 30% increased risk of dying from lung disease. [ 109 ] [ 110 ]
Air quality guidelines such as those from the World Health Organization , the U.S. Environmental Protection Agency (EPA), and the European Union are based on detailed studies designed to identify the levels that can cause measurable ill health effects .
According to scientists with the EPA, susceptible people can be adversely affected by ozone levels as low as 40 nmol/mol. [ 108 ] [ 111 ] [ 112 ] In the EU, the current target value for ozone concentrations is 120 μg/m 3 which is about 60 nmol/mol. This target applies to all member states in accordance with Directive 2008/50/EC. [ 113 ] Ozone concentration is measured as a maximum daily mean of 8 hour averages and the target should not be exceeded on more than 25 calendar days per year, starting from January 2010. While the directive requires in the future a strict compliance with 120 μg/m 3 limit (i.e. mean ozone concentration not to be exceeded on any day of the year), there is no date set for this requirement and this is treated as a long-term objective. [ 114 ]
In the US, the Clean Air Act directs the EPA to set National Ambient Air Quality Standards for several pollutants, including ground-level ozone, and counties out of compliance with these standards are required to take steps to reduce their levels. In May 2008, under a court order, the EPA lowered its ozone standard from 80 nmol/mol to 75 nmol/mol. The move proved controversial, since the Agency's own scientists and advisory board had recommended lowering the standard to 60 nmol/mol. [ 108 ] Many public health and environmental groups also supported the 60 nmol/mol standard, [ 115 ] and the World Health Organization recommends 100 μg/m 3 (51 nmol/mol). [ 116 ]
On January 7, 2010, the U.S. Environmental Protection Agency (EPA) announced proposed revisions to the National Ambient Air Quality Standard (NAAQS) for the pollutant ozone, the principal component of smog:
... EPA proposes that the level of the 8-hour primary standard, which was set at 0.075 μmol/mol in the 2008 final rule, should instead be set at a lower level within the range of 0.060 to 0.070 μmol/mol, to provide increased protection for children and other at risk populations against an array of O 3 – related adverse health effects that range from decreased lung function and increased respiratory symptoms to serious indicators of respiratory morbidity including emergency department visits and hospital admissions for respiratory causes, and possibly cardiovascular-related morbidity as well as total non- accidental and cardiopulmonary mortality ... [ 117 ]
On October 26, 2015, the EPA published a final rule with an effective date of December 28, 2015, that revised the 8-hour primary NAAQS from 0.075 ppm to 0.070 ppm. [ 118 ]
The EPA has developed an air quality index (AQI) to help explain air pollution levels to the general public. Under the current standards, eight-hour average ozone mole fractions of 85 to 104 nmol/mol are described as "unhealthy for sensitive groups", 105 nmol/mol to 124 nmol/mol as "unhealthy", and 125 nmol/mol to 404 nmol/mol as "very unhealthy". [ 119 ]
Ozone can also be present in indoor air pollution , partly as a result of electronic equipment such as photocopiers. A connection has also been known to exist between the increased pollen, fungal spores, and ozone caused by thunderstorms and hospital admissions of asthma sufferers. [ 120 ]
In the Victorian era , one British folk myth held that the smell of the sea was caused by ozone. In fact, the characteristic "smell of the sea" is caused by dimethyl sulfide , a chemical generated by phytoplankton . Victorian Britons considered the resulting smell "bracing". [ 121 ]
An investigation to assess the joint mortality effects of ozone and heat during the European heat waves in 2003, concluded that these appear to be additive. [ 122 ]
Ozone, along with reactive forms of oxygen such as superoxide , singlet oxygen , hydrogen peroxide , and hypochlorite ions, is produced by white blood cells and other biological systems (such as the roots of marigolds ) as a means of destroying foreign bodies. Ozone reacts directly with organic double bonds. Also, when ozone breaks down to dioxygen it gives rise to oxygen free radicals , which are highly reactive and capable of damaging many organic molecules . Moreover, it is believed that the powerful oxidizing properties of ozone may be a contributing factor of inflammation . The cause-and-effect relationship of how the ozone is created in the body and what it does is still under consideration and still subject to various interpretations, since other body chemical processes can trigger some of the same reactions. There is evidence linking the antibody-catalyzed water-oxidation pathway of the human immune response to the production of ozone. In this system, ozone is produced by antibody-catalyzed production of trioxidane from water and neutrophil-produced singlet oxygen. [ 123 ]
When inhaled, ozone reacts with compounds lining the lungs to form specific, cholesterol-derived metabolites that are thought to facilitate the build-up and pathogenesis of atherosclerotic plaques (a form of heart disease ). These metabolites have been confirmed as naturally occurring in human atherosclerotic arteries and are categorized into a class of secosterols termed atheronals , generated by ozonolysis of cholesterol's double bond to form a 5,6 secosterol [ 124 ] as well as a secondary condensation product via aldolization. [ 125 ]
Ozone has been implicated to have an adverse effect on plant growth: "... ozone reduced total chlorophylls, carotenoid and carbohydrate concentration, and increased 1-aminocyclopropane-1-carboxylic acid (ACC) content and ethylene production. In treated plants, the ascorbate leaf pool was decreased, while lipid peroxidation and solute leakage were significantly higher than in ozone-free controls. The data indicated that ozone triggered protective mechanisms against oxidative stress in citrus." [ 126 ] Studies that have used pepper plants as a model have shown that ozone decreased fruit yield and changed fruit quality. [ 127 ] [ 128 ] Furthermore, it was also observed a decrease in chlorophylls levels and antioxidant defences on the leaves, as well as increased the reactive oxygen species (ROS) levels and lipid and protein damages. [ 127 ] [ 128 ]
A 2022 study concludes that East Asia loses 63 billion dollars in crops per year due to ozone pollution, a byproduct of fossil fuel combustion. China loses about one-third of its potential wheat production and one-fourth of its rice production. [ 129 ] [ 130 ]
Because of the strongly oxidizing properties of ozone, ozone is a primary irritant, affecting especially the eyes and respiratory systems and can be hazardous at even low concentrations. The Canadian Centre for Occupation Safety and Health reports that:
Even very low concentrations of ozone can be harmful to the upper respiratory tract and the lungs. The severity of injury depends on both the concentration of ozone and the duration of exposure. Severe and permanent lung injury or death could result from even a very short-term exposure to relatively low concentrations." [ 131 ]
To protect workers potentially exposed to ozone, U.S. Occupational Safety and Health Administration has established a permissible exposure limit (PEL) of 0.1 μmol/mol (29 CFR 1910.1000 table Z-1), calculated as an 8-hour time weighted average. Higher concentrations are especially hazardous and NIOSH has established an Immediately Dangerous to Life and Health Limit (IDLH) of 5 μmol/mol. [ 132 ] Work environments where ozone is used or where it is likely to be produced should have adequate ventilation and it is prudent to have a monitor for ozone that will alarm if the concentration exceeds the OSHA PEL. Continuous monitors for ozone are available from several suppliers.
Elevated ozone exposure can occur on passenger aircraft , with levels depending on altitude and atmospheric turbulence. [ 133 ] U.S. Federal Aviation Administration regulations set a limit of 250 nmol/mol with a maximum four-hour average of 100 nmol/mol. [ 134 ] Some planes are equipped with ozone converters in the ventilation system to reduce passenger exposure. [ 133 ]
Ozone generators , or ozonators , [ 135 ] are used to produce ozone for cleaning air or removing smoke odours in unoccupied rooms. These ozone generators can produce over 3 g of ozone per hour. Ozone often forms in nature under conditions where O 2 will not react. [ 29 ] Ozone used in industry is measured in μmol/mol (ppm, parts per million), nmol/mol (ppb, parts per billion), μg/m 3 , mg/h (milligrams per hour) or weight percent. The regime of applied concentrations ranges from 1% to 5% (in air) and from 6% to 14% (in oxygen) for older generation methods. New electrolytic methods can achieve up 20% to 30% dissolved ozone concentrations in output water.
Temperature and humidity play a large role in how much ozone is being produced using traditional generation methods (such as corona discharge and ultraviolet light). Old generation methods will produce less than 50% of nominal capacity if operated with humid ambient air, as opposed to very dry air. New generators, using electrolytic methods, can achieve higher purity and dissolution through using water molecules as the source of ozone production.
This is the most common type of ozone generator for most industrial and personal uses. While variations of the "hot spark" coronal discharge method of ozone production exist, including medical grade and industrial grade ozone generators, these units usually work by means of a corona discharge tube or ozone plate. [ 136 ] [ 137 ] They are typically cost-effective and do not require an oxygen source other than the ambient air to produce ozone concentrations of 3–6%. Fluctuations in ambient air, due to weather or other environmental conditions, cause variability in ozone production. However, they also produce nitrogen oxides as a by-product. Use of an air dryer can reduce or eliminate nitric acid formation by removing water vapor and increase ozone production. At room temperature, nitric acid will form into a vapour that is hazardous if inhaled. Symptoms can include chest pain, shortness of breath, headaches and a dry nose and throat causing a burning sensation. Use of an oxygen concentrator can further increase the ozone production and further reduce the risk of nitric acid formation by removing not only the water vapor, but also the bulk of the nitrogen.
UV ozone generators, or vacuum-ultraviolet (VUV) ozone generators, employ a light source that generates a narrow-band ultraviolet light, a subset of that produced by the Sun. The Sun's UV sustains the ozone layer in the stratosphere of Earth. [ 138 ]
UV ozone generators use ambient air for ozone production, no air prep systems are used (air dryer or oxygen concentrator), therefore these generators tend to be less expensive. However, UV ozone generators usually produce ozone with a concentration of about 0.5% or lower which limits the potential ozone production rate. Another disadvantage of this method is that it requires the ambient air (oxygen) to be exposed to the UV source for a longer amount of time, and any gas that is not exposed to the UV source will not be treated. This makes UV generators impractical for use in situations that deal with rapidly moving air or water streams (in-duct air sterilization , for example). Production of ozone is one of the potential dangers of ultraviolet germicidal irradiation . VUV ozone generators are used in swimming pools and spa applications ranging to millions of gallons of water. VUV ozone generators, unlike corona discharge generators, do not produce harmful nitrogen by-products and also unlike corona discharge systems, VUV ozone generators work extremely well in humid air environments. There is also not normally a need for expensive off-gas mechanisms, and no need for air driers or oxygen concentrators which require extra costs and maintenance.
In the cold plasma method, pure oxygen gas is exposed to a plasma created by DBD . The diatomic oxygen is split into single atoms, which then recombine in triplets to form ozone.
It is common in the industry to mislabel some DBD ozone generators as CD Corona Discharge generators. Typically all solid flat metal electrode ozone generators produce ozone using the dielectric barrier discharge method. Cold plasma machines use pure oxygen as the input source and produce a maximum concentration of about 24% ozone. They produce far greater quantities of ozone in a given time compared to ultraviolet production that has about 2% efficiency. The discharges manifest as filamentary transfer of electrons (micro discharges) in a gap between two electrodes. In order to evenly distribute the micro discharges, a dielectric insulator must be used to separate the metallic electrodes and to prevent arcing.
Electrolytic ozone generation (EOG) splits water molecules into H 2 , O 2 , and O 3 .
In most EOG methods, the hydrogen gas will be removed to leave oxygen and ozone as the only reaction products. Therefore, EOG can achieve higher dissolution in water without other competing gases found in corona discharge method, such as nitrogen gases present in ambient air. This method of generation can achieve concentrations of 20–30% and is independent of air quality because water is used as the source material. Production of ozone electrolytically is typically unfavorable because of the high overpotential required to produce ozone as compared to oxygen. This is why ozone is not produced during typical water electrolysis. However, it is possible to increase the overpotential of oxygen by careful catalyst selection such that ozone is preferentially produced under electrolysis. Catalysts typically chosen for this approach are lead dioxide [ 139 ] or boron-doped diamond. [ 140 ]
The ozone-to-oxygen ratio is improved by increasing current density at the anode, cooling the electrolyte around the anode close to 0 °C, using an acidic electrolyte (such as dilute sulfuric acid) instead of a basic solution, and by applying pulsed current instead of DC. [ 141 ]
Ozone cannot be stored and transported like other industrial gases (because it quickly decays into diatomic oxygen) and must therefore be produced on site. Available ozone generators vary in the arrangement and design of the high-voltage electrodes. At production capacities higher than 20 kg per hour, a gas/water tube heat-exchanger may be utilized as ground electrode and assembled with tubular high-voltage electrodes on the gas-side. The regime of typical gas pressures is around 2 bars (200 kPa ) absolute in oxygen and 3 bars (300 kPa) absolute in air. Several megawatts of electrical power may be installed in large facilities, applied as single phase AC current at 50 to 8000 Hz and peak voltages between 3,000 and 20,000 volts. Applied voltage is usually inversely related to the applied frequency.
The dominating parameter influencing ozone generation efficiency is the gas temperature, which is controlled by cooling water temperature and/or gas velocity. The cooler the water, the better the ozone synthesis. The lower the gas velocity, the higher the concentration (but the lower the net ozone produced). At typical industrial conditions, almost 90% of the effective power is dissipated as heat and needs to be removed by a sufficient cooling water flow.
Because of the high reactivity of ozone, only a few materials may be used like stainless steel (quality 316L), titanium , aluminium (as long as no moisture is present), glass , polytetrafluorethylene , or polyvinylidene fluoride . Viton may be used with the restriction of constant mechanical forces and absence of humidity (humidity limitations apply depending on the formulation). Hypalon may be used with the restriction that no water comes in contact with it, except for normal atmospheric levels. Embrittlement or shrinkage is the common mode of failure of elastomers with exposure to ozone. Ozone cracking is the common mode of failure of elastomer seals like O-rings .
Silicone rubbers are usually adequate for use as gaskets in ozone concentrations below 1 wt%, such as in equipment for accelerated aging of rubber samples.
Ozone may be formed from O 2 by electrical discharges and by action of high energy electromagnetic radiation . Unsuppressed arcing in electrical contacts, motor brushes, or mechanical switches breaks down the chemical bonds of the atmospheric oxygen surrounding the contacts [ O 2 -> 2O]. Free radicals of oxygen in and around the arc recombine to create ozone [ O 3 ]. [ 142 ] Certain electrical equipment generate significant levels of ozone. This is especially true of devices using high voltages , such as ionic air purifiers , laser printers , photocopiers , tasers , and arc welders . Electric motors using brushes can generate ozone from repeated sparking inside the unit. Large motors that use brushes, such as those used by elevators or hydraulic pumps, will generate more ozone than smaller motors.
Ozone is similarly formed in the Catatumbo lightning storms phenomenon on the Catatumbo River in Venezuela , though ozone's instability makes it dubious that it has any effect on the ozonosphere. [ 143 ] It is the world's largest single natural generator of ozone, lending calls for it to be designated a UNESCO World Heritage Site . [ 144 ]
In the laboratory, ozone can be produced by electrolysis using a 9 volt battery , a pencil graphite rod cathode , a platinum wire anode , and a 3 molar sulfuric acid electrolyte . [ 145 ] The half cell reactions taking place are:
where E° represents the standard electrode potential .
In the net reaction, three equivalents of water are converted into one equivalent of ozone and three equivalents of hydrogen . Oxygen formation is a competing reaction.
It can also be generated by a high voltage arc . In its simplest form, high voltage AC, such as the output of a neon-sign transformer is connected to two metal rods with the ends placed sufficiently close to each other to allow an arc. The resulting arc will convert atmospheric oxygen to ozone.
It is often desirable to contain the ozone. This can be done with an apparatus consisting of two concentric glass tubes sealed together at the top with gas ports at the top and bottom of the outer tube. The inner core should have a length of metal foil inserted into it connected to one side of the power source. The other side of the power source should be connected to another piece of foil wrapped around the outer tube. A source of dry O 2 is applied to the bottom port. When high voltage is applied to the foil leads, electricity will discharge between the dry dioxygen in the middle and form O 3 and O 2 which will flow out the top port. This is called a Siemen's ozoniser. The reaction can be summarized as follows: [ 29 ]
The largest use of ozone is in the preparation of pharmaceuticals , synthetic lubricants , and many other commercially useful organic compounds , where it is used to sever carbon -carbon bonds. [ 29 ] It can also be used for bleaching substances and for killing microorganisms in air and water sources. [ 146 ] Many municipal drinking water systems kill bacteria with ozone instead of the more common chlorine . [ 147 ] Ozone has a very high oxidation potential . [ 148 ] Ozone does not form organochlorine compounds, nor does it remain in the water after treatment. Ozone can form the suspected carcinogen bromate in source water with high bromide concentrations. The U.S. Safe Drinking Water Act mandates that these systems introduce an amount of chlorine to maintain a minimum of 0.2 μmol/mol residual free chlorine in the pipes, based on results of regular testing. Where electrical power is abundant, ozone is a cost-effective method of treating water, since it is produced on demand and does not require transportation and storage of hazardous chemicals. Once it has decayed, it leaves no taste or odour in drinking water.
Although low levels of ozone have been advertised to be of some disinfectant use in residential homes, the concentration of ozone in dry air required to have a rapid, substantial effect on airborne pathogens exceeds safe levels recommended by the U.S. Occupational Safety and Health Administration and Environmental Protection Agency . Humidity control can vastly improve both the killing power of the ozone and the rate at which it decays back to oxygen (more humidity allows more effectiveness). Spore forms of most pathogens are very tolerant of atmospheric ozone in concentrations at which asthma patients start to have issues.
In 1908 artificial ozonisation of the Central Line of the London Underground was introduced for aerial disinfection. The process was found to be worthwhile, but was phased out by 1956. However the beneficial effect was maintained by the ozone created incidentally from the electrical discharges of the train motors (see above: Incidental production ). [ 149 ]
Ozone generators were made available to schools and universities in Wales for the Autumn term 2021, to disinfect classrooms after COVID-19 outbreaks. [ 150 ]
Industrially, ozone is used to:
Ozone is a reagent in many organic reactions in the laboratory and in industry. Ozonolysis is the cleavage of an alkene to carbonyl compounds.
Many hospitals around the world use large ozone generators to decontaminate operating rooms between surgeries. The rooms are cleaned and then sealed airtight before being filled with ozone which effectively kills or neutralizes all remaining bacteria. [ 156 ]
Ozone is used as an alternative to chlorine or chlorine dioxide in the bleaching of wood pulp . [ 157 ] It is often used in conjunction with oxygen and hydrogen peroxide to eliminate the need for chlorine-containing compounds in the manufacture of high-quality, white paper . [ 158 ]
Ozone can be used to detoxify cyanide wastes (for example from gold and silver mining ) by oxidizing cyanide to cyanate and eventually to carbon dioxide . [ 159 ]
Since the invention of dielectric barrier discharge (DBD) plasma reactors, it has been employed for water treatment with ozone. [ 160 ] However, with cheaper alternative disinfectants like chlorine, such applications of DBD ozone water decontamination have been limited by high power consumption and bulky equipment. [ 161 ] [ 162 ] Despite this, with research revealing the negative impacts of common disinfectants like chlorine with respect to toxic residuals and ineffectiveness in killing certain micro-organisms, [ 163 ] DBD plasma-based ozone decontamination is of interest in current available technologies. Although ozonation of water with a high concentration of bromide does lead to the formation of undesirable brominated disinfection byproducts, unless drinking water is produced by desalination, ozonation can generally be applied without concern for these byproducts. [ 162 ] [ 164 ] [ 165 ] [ 166 ] Advantages of ozone include high thermodynamic oxidation potential, less sensitivity to organic material and better tolerance for pH variations while retaining the ability to kill bacteria, fungi, viruses, as well as spores and cysts. [ 167 ] [ 168 ] [ 169 ] Although, ozone has been widely accepted in Europe for decades, it is sparingly used for decontamination in the U.S. due to limitations of high-power consumption, bulky installation and stigma attached with ozone toxicity. [ 161 ] [ 170 ] Considering this, recent research efforts have been directed toward the study of effective ozone water treatment systems. [ 171 ] Researchers have looked into lightweight and compact low power surface DBD reactors, [ 172 ] [ 173 ] energy efficient volume DBD reactors [ 174 ] and low power micro-scale DBD reactors. [ 175 ] [ 176 ] Such studies can help pave the path to re-acceptance of DBD plasma-based ozone decontamination of water, especially in the U.S.
Ozone levels which are safe for people are ineffective at killing fungi and bacteria. [ 177 ] Some consumer disinfection and cosmetic products emit ozone at levels harmful to human health. [ 177 ]
Devices generating high levels of ozone, some of which use ionization, are used to sanitize and deodorize uninhabited buildings, rooms, ductwork, woodsheds, boats, and other vehicles.
Ozonated water is used to launder clothes and to sanitize food, drinking water, and surfaces in the home. According to the U.S. Food and Drug Administration (FDA), it is "amending the food additive regulations to provide for the safe use of ozone in gaseous and aqueous phases as an antimicrobial agent on food, including meat and poultry." Studies at California Polytechnic University demonstrated that 0.3 μmol/mol levels of ozone dissolved in filtered tapwater can produce a reduction of more than 99.99% in such food-borne microorganisms as salmonella, E. coli 0157:H7 and Campylobacter . This quantity is 20,000 times the WHO -recommended limits stated above. [ 152 ] [ 178 ]
Ozone can be used to remove pesticide residues from fruits and vegetables . [ 179 ] [ 180 ]
Ozone is used in homes and hot tubs to kill bacteria in the water and to reduce the amount of chlorine or bromine required by reactivating them to their free state. Since ozone does not remain in the water long enough, ozone by itself is ineffective at preventing cross-contamination among bathers and must be used in conjunction with halogens . Gaseous ozone created by ultraviolet light or by corona discharge is injected into the water. [ 181 ]
Ozone is also widely used in the treatment of water in aquariums and fishponds. Its use can minimize bacterial growth, control parasites, eliminate transmission of some diseases, and reduce or eliminate "yellowing" of the water. Ozone must not come in contact with fishes' gill structures. Natural saltwater (with life forms) provides enough "instantaneous demand" that controlled amounts of ozone activate bromide ions to hypobromous acid , and the ozone entirely decays in a few seconds to minutes. If oxygen-fed ozone is used, the water will be higher in dissolved oxygen and fishes' gill structures will atrophy, making them dependent on oxygen-enriched water.
Ozonation – a process of infusing water with ozone – can be used in aquaculture to facilitate organic breakdown. Ozone is also added to recirculating systems to reduce nitrite levels [ 182 ] through conversion into nitrate . If nitrite levels in the water are high, nitrites will also accumulate in the blood and tissues of fish, where it interferes with oxygen transport (it causes oxidation of the heme-group of haemoglobin from ferrous ( Fe 2+ ) to ferric ( Fe 3+ ), making haemoglobin unable to bind O 2 ). [ 183 ] Despite these apparent positive effects, ozone use in recirculation systems has been linked to reducing the level of bioavailable iodine in salt water systems, resulting in iodine deficiency symptoms such as goitre and decreased growth in Senegalese sole ( Solea senegalensis ) larvae. [ 184 ]
Ozonate seawater is used for surface disinfection of haddock and Atlantic halibut eggs against nodavirus. Nodavirus is a lethal and vertically transmitted virus which causes severe mortality in fish. Haddock eggs should not be treated with high ozone level as eggs so treated did not hatch and died after 3–4 days. [ 185 ]
Ozone application on freshly cut pineapple and banana shows increase in flavonoids and total phenol contents when exposure is up to 20 minutes. Decrease in ascorbic acid (one form of vitamin C ) content is observed but the positive effect on total phenol content and flavonoids can overcome the negative effect. [ 186 ] Tomatoes upon treatment with ozone show an increase in β-carotene, lutein and lycopene. [ 187 ] However, ozone application on strawberries in pre-harvest period shows decrease in ascorbic acid content. [ 188 ]
Ozone facilitates the extraction of some heavy metals from soil using EDTA . EDTA forms strong, water-soluble coordination compounds with some heavy metals ( Pb and Zn ) thereby making it possible to dissolve them out from contaminated soil. If contaminated soil is pre-treated with ozone, the extraction efficacy of Pb , Am , and Pu increases by 11.0–28.9%, [ 189 ] 43.5% [ 190 ] and 50.7% [ 190 ] respectively.
Crop pollination is an essential part of an ecosystem. Ozone can have detrimental effects on plant-pollinator interactions. [ 191 ] Pollinators carry pollen from one plant to another. This is an essential cycle inside of an ecosystem. Causing changes in certain atmospheric conditions around pollination sites or with xenobiotics could cause unknown changes to the natural cycles of pollinators and flowering plants. In a study conducted in North-Western Europe, crop pollinators were negatively affected more when ozone levels were higher. [ 192 ]
The use of ozone for the treatment of medical conditions is not supported by high quality evidence, and is generally considered alternative medicine . [ 193 ]
Footnotes
Citations
Nascent oxygen O
Dioxygen ( singlet and triplet ) O 2
Trioxygen ( ozone and cyclic ozone ) O 3
Tetraoxygen O 4
Octaoxygen O 8 | https://en.wikipedia.org/wiki/O⚍O⚎O |
Dinitrogen tetroxide Dinitrogen trioxide Nitric oxide Nitrous oxide
Nitrogen dioxide is a chemical compound with the formula NO 2 . One of several nitrogen oxides , nitrogen dioxide is a reddish-brown gas. It is a paramagnetic , bent molecule with C 2v point group symmetry . Industrially, NO 2 is an intermediate in the synthesis of nitric acid , millions of tons of which are produced each year, primarily for the production of fertilizers .
Nitrogen dioxide is poisonous and can be fatal if inhaled in large quantities. [ 8 ] Cooking with a gas stove produces nitrogen dioxide which causes poorer indoor air quality . Combustion of gas can lead to increased concentrations of nitrogen dioxide throughout the home environment which is linked to respiratory issues and diseases . [ 9 ] [ 10 ] The LC 50 ( median lethal dose ) for humans has been estimated to be 174 ppm for a 1-hour exposure. [ 11 ] It is also included in the NO x family of atmospheric pollutants .
Nitrogen dioxide is a reddish-brown gas with a pungent, acrid odor above 21.2 °C (70.2 °F; 294.3 K) and becomes a yellowish-brown liquid below 21.2 °C (70.2 °F; 294.3 K). It forms an equilibrium with its dimer , dinitrogen tetroxide ( N 2 O 4 ), and converts almost entirely to N 2 O 4 below −11.2 °C (11.8 °F; 261.9 K). [ 6 ]
The bond length between the nitrogen atom and the oxygen atom is 119.7 pm . This bond length is consistent with a bond order between one and two.
Unlike ozone ( O 3 ) the ground electronic state of nitrogen dioxide is a doublet state , since nitrogen has one unpaired electron, [ 12 ] which decreases the alpha effect compared with nitrite and creates a weak bonding interaction with the oxygen lone pairs. The lone electron in NO 2 also means that this compound is a free radical , so the formula for nitrogen dioxide is often written as • NO 2 .
The reddish-brown color is a consequence of preferential absorption of light in the blue region of the spectrum (400–500 nm), although the absorption extends throughout the visible (at shorter wavelengths) and into the infrared (at longer wavelengths). Absorption of light at wavelengths shorter than about 400 nm results in photolysis (to form NO + O , atomic oxygen); in the atmosphere the addition of the oxygen atom so formed to O 2 results in ozone.
Industrially, nitrogen dioxide is produced and transported as its cryogenic liquid dimer, dinitrogen tetroxide . It is produced industrially by the oxidation of ammonia, the Ostwald Process . This reaction is the first step in the production of nitric acid: [ 13 ]
It can also be produced by the oxidation of nitrosyl chloride :
Instead, most laboratory syntheses stabilize and then heat the nitric acid to accelerate the decomposition. For example, the thermal decomposition of some metal nitrates generates NO 2 : [ 14 ]
Alternatively, dehydration of nitric acid produces nitronium nitrate ...
...which subsequently undergoes thermal decomposition:
NO 2 is generated by the reduction of concentrated nitric acid with a metal (such as copper):
Nitric acid decomposes slowly to nitrogen dioxide by the overall reaction:
The nitrogen dioxide so formed confers the characteristic yellow color often exhibited by this acid. However, the reaction is too slow to be a practical source of NO 2 .
At low temperatures, NO 2 reversibly converts to the colourless gas dinitrogen tetroxide ( N 2 O 4 ):
The exothermic equilibrium has enthalpy change Δ H = −57.23 kJ/mol . [ 15 ]
At 150 °C (302 °F; 423 K), NO 2 decomposes with release of oxygen via an endothermic process ( Δ H = 14 kJ/mol ):
As suggested by the weakness of the N–O bond, NO 2 is a good oxidizer. Consequently, it will combust, sometimes explosively, in the presence of hydrocarbons . [ 16 ]
NO 2 reacts with water to give nitric acid and nitrous acid :
This reaction is one of the steps in the Ostwald process for the industrial production of nitric acid from ammonia. [ 13 ] This reaction is negligibly slow at low concentrations of NO 2 characteristic of the ambient atmosphere, although it does proceed upon NO 2 uptake to surfaces. Such surface reaction is thought to produce gaseous HNO 2 (often written as HONO ) in outdoor and indoor environments. [ 17 ]
NO 2 is used to generate anhydrous metal nitrates from the oxides: [ 15 ]
Alkyl and metal iodides give the corresponding nitrates: [ 12 ]
The reactivity of nitrogen dioxide toward organic compounds has long been known. [ 18 ] For example, it reacts with amides to give N-nitroso derivatives. [ 19 ] It is used for nitrations under anhydrous conditions. [ 20 ]
NO 2 is used as an intermediate in the manufacturing of nitric acid , as a nitrating agent in the manufacturing of chemical explosives , as a polymerization inhibitor for acrylates , as a flour bleaching agent , [ 21 ] : 223 and as a room temperature sterilization agent. [ 22 ] It is also used as an oxidizer in rocket fuel , for example in red fuming nitric acid ; it was used in the Titan rockets , to launch Project Gemini , in the maneuvering thrusters of the Space Shuttle , and in uncrewed space probes sent to various planets. [ 23 ]
Nitrogen dioxide typically arises via the oxidation of nitric oxide by oxygen in air (e.g. as result of corona discharge ): [ 15 ]
NO 2 is introduced into the environment by natural causes, including entry from the stratosphere , bacterial respiration, volcanos, and lightning. These sources make NO 2 a trace gas in the atmosphere of Earth , where it plays a role in absorbing sunlight and regulating the chemistry of the troposphere , especially in determining ozone concentrations. [ 24 ]
Nitrogen dioxide also forms in most combustion processes. At elevated temperatures nitrogen combines with oxygen to form nitrogen dioxide:
For the general public, the most prominent sources of NO 2 are internal combustion engines , as combustion temperatures are high enough to thermally combine some of the nitrogen and oxygen in the air to form NO 2 . [ 8 ] Nitrogen dioxide accounts for a small fraction (generally well under 0.1) of NOx auto emissions. [ 25 ]
Outdoors, NO 2 can be a result of traffic from motor vehicles. [ 26 ] Indoors, exposure arises from cigarette smoke, [ 27 ] and butane and kerosene heaters and stoves. [ 28 ] Indoor exposure levels of NO 2 are, on average, at least three times higher in homes with gas stoves compared to electric stove. [ 29 ] [ 30 ]
Workers in industries where NO 2 is used are also exposed and are at risk for occupational lung diseases , and NIOSH has set exposure limits and safety standards. [ 6 ] Workers in high voltage areas especially those with spark or plasma creation are at risk. [ citation needed ] Agricultural workers can be exposed to NO 2 arising from grain decomposing in silos; chronic exposure can lead to lung damage in a condition called " silo-filler's disease ". [ 31 ] [ 32 ]
NO 2 diffuses into the epithelial lining fluid (ELF) of the respiratory epithelium and dissolves. There, it chemically reacts with antioxidant and lipid molecules in the ELF. The health effects of NO 2 are caused by the reaction products or their metabolites, which are reactive nitrogen species and reactive oxygen species that can drive bronchoconstriction , inflammation, reduced immune response, and may have effects on the heart. [ 33 ]
Acute harm due to NO 2 exposure is rare. 100–200 ppm can cause mild irritation of the nose and throat, 250–500 ppm can cause edema , leading to bronchitis or pneumonia , and levels above 1000 ppm can cause death due to asphyxiation from fluid in the lungs. There are often no symptoms at the time of exposure other than transient cough, fatigue or nausea, but over hours inflammation in the lungs causes edema. [ 34 ] [ 35 ]
For skin or eye exposure, the affected area is flushed with saline. For inhalation, oxygen is administered, bronchodilators may be administered, and if there are signs of methemoglobinemia , a condition that arises when nitrogen-based compounds affect the hemoglobin in red blood cells, methylene blue may be administered. [ 36 ] [ 37 ]
It is classified as an extremely hazardous substance in the United States as defined in Section 302 of the U.S. Emergency Planning and Community Right-to-Know Act (42 U.S.C. 11002), and it is subject to strict reporting requirements by facilities which produce, store, or use it in significant quantities. [ 38 ]
Exposure to low levels of NO 2 over time can cause changes in lung function. [ 39 ] Cooking with a gas stove is associated with poorer indoor air quality . Combustion of gas can lead to increased concentrations of nitrogen dioxide throughout the home environment which is linked to respiratory issues and diseases . [ 9 ] [ 10 ] Children exposed to NO 2 are more likely to be admitted to hospital with asthma . [ 40 ]
In 2019, the Court of Justice of the EU , found that France did not comply with the limit values of the EU air quality standards applicable to the concentrations of nitrogen dioxide (NO 2 ) in 12 air quality zones. [ 41 ]
Interaction of NO 2 and other NO x with water, oxygen and other chemicals in the atmosphere can form acid rain which harms sensitive ecosystems such as lakes and forests. [ 42 ] Elevated levels of NO 2 can also harm vegetation, decreasing growth, and reduce crop yields. [ 43 ] | https://en.wikipedia.org/wiki/O⚎N⚍O |
Dinitrogen tetroxide Dinitrogen trioxide Nitric oxide Nitrous oxide
Nitrogen dioxide is a chemical compound with the formula NO 2 . One of several nitrogen oxides , nitrogen dioxide is a reddish-brown gas. It is a paramagnetic , bent molecule with C 2v point group symmetry . Industrially, NO 2 is an intermediate in the synthesis of nitric acid , millions of tons of which are produced each year, primarily for the production of fertilizers .
Nitrogen dioxide is poisonous and can be fatal if inhaled in large quantities. [ 8 ] Cooking with a gas stove produces nitrogen dioxide which causes poorer indoor air quality . Combustion of gas can lead to increased concentrations of nitrogen dioxide throughout the home environment which is linked to respiratory issues and diseases . [ 9 ] [ 10 ] The LC 50 ( median lethal dose ) for humans has been estimated to be 174 ppm for a 1-hour exposure. [ 11 ] It is also included in the NO x family of atmospheric pollutants .
Nitrogen dioxide is a reddish-brown gas with a pungent, acrid odor above 21.2 °C (70.2 °F; 294.3 K) and becomes a yellowish-brown liquid below 21.2 °C (70.2 °F; 294.3 K). It forms an equilibrium with its dimer , dinitrogen tetroxide ( N 2 O 4 ), and converts almost entirely to N 2 O 4 below −11.2 °C (11.8 °F; 261.9 K). [ 6 ]
The bond length between the nitrogen atom and the oxygen atom is 119.7 pm . This bond length is consistent with a bond order between one and two.
Unlike ozone ( O 3 ) the ground electronic state of nitrogen dioxide is a doublet state , since nitrogen has one unpaired electron, [ 12 ] which decreases the alpha effect compared with nitrite and creates a weak bonding interaction with the oxygen lone pairs. The lone electron in NO 2 also means that this compound is a free radical , so the formula for nitrogen dioxide is often written as • NO 2 .
The reddish-brown color is a consequence of preferential absorption of light in the blue region of the spectrum (400–500 nm), although the absorption extends throughout the visible (at shorter wavelengths) and into the infrared (at longer wavelengths). Absorption of light at wavelengths shorter than about 400 nm results in photolysis (to form NO + O , atomic oxygen); in the atmosphere the addition of the oxygen atom so formed to O 2 results in ozone.
Industrially, nitrogen dioxide is produced and transported as its cryogenic liquid dimer, dinitrogen tetroxide . It is produced industrially by the oxidation of ammonia, the Ostwald Process . This reaction is the first step in the production of nitric acid: [ 13 ]
It can also be produced by the oxidation of nitrosyl chloride :
Instead, most laboratory syntheses stabilize and then heat the nitric acid to accelerate the decomposition. For example, the thermal decomposition of some metal nitrates generates NO 2 : [ 14 ]
Alternatively, dehydration of nitric acid produces nitronium nitrate ...
...which subsequently undergoes thermal decomposition:
NO 2 is generated by the reduction of concentrated nitric acid with a metal (such as copper):
Nitric acid decomposes slowly to nitrogen dioxide by the overall reaction:
The nitrogen dioxide so formed confers the characteristic yellow color often exhibited by this acid. However, the reaction is too slow to be a practical source of NO 2 .
At low temperatures, NO 2 reversibly converts to the colourless gas dinitrogen tetroxide ( N 2 O 4 ):
The exothermic equilibrium has enthalpy change Δ H = −57.23 kJ/mol . [ 15 ]
At 150 °C (302 °F; 423 K), NO 2 decomposes with release of oxygen via an endothermic process ( Δ H = 14 kJ/mol ):
As suggested by the weakness of the N–O bond, NO 2 is a good oxidizer. Consequently, it will combust, sometimes explosively, in the presence of hydrocarbons . [ 16 ]
NO 2 reacts with water to give nitric acid and nitrous acid :
This reaction is one of the steps in the Ostwald process for the industrial production of nitric acid from ammonia. [ 13 ] This reaction is negligibly slow at low concentrations of NO 2 characteristic of the ambient atmosphere, although it does proceed upon NO 2 uptake to surfaces. Such surface reaction is thought to produce gaseous HNO 2 (often written as HONO ) in outdoor and indoor environments. [ 17 ]
NO 2 is used to generate anhydrous metal nitrates from the oxides: [ 15 ]
Alkyl and metal iodides give the corresponding nitrates: [ 12 ]
The reactivity of nitrogen dioxide toward organic compounds has long been known. [ 18 ] For example, it reacts with amides to give N-nitroso derivatives. [ 19 ] It is used for nitrations under anhydrous conditions. [ 20 ]
NO 2 is used as an intermediate in the manufacturing of nitric acid , as a nitrating agent in the manufacturing of chemical explosives , as a polymerization inhibitor for acrylates , as a flour bleaching agent , [ 21 ] : 223 and as a room temperature sterilization agent. [ 22 ] It is also used as an oxidizer in rocket fuel , for example in red fuming nitric acid ; it was used in the Titan rockets , to launch Project Gemini , in the maneuvering thrusters of the Space Shuttle , and in uncrewed space probes sent to various planets. [ 23 ]
Nitrogen dioxide typically arises via the oxidation of nitric oxide by oxygen in air (e.g. as result of corona discharge ): [ 15 ]
NO 2 is introduced into the environment by natural causes, including entry from the stratosphere , bacterial respiration, volcanos, and lightning. These sources make NO 2 a trace gas in the atmosphere of Earth , where it plays a role in absorbing sunlight and regulating the chemistry of the troposphere , especially in determining ozone concentrations. [ 24 ]
Nitrogen dioxide also forms in most combustion processes. At elevated temperatures nitrogen combines with oxygen to form nitrogen dioxide:
For the general public, the most prominent sources of NO 2 are internal combustion engines , as combustion temperatures are high enough to thermally combine some of the nitrogen and oxygen in the air to form NO 2 . [ 8 ] Nitrogen dioxide accounts for a small fraction (generally well under 0.1) of NOx auto emissions. [ 25 ]
Outdoors, NO 2 can be a result of traffic from motor vehicles. [ 26 ] Indoors, exposure arises from cigarette smoke, [ 27 ] and butane and kerosene heaters and stoves. [ 28 ] Indoor exposure levels of NO 2 are, on average, at least three times higher in homes with gas stoves compared to electric stove. [ 29 ] [ 30 ]
Workers in industries where NO 2 is used are also exposed and are at risk for occupational lung diseases , and NIOSH has set exposure limits and safety standards. [ 6 ] Workers in high voltage areas especially those with spark or plasma creation are at risk. [ citation needed ] Agricultural workers can be exposed to NO 2 arising from grain decomposing in silos; chronic exposure can lead to lung damage in a condition called " silo-filler's disease ". [ 31 ] [ 32 ]
NO 2 diffuses into the epithelial lining fluid (ELF) of the respiratory epithelium and dissolves. There, it chemically reacts with antioxidant and lipid molecules in the ELF. The health effects of NO 2 are caused by the reaction products or their metabolites, which are reactive nitrogen species and reactive oxygen species that can drive bronchoconstriction , inflammation, reduced immune response, and may have effects on the heart. [ 33 ]
Acute harm due to NO 2 exposure is rare. 100–200 ppm can cause mild irritation of the nose and throat, 250–500 ppm can cause edema , leading to bronchitis or pneumonia , and levels above 1000 ppm can cause death due to asphyxiation from fluid in the lungs. There are often no symptoms at the time of exposure other than transient cough, fatigue or nausea, but over hours inflammation in the lungs causes edema. [ 34 ] [ 35 ]
For skin or eye exposure, the affected area is flushed with saline. For inhalation, oxygen is administered, bronchodilators may be administered, and if there are signs of methemoglobinemia , a condition that arises when nitrogen-based compounds affect the hemoglobin in red blood cells, methylene blue may be administered. [ 36 ] [ 37 ]
It is classified as an extremely hazardous substance in the United States as defined in Section 302 of the U.S. Emergency Planning and Community Right-to-Know Act (42 U.S.C. 11002), and it is subject to strict reporting requirements by facilities which produce, store, or use it in significant quantities. [ 38 ]
Exposure to low levels of NO 2 over time can cause changes in lung function. [ 39 ] Cooking with a gas stove is associated with poorer indoor air quality . Combustion of gas can lead to increased concentrations of nitrogen dioxide throughout the home environment which is linked to respiratory issues and diseases . [ 9 ] [ 10 ] Children exposed to NO 2 are more likely to be admitted to hospital with asthma . [ 40 ]
In 2019, the Court of Justice of the EU , found that France did not comply with the limit values of the EU air quality standards applicable to the concentrations of nitrogen dioxide (NO 2 ) in 12 air quality zones. [ 41 ]
Interaction of NO 2 and other NO x with water, oxygen and other chemicals in the atmosphere can form acid rain which harms sensitive ecosystems such as lakes and forests. [ 42 ] Elevated levels of NO 2 can also harm vegetation, decreasing growth, and reduce crop yields. [ 43 ] | https://en.wikipedia.org/wiki/O⚎N⚎O |
Ozone ( / ˈ oʊ z oʊ n / ) (or trioxygen ) is an inorganic molecule with the chemical formula O 3 . It is a pale blue gas with a distinctively pungent smell. It is an allotrope of oxygen that is much less stable than the diatomic allotrope O 2 , breaking down in the lower atmosphere to O 2 ( dioxygen ). Ozone is formed from dioxygen by the action of ultraviolet (UV) light and electrical discharges within the Earth's atmosphere . It is present in very low concentrations throughout the atmosphere, with its highest concentration high in the ozone layer of the stratosphere , which absorbs most of the Sun 's ultraviolet (UV) radiation.
Ozone's odor is reminiscent of chlorine , and detectable by many people at concentrations of as little as 0.1 ppm in air. Ozone's O 3 structure was determined in 1865. The molecule was later proven to have a bent structure and to be weakly diamagnetic . At standard temperature and pressure , ozone is a pale blue gas that condenses at cryogenic temperatures to a dark blue liquid and finally a violet-black solid . Ozone's instability with regard to more common dioxygen is such that both concentrated gas and liquid ozone may decompose explosively at elevated temperatures, physical shock, or fast warming to the boiling point. [ 5 ] [ 6 ] It is therefore used commercially only in low concentrations.
Ozone is a powerful oxidizing agent (far more so than dioxygen) and has many industrial and consumer applications related to oxidation. This same high oxidizing potential, however, causes ozone to damage mucous and respiratory tissues in animals, and also tissues in plants, above concentrations of about 0.1 ppm . While this makes ozone a potent respiratory hazard and pollutant near ground level , a higher concentration in the ozone layer (from two to eight ppm) is beneficial, preventing damaging UV light from reaching the Earth's surface.
The trivial name ozone is the most commonly used and preferred IUPAC name . The systematic names 2λ 4 -trioxidiene [ dubious – discuss ] and catena-trioxygen , valid IUPAC names, are constructed according to the substitutive and additive nomenclatures , respectively. The name ozone derives from ozein (ὄζειν), the Greek neuter present participle for smell, [ 7 ] referring to ozone's distinctive smell.
In appropriate contexts, ozone can be viewed as trioxidane with two hydrogen atoms removed, and as such, trioxidanylidene may be used as a systematic name, according to substitutive nomenclature. By default, these names pay no regard to the radicality of the ozone molecule. In an even more specific context, this can also name the non-radical singlet ground state, whereas the diradical state is named trioxidanediyl .
Trioxidanediyl (or ozonide ) is used, non-systematically, to refer to the substituent group (-OOO-). Care should be taken to avoid confusing the name of the group for the context-specific name for the ozone given above.
In 1785, Dutch chemist Martinus van Marum was conducting experiments involving electrical sparking above water when he noticed an unusual smell, which he attributed to the electrical reactions, failing to realize that he had in fact produced ozone. [ 8 ] [ 9 ]
A half century later, Christian Friedrich Schönbein noticed the same pungent odour and recognized it as the smell often following a bolt of lightning . In 1839, he succeeded in isolating the gaseous chemical and named it "ozone", from the Greek word ozein ( ὄζειν ) meaning "to smell". [ 10 ] [ 11 ] For this reason, Schönbein is generally credited with the discovery of ozone. [ 12 ] [ 13 ] [ 14 ] [ 8 ] He also noted the similarity of ozone smell to the smell of phosphorus, and in 1844 proved that the product of reaction of white phosphorus with air is identical. [ 10 ] A subsequent effort to call ozone "electrified oxygen" he ridiculed by proposing to call the ozone from white phosphorus "phosphorized oxygen". [ 10 ] The chemical formula for ozone, O 3 , was not determined until 1865 by Jacques-Louis Soret [ 15 ] and confirmed by Schönbein in 1867. [ 10 ] [ 16 ]
For much of the second half of the 19th century and well into the 20th, ozone was considered a healthy component of the environment by naturalists and health-seekers. Beaumont, California , had as its official slogan "Beaumont: Zone of Ozone", as evidenced on postcards and Chamber of Commerce letterhead. [ 17 ] Naturalists working outdoors often considered the higher elevations beneficial because of their ozone content which was readily monitored. [ 18 ] "There is quite a different atmosphere [at higher elevation] with enough ozone to sustain the necessary energy [to work]", wrote naturalist Henry Henshaw , working in Hawaii. [ 19 ] Seaside air was considered to be healthy because of its believed ozone content. The smell giving rise to this belief is in fact that of halogenated seaweed metabolites [ 20 ] and dimethyl sulfide . [ 21 ]
Much of ozone's appeal seems to have resulted from its "fresh" smell, which evoked associations with purifying properties. Scientists noted its harmful effects. In 1873 James Dewar and John Gray McKendrick documented that frogs grew sluggish, birds gasped for breath, and rabbits' blood showed decreased levels of oxygen after exposure to "ozonized air", which "exercised a destructive action". [ 22 ] [ 12 ] Schönbein himself reported that chest pains, irritation of the mucous membranes , and difficulty breathing occurred as a result of inhaling ozone, and small mammals died. [ 23 ] In 1911, Leonard Hill and Martin Flack stated in the Proceedings of the Royal Society B that ozone's healthful effects "have, by mere iteration, become part and parcel of common belief; and yet exact physiological evidence in favour of its good effects has been hitherto almost entirely wanting ... The only thoroughly well-ascertained knowledge concerning the physiological effect of ozone, so far attained, is that it causes irritation and œdema of the lungs, and death if inhaled in relatively strong concentration for any time." [ 12 ] [ 24 ]
During World War I , ozone was tested at Queen Alexandra Military Hospital in London as a possible disinfectant for wounds. The gas was applied directly to wounds for as long as 15 minutes. This resulted in damage to both bacterial cells and human tissue. Other sanitizing techniques, such as irrigation with antiseptics , were found preferable. [ 12 ] [ 25 ]
Until the 1920s, it was not certain whether small amounts of oxozone , O 4 , were also present in ozone samples due to the difficulty of applying analytical chemistry techniques to the explosive concentrated chemical. [ 26 ] [ 27 ] In 1923, Georg-Maria Schwab (working for his doctoral thesis under Ernst Hermann Riesenfeld ) was the first to successfully solidify ozone and perform accurate analysis which conclusively refuted the oxozone hypothesis. [ 26 ] [ 27 ] Further hitherto unmeasured physical properties of pure concentrated ozone were determined by the Riesenfeld group in the 1920s. [ 26 ]
Ozone is a colourless or pale blue gas, slightly soluble in water, and much more soluble in inert non-polar solvents such as carbon tetrachloride or fluorocarbons, in which it forms a blue solution. At 161 K (−112 °C; −170 °F), it condenses to form a dark blue liquid . It is dangerous to allow this liquid to warm to its boiling point, because both concentrated gaseous ozone and liquid ozone can detonate. At temperatures below 80 K (−193.2 °C; −315.7 °F), it forms a violet-black solid . [ 28 ]
Ozone has a very specific sharp odour somewhat resembling chlorine bleach . Most people can detect it at the 0.01 μmol/mol level in air. Exposure of 0.1 to 1 μmol/mol produces headaches and burning eyes and irritates the respiratory passages. [ 29 ] Even low concentrations of ozone in air are very destructive to organic materials such as latex, plastics, and animal lung tissue.
The ozone molecule is weakly diamagnetic . [ 30 ]
According to experimental evidence from microwave spectroscopy , ozone is a bent molecule, with C 2v symmetry (similar to the water molecule). [ 31 ] The O–O distances are 127.2 pm (1.272 Å ). The O–O–O angle is 116.78°. [ 32 ] The central atom is sp ² hybridized with one lone pair. Ozone is a polar molecule with a dipole moment of 0.53 D . [ 33 ] The molecule can be represented as a resonance hybrid with two contributing structures, each with a single bond on one side and double bond on the other. The arrangement possesses an overall bond order of 1.5 for both sides. It is isoelectronic with the nitrite anion . Naturally occurring ozone can be composed of substituted isotopes ( 16 O, 17 O, 18 O). A cyclic form has been predicted but not observed.
Ozone is among the most powerful oxidizing agents known, far stronger than O 2 . It is also unstable at high concentrations, decaying into ordinary diatomic oxygen. Its half-life varies with atmospheric conditions such as temperature, humidity, and air movement. Under laboratory conditions, the half-life will average ~1500 minutes (25 hours) in still air at room temperature (24 °C), zero humidity with zero air changes per hour. [ 34 ]
This reaction proceeds more rapidly with increasing temperature. Deflagration of ozone can be triggered by a spark and can occur in ozone concentrations of 10 wt% or higher. [ 35 ]
Ozone can also be produced from oxygen at the anode of an electrochemical cell. This reaction can create smaller quantities of ozone for research purposes. [ 36 ]
This can be observed as an unwanted reaction in a Hoffman apparatus during the electrolysis of water when the voltage is set above the necessary voltage.
Ozone oxidizes most metals (except gold , platinum , and iridium ) into oxides of the metals in their highest oxidation state . For example:
Ozone oxidizes nitric oxide to nitrogen dioxide :
This reaction is accompanied by chemiluminescence .
The NO 2 can be further oxidized to nitrate radical :
The NO 3 formed can react with NO 2 to form dinitrogen pentoxide ( N 2 O 5 ).
Solid nitronium perchlorate can be made from NO 2 , ClO 2 , and O 3 gases:
Ozone does not react with ammonium salts , but it oxidizes ammonia to ammonium nitrate :
Ozone reacts with carbon to form carbon dioxide , even at room temperature:
Ozone oxidizes sulfides to sulfates . For example, lead(II) sulfide is oxidized to lead(II) sulfate :
Sulfuric acid can be produced from ozone, water and either elemental sulfur or sulfur dioxide :
In the gas phase , ozone reacts with hydrogen sulfide to form sulfur dioxide:
In an aqueous solution, however, two competing simultaneous reactions occur, one to produce elemental sulfur, and one to produce sulfuric acid :
Alkenes can be oxidatively cleaved by ozone, in a process called ozonolysis , giving alcohols, aldehydes, ketones, and carboxylic acids, depending on the second step of the workup.
Ozone can also cleave alkynes to form an acid anhydride or diketone product. [ 38 ] If the reaction is performed in the presence of water, the anhydride hydrolyzes to give two carboxylic acids .
Usually ozonolysis is carried out in a solution of dichloromethane , at a temperature of −78 °C. After a sequence of cleavage and rearrangement, an organic ozonide is formed. With reductive workup (e.g. zinc in acetic acid or dimethyl sulfide ), ketones and aldehydes will be formed, with oxidative workup (e.g. aqueous or alcoholic hydrogen peroxide ), carboxylic acids will be formed. [ 39 ]
All three atoms of ozone may also react, as in the reaction of tin(II) chloride with hydrochloric acid and ozone:
Iodine perchlorate can be made by treating iodine dissolved in cold anhydrous perchloric acid with ozone:
Ozone could also react with potassium iodide to give oxygen and iodine gas that can be titrated for quantitative determination: [ 40 ]
Ozone can be used for combustion reactions and combustible gases; ozone provides higher temperatures than burning in dioxygen ( O 2 ). The following is a reaction for the combustion of carbon subnitride which can also cause higher temperatures:
Ozone can react at cryogenic temperatures. At 77 K (−196.2 °C; −321.1 °F), atomic hydrogen reacts with liquid ozone to form a hydrogen superoxide radical , which dimerizes : [ 41 ]
Ozone is a toxic substance, [ 42 ] [ 43 ] commonly found or generated in human environments (aircraft cabins, offices with photocopiers, laser printers, sterilizers, ...).
The catalytic decomposition of ozone is very important to reduce pollution. This type of decomposition is the most widely used, especially with solid catalysts, and it has many advantages such as a higher conversion with a lower temperature. Furthermore, the product and the catalyst can be instantaneously separated, and this way the catalyst can be easily recovered without using any separation operation. The most-used materials in the catalytic decomposition of ozone in the gas phase are manganese dioxide , transition metals such as Mn, Co, Cu, Fe, Ni, or Ag, and noble metals such as Pt, Rh, or Pd.
Free radicals of chlorine (Cl · ), formed by the action of ultraviolet radiation on chlorofluorocarbons (CFCs) and sea salt, are known to catalyze the breakdown of ozone in the atmosphere.
There are two other possibilities for decomposing ozone in the gas phase:
The uncatalyzed process of ozone decomposition in the gas phase is a complex reaction involving two elementary reactions that finally lead to molecular oxygen, [ 45 ] and this means that the reaction order and the rate law cannot be determined by the stoichiometry of the overall reaction.
Overall reaction: 2 O 3 ⟶ 3 O 2 {\displaystyle {\ce {2 O3 -> 3 O2}}}
Rate law (observed): V = K o b s ⋅ [ O 3 ] 2 [ O 2 ] {\displaystyle V={\frac {K_{obs}\cdot [{\ce {O3}}]^{2}}{[{\ce {O2}}]}}}
where K o b s {\displaystyle K_{obs}} is the observed rate constant and V {\displaystyle V} is the reaction rate. From the rate law above it can be determined that the partial order respect to molecular oxygen is −1 and respect to ozone is 2; therefore, the global reaction order is 1.
The first step is a unimolecular reaction wherein one molecule of ozone decomposes into two products (molecular oxygen and oxygen). The oxygen atom from the first step is a reactive intermediate because it participates as a reactant in the second step, which is a bimolecular reaction because there are two different reactants (ozone and oxygen) that give rise to molecular oxygen.
Step 1: Unimolecular reaction O 3 ⟶ O 2 + O {\displaystyle {\ce {O3 -> O2 + O}}}
Step 2: Bimolecular reaction O 3 + O ⟶ 2 O 2 {\displaystyle {\ce {O3 + O -> 2 O2}}}
These two steps have different reaction rates and rate constants. The reaction rate laws for each of these steps are shown below:
The following mechanism allows to explain the rate law of the ozone decomposition observed experimentally, and also it allows to determine the reaction orders with respect to ozone and oxygen, with which the overall reaction order will be determined.
The first step is assumed reversible and faster than the second reaction, which means that the slower rate determining step is the second reaction. This step determines the rate of product formation, and so V = V 2 {\displaystyle V=V_{2}} . However, this equation depends on the concentration of oxygen (intermediate), which does not appear in the observed rate law. Since the first step is a rapid equilibrium, the concentration of the intermediate can be determined as follows:
Then using these equations, the formation rate of molecular oxygen is as shown below:
The mechanism is consistent with the rate law observed experimentally if the rate constant ( K obs ) is given in terms of the individual mechanistic steps' rate constants as follows: [ 46 ]
where K obs = K 2 ⋅ K 1 K − 1 {\displaystyle K_{\text{obs}}={K_{2}\cdot K_{1} \over K_{-1}}}
Reduction of ozone gives the ozonide anion, O − 3 . Derivatives of this anion are explosive and must be stored at cryogenic temperatures. Ozonides for all the alkali metals are known. KO 3 , RbO 3 , and CsO 3 can be prepared from their respective superoxides:
Although KO 3 can be formed as above, it can also be formed from potassium hydroxide and ozone: [ 47 ]
NaO 3 and LiO 3 must be prepared by action of CsO 3 in liquid NH 3 on an ion-exchange resin containing Na + or Li + ions: [ 48 ]
A solution of calcium in ammonia reacts with ozone to give ammonium ozonide and not calcium ozonide: [ 41 ]
Ozone can be used to remove iron and manganese from water , forming a precipitate which can be filtered:
Ozone oxidizes dissolved hydrogen sulfide in water to sulfurous acid :
These three reactions are central in the use of ozone-based well water treatment.
Ozone detoxifies cyanides by converting them to cyanates .
Ozone completely decomposes urea : [ 49 ]
Ozone is a bent triatomic molecule with three vibrational modes: the symmetric stretch (1103.157 cm −1 ), bend (701.42 cm −1 ) and antisymmetric stretch (1042.096 cm −1 ). [ 50 ] The symmetric stretch and bend are weak absorbers, but the antisymmetric stretch is strong and responsible for ozone being an important minor greenhouse gas . This IR band is also used to detect ambient and atmospheric ozone although UV-based measurements are more common. [ 51 ]
The electromagnetic spectrum of ozone is quite complex. An overview can be seen at the MPI Mainz UV/VIS Spectral Atlas of Gaseous Molecules of Atmospheric Interest. [ 52 ]
All of the bands are dissociative, meaning that the molecule falls apart to O + O 2 after absorbing a photon. The most important absorption is the Hartley band, extending from slightly above 300 nm down to slightly above 200 nm. It is this band that is responsible for absorbing UV C in the stratosphere.
On the high wavelength side, the Hartley band transitions to the so-called Huggins band, which falls off rapidly until disappearing by ~360 nm. Above 400 nm, extending well out into the NIR, are the Chappius and Wulf bands. There, unstructured absorption bands are useful for detecting high ambient concentrations of ozone, but are so weak that they do not have much practical effect.
There are additional absorption bands in the far UV, which increase slowly from 200 nm down to reaching a maximum at ~120 nm.
The standard way to express total ozone levels (the amount of ozone in a given vertical column) in the atmosphere is by using Dobson units . Point measurements are reported as mole fractions in nmol/mol (parts per billion, ppb) or as concentrations in μg/m 3 . The study of ozone concentration in the atmosphere started in the 1920s. [ 53 ]
The highest levels of ozone in the atmosphere are in the stratosphere , in a region also known as the ozone layer between about 10 and 50 km above the surface (or between about 6 and 31 miles). However, even in this "layer", the ozone concentrations are only two to eight parts per million, so most of the oxygen there is dioxygen, O 2 , at about 210,000 parts per million by volume. [ 54 ]
Ozone in the stratosphere is mostly produced from short-wave ultraviolet rays between 240 and 160 nm. Oxygen starts to absorb weakly at 240 nm in the Herzberg bands, but most of the oxygen is dissociated by absorption in the strong Schumann–Runge bands between 200 and 160 nm where ozone does not absorb. While shorter wavelength light, extending to even the X-Ray limit, is energetic enough to dissociate molecular oxygen, there is relatively little of it, and, the strong solar emission at Lyman-alpha, 121 nm, falls at a point where molecular oxygen absorption is a minimum. [ 55 ]
The process of ozone creation and destruction is called the Chapman cycle and starts with the photolysis of molecular oxygen
followed by reaction of the oxygen atom with another molecule of oxygen to form ozone.
where "M" denotes the third body that carries off the excess energy of the reaction. The ozone molecule can then absorb a UV-C photon and dissociate
The excess kinetic energy heats the stratosphere when the O atoms and the molecular oxygen fly apart and collide with other molecules. This conversion of UV light into kinetic energy warms the stratosphere. The oxygen atoms produced in the photolysis of ozone then react back with other oxygen molecule as in the previous step to form more ozone. In the clear atmosphere, with only nitrogen and oxygen, ozone can react with the atomic oxygen to form two molecules of O 2 :
An estimate of the rate of this termination step to the cycling of atomic oxygen back to ozone can be found simply by taking the ratios of the concentration of O 2 to O 3 . The termination reaction is catalysed by the presence of certain free radicals, of which the most important are hydroxyl (OH), nitric oxide (NO) and atomic chlorine (Cl) and bromine (Br). In the second half of the 20th century, the amount of ozone in the stratosphere was discovered to be declining , mostly because of increasing concentrations of chlorofluorocarbons (CFC) and similar chlorinated and brominated organic molecules . The concern over the health effects of the decline led to the 1987 Montreal Protocol , the ban on the production of many ozone-depleting chemicals and in the first and second decade of the 21st century the beginning of the recovery of stratospheric ozone concentrations.
Ozone in the ozone layer filters out sunlight wavelengths from about 200 nm UV rays to 315 nm, with ozone peak absorption at about 250 nm. [ 56 ] This ozone UV absorption is important to life, since it extends the absorption of UV by ordinary oxygen and nitrogen in air (which absorb all wavelengths < 200 nm) through the lower UV-C (200–280 nm) and the entire UV-B band (280–315 nm). The small unabsorbed part that remains of UV-B after passage through ozone causes sunburn in humans, and direct DNA damage in living tissues in both plants and animals. Ozone's effect on mid-range UV-B rays is illustrated by its effect on UV-B at 290 nm, which has a radiation intensity 350 million times as powerful at the top of the atmosphere as at the surface. Nevertheless, enough of UV-B radiation at similar frequency reaches the ground to cause some sunburn, and these same wavelengths are also among those responsible for the production of vitamin D in humans.
The ozone layer has little effect on the longer UV wavelengths called UV-A (315–400 nm), but this radiation does not cause sunburn or direct DNA damage. While UV-A probably does cause long-term skin damage in certain humans, it is not as dangerous to plants and to the health of surface-dwelling organisms on Earth in general (see ultraviolet for more information on near ultraviolet).
Ground-level ozone (or tropospheric ozone) is an atmospheric pollutant. [ 57 ] It is not emitted directly by car engines or by industrial operations, but formed by the reaction of sunlight on air containing hydrocarbons and nitrogen oxides that react to form ozone directly at the source of the pollution or many kilometers downwind.
Ozone reacts directly with some hydrocarbons such as aldehydes and thus begins their removal from the air, but the products are themselves key components of smog . Ozone photolysis by UV light leads to production of the hydroxyl radical HO• and this plays a part in the removal of hydrocarbons from the air, but is also the first step in the creation of components of smog such as peroxyacyl nitrates , which can be powerful eye irritants. The atmospheric lifetime of tropospheric ozone is about 22 days; its main removal mechanisms are being deposited to the ground, the above-mentioned reaction giving HO•, and by reactions with OH and the peroxy radical HO 2 •. [ 58 ]
There is evidence of significant reduction in agricultural yields because of increased ground-level ozone and pollution which interferes with photosynthesis and stunts overall growth of some plant species. [ 59 ] [ 60 ] The United States Environmental Protection Agency (EPA) has proposed a secondary regulation to reduce crop damage, in addition to the primary regulation designed for the protection of human health.
Certain examples of cities with elevated ozone readings are Denver, Colorado ; Houston, Texas ; and Mexico City , Mexico . Houston has a reading of around 41 nmol/mol, while Mexico City is far more hazardous, with a reading of about 125 nmol/mol. [ 60 ]
Ground-level ozone, or tropospheric ozone, is the most concerning type of ozone pollution in urban areas and is increasing in general. [ 61 ] Ozone pollution in urban areas affects denser populations, and is worsened by high populations of vehicles, which emit pollutants NO 2 and VOCs , the main contributors to problematic ozone levels. [ 62 ] Ozone pollution in urban areas is especially concerning with increasing temperatures, raising heat-related mortality during heat waves . [ 63 ] During heat waves in urban areas, ground level ozone pollution can be 20% higher than usual. [ 64 ] Ozone pollution in urban areas reaches higher levels of exceedance in the summer and autumn, which may be explained by weather patterns and traffic patterns. [ 62 ] People experiencing poverty are more affected by pollution in general, even though these populations are less likely to be contributing to pollution levels. [ 65 ]
As mentioned above, Denver, Colorado, is one of the many cities in the U.S. that have high amounts of ozone. According to the American Lung Association , the Denver–Aurora area is the 14th most ozone-polluted area in the U.S. [ 66 ] The problem of high ozone levels is not new to this area. In 2004, the EPA allotted the Denver Metro /North Front Range [ b ] as non-attainment areas per 1997's 8-hour ozone standard, [ 67 ] but later deferred this status until 2007. The non-attainment standard indicates that an area does not meet the EPA's air quality standards. The Colorado Ozone Action Plan was created in response, and numerous changes were implemented from this plan. The first major change was that car emission testing was expanded across the state to more counties that did not previously mandate emissions testing, like areas of Larimer and Weld County. There have also been changes made to decrease Nitrogen Oxides (NOx) and Volatile Organic Compound (VOC) emissions, which should help lower ozone levels.
One large contributor to high ozone levels in the area is the oil and natural gas industry situated in the Denver-Julesburg Basin (DJB) which overlaps with a majority of Colorado's metropolitan areas. Ozone is produced naturally in the Earth's stratosphere, but is also produced in the troposphere from human efforts. Briefly mentioned above, NOx and VOCs react with sunlight to create ozone through a process called photochemistry. One hour elevated ozone events (<75 ppb) "occur during June–August indicating that elevated ozone levels are driven by regional photochemistry". [ 68 ] According to an article from the University of Colorado-Boulder, "Oil and natural gas VOC emission have a major role in ozone production and bear the potential to contribute to elevated O 3 levels in the Northern Colorado Front Range (NCFR)". [ 68 ] Using complex analyses to research wind patterns and emissions from large oil and natural gas operations, the authors concluded that "elevated O 3 levels in the NCFR are predominantly correlated with air transport from N– ESE, which are the upwind sectors where the O&NG operations in the Wattenberg Field area of the DJB are located". [ 68 ]
Contained in the Colorado Ozone Action Plan, created in 2008, plans exist to evaluate "emission controls for large industrial sources of NOx" and "statewide control requirements for new oil and gas condensate tanks and pneumatic valves". [ 69 ] In 2011, the Regional Haze Plan was released that included a more specific plan to help decrease NOx emissions. These efforts are increasingly difficult to implement and take many years to come to pass. Of course there are also other reasons that ozone levels remain high. These include: a growing population meaning more car emissions, and the mountains along the NCFR that can trap emissions. If interested, daily air quality readings can be found at the Colorado Department of Public Health and Environment's website. [ 70 ] As noted earlier, Denver continues to experience high levels of ozone to this day. It will take many years and a systems-thinking approach to combat this issue of high ozone levels in the Front Range of Colorado.
Ozone gas attacks any polymer possessing olefinic or double bonds within its chain structure, such as natural rubber , nitrile rubber , and styrene-butadiene rubber. Products made using these polymers are especially susceptible to attack, which causes cracks to grow longer and deeper with time, the rate of crack growth depending on the load carried by the rubber component and the concentration of ozone in the atmosphere. Such materials can be protected by adding antiozonants , such as waxes, which bond to the surface to create a protective film or blend with the material and provide long term protection. Ozone cracking used to be a serious problem in car tires, [ 71 ] for example, but it is not an issue with modern tires. On the other hand, many critical products, like gaskets and O-rings , may be attacked by ozone produced within compressed air systems. Fuel lines made of reinforced rubber are also susceptible to attack, especially within the engine compartment, where some ozone is produced by electrical components. Storing rubber products in close proximity to a DC electric motor can accelerate ozone cracking. The commutator of the motor generates sparks which in turn produce ozone.
Although ozone was present at ground level before the Industrial Revolution , peak concentrations are now far higher than the pre-industrial levels, and even background concentrations well away from sources of pollution are substantially higher. [ 73 ] [ 74 ] Ozone acts as a greenhouse gas , absorbing some of the infrared energy emitted by the earth. Quantifying the greenhouse gas potency of ozone is difficult because it is not present in uniform concentrations across the globe. However, the most widely accepted scientific assessments relating to climate change (e.g. the Intergovernmental Panel on Climate Change Third Assessment Report ) [ 75 ] suggest that the radiative forcing of tropospheric ozone is about 25% that of carbon dioxide .
The annual global warming potential of tropospheric ozone is between 918 and 1022 tons carbon dioxide equivalent /tons tropospheric ozone. This means on a per-molecule basis, ozone in the troposphere has a radiative forcing effect roughly 1,000 times as strong as carbon dioxide . However, tropospheric ozone is a short-lived greenhouse gas, which decays in the atmosphere much more quickly than carbon dioxide . This means that over a 20-year span, the global warming potential of tropospheric ozone is much less, roughly 62 to 69 tons carbon dioxide equivalent / ton tropospheric ozone. [ 76 ]
Because of its short-lived nature, tropospheric ozone does not have strong global effects, but has very strong radiative forcing effects on regional scales. In fact, there are regions of the world where tropospheric ozone has a radiative forcing up to 150% of carbon dioxide . [ 77 ] For example, ozone increase in the troposphere is shown to be responsible for ~30% of upper Southern Ocean interior warming between 1955 and 2000. [ 78 ]
Filters containing an adsorbent or catalyst such as charcoal (carbon) may be used to remove odors and gaseous pollutants such as volatile organic compounds or ozone. [ 79 ]
For the last few decades, scientists studied the effects of acute and chronic ozone exposure on human health. Hundreds of studies suggest that ozone is harmful to people at levels currently found in urban areas. [ 80 ] [ 81 ] Ozone has been shown to affect the respiratory, cardiovascular and central nervous system. Early death and problems in reproductive health and development are also shown to be associated with ozone exposure. [ 82 ]
The American Lung Association has identified five populations who are especially vulnerable to the effects of breathing ozone: [ 83 ]
Additional evidence suggests that women, those with obesity and low-income populations may also face higher risk from ozone, although more research is needed. [ 83 ]
Acute ozone exposure ranges from hours to a few days. Because ozone is a gas, it directly affects the lungs and the entire respiratory system. Inhaled ozone causes inflammation and acute—but reversible—changes in lung function, as well as airway hyperresponsiveness. [ 84 ] These changes lead to shortness of breath, wheezing, and coughing which may exacerbate lung diseases, like asthma or chronic obstructive pulmonary disease (COPD) resulting in the need to receive medical treatment. [ 85 ] [ 86 ] Acute and chronic exposure to ozone has been shown to cause an increased risk of respiratory infections, due to the following mechanism. [ 87 ]
Multiple studies have been conducted to determine the mechanism behind ozone's harmful effects, particularly in the lungs. These studies have shown that exposure to ozone causes changes in the immune response within the lung tissue, resulting in disruption of both the innate and adaptive immune response, as well as altering the protective function of lung epithelial cells. [ 88 ] It is thought that these changes in immune response and the related inflammatory response are factors that likely contribute to the increased risk of lung infections, and worsening or triggering of asthma and reactive airways after exposure to ground-level ozone pollution. [ 88 ] [ 89 ]
The innate (cellular) immune system consists of various chemical signals and cell types that work broadly and against multiple pathogen types, typically bacteria or foreign bodies/substances in the host. [ 89 ] [ 90 ] The cells of the innate system include phagocytes, neutrophils, [ 90 ] both thought to contribute to the mechanism of ozone pathology in the lungs, as the functioning of these cell types have been shown to change after exposure to ozone. [ 89 ] Macrophages, cells that serve the purpose of eliminating pathogens or foreign material through the process of "phagocytosis", [ 90 ] have been shown to change the level of inflammatory signals they release in response to ozone, either up-regulating and resulting in an inflammatory response in the lung, or down-regulating and reducing immune protection. [ 88 ] Neutrophils, another important cell type of the innate immune system that primarily targets bacterial pathogens, [ 90 ] are found to be present in the airways within 6 hours of exposure to high ozone levels. Despite high levels in the lung tissues, however, their ability to clear bacteria appears impaired by exposure to ozone. [ 88 ]
The adaptive immune system is the branch of immunity that provides long-term protection via the development of antibodies targeting specific pathogens and is also impacted by high ozone exposure. [ 89 ] [ 90 ] Lymphocytes, a cellular component of the adaptive immune response, produce an increased amount of inflammatory chemicals called "cytokines" after exposure to ozone, which may contribute to airway hyperreactivity and worsening asthma symptoms. [ 88 ]
The airway epithelial cells also play an important role in protecting individuals from pathogens. In normal tissue, the epithelial layer forms a protective barrier, and also contains specialized ciliary structures that work to clear foreign bodies, mucus and pathogens from the lungs. When exposed to ozone, the cilia become damaged and mucociliary clearance of pathogens is reduced. Furthermore, the epithelial barrier becomes weakened, allowing pathogens to cross the barrier, proliferate and spread into deeper tissues. Together, these changes in the epithelial barrier help make individuals more susceptible to pulmonary infections. [ 88 ]
Inhaling ozone not only affects the immune system and lungs, but it may also affect the heart as well. Ozone causes short-term autonomic imbalance leading to changes in heart rate and reduction in heart rate variability; [ 91 ] and high levels exposure for as little as one-hour results in a supraventricular arrhythmia in the elderly, [ 92 ] both increase the risk of premature death and stroke. Ozone may also lead to vasoconstriction resulting in increased systemic arterial pressure contributing to increased risk of cardiac morbidity and mortality in patients with pre-existing cardiac diseases. [ 93 ] [ 94 ]
Breathing ozone for periods longer than eight hours at a time for weeks, months or years defines chronic exposure. Numerous studies suggest a serious impact on the health of various populations from this exposure.
One study finds significant positive associations between chronic ozone and all-cause, circulatory, and respiratory mortality with 2%, 3%, and 12% increases in risk per 10 ppb [ 95 ] and report an association (95% CI) of annual ozone and all-cause mortality with a hazard ratio of 1.02 (1.01–1.04), and with cardiovascular mortality of 1.03 (1.01–1.05). A similar study finds similar associations with all-cause mortality and even larger effects for cardiovascular mortality. [ 96 ] An increased risk of mortality from respiratory causes is associated with long-term chronic exposure to ozone. [ 97 ]
Chronic ozone has detrimental effects on children, especially those with asthma. The risk for hospitalization in children with asthma increases with chronic exposure to ozone; younger children and those with low-income status are even at greater risk. [ 98 ]
Adults suffering from respiratory diseases (asthma, [ 99 ] COPD, [ 100 ] lung cancer [ 101 ] ) are at a higher risk of mortality and morbidity and critically ill patients have an increased risk of developing acute respiratory distress syndrome with chronic ozone exposure as well. [ 102 ]
Ozone generators sold as air cleaners intentionally produce the gas ozone. [ 43 ] These are often marketed to control indoor air pollution , and use misleading terms to describe ozone. Some examples are describing it as "energized oxygen" or "pure air", suggesting that ozone is a healthy or "better" kind of oxygen. [ 43 ] However, according to the EPA , "There is evidence to show that at concentrations that do not exceed public health standards, ozone is not effective at removing many odor-causing chemicals", and "If used at concentrations that do not exceed public health standards, ozone applied to indoor air does not effectively remove viruses, bacteria, mold, or other biological pollutants.". [ 43 ] Furthermore, another report states that "results of some controlled studies show that concentrations of ozone considerably higher than these [human safety] standards are possible even when a user follows the manufacturer's operating instructions". [ 103 ]
The California Air Resources Board has a page listing air cleaners (many with ionizers ) meeting their indoor ozone limit of 0.050 parts per million. [ 104 ] From that article:
All portable indoor air cleaning devices sold in California must be certified by the California Air Resources Board (CARB). To be certified, air cleaners must be tested for electrical safety and ozone emissions, and meet an ozone emission concentration limit of 0.050 parts per million. For more information about the regulation, visit the air cleaner regulation .
Ozone precursors are a group of pollutants, predominantly those emitted during the combustion of fossil fuels . Ground-level ozone pollution (tropospheric ozone) is produced near the Earth's surface by the action of daylight UV rays on these precursors. The ozone at ground level is primarily from fossil fuel precursors, but methane is a natural precursor, and the very low natural background level of ozone at ground level is considered safe. This section examines the health impacts of fossil fuel burning, which raises ground level ozone far above background levels.
There is a great deal of evidence to show that ground-level ozone can harm lung function and irritate the respiratory system . [ 57 ] [ 106 ] Exposure to ozone (and the pollutants that produce it) is linked to premature death , asthma , bronchitis , heart attack , and other cardiopulmonary problems. [ 107 ] [ 108 ]
Long-term exposure to ozone has been shown to increase risk of death from respiratory illness . [ 43 ] A study of 450,000 people living in U.S. cities saw a significant correlation between ozone levels and respiratory illness over the 18-year follow-up period. The study revealed that people living in cities with high ozone levels, such as Houston or Los Angeles, had an over 30% increased risk of dying from lung disease. [ 109 ] [ 110 ]
Air quality guidelines such as those from the World Health Organization , the U.S. Environmental Protection Agency (EPA), and the European Union are based on detailed studies designed to identify the levels that can cause measurable ill health effects .
According to scientists with the EPA, susceptible people can be adversely affected by ozone levels as low as 40 nmol/mol. [ 108 ] [ 111 ] [ 112 ] In the EU, the current target value for ozone concentrations is 120 μg/m 3 which is about 60 nmol/mol. This target applies to all member states in accordance with Directive 2008/50/EC. [ 113 ] Ozone concentration is measured as a maximum daily mean of 8 hour averages and the target should not be exceeded on more than 25 calendar days per year, starting from January 2010. While the directive requires in the future a strict compliance with 120 μg/m 3 limit (i.e. mean ozone concentration not to be exceeded on any day of the year), there is no date set for this requirement and this is treated as a long-term objective. [ 114 ]
In the US, the Clean Air Act directs the EPA to set National Ambient Air Quality Standards for several pollutants, including ground-level ozone, and counties out of compliance with these standards are required to take steps to reduce their levels. In May 2008, under a court order, the EPA lowered its ozone standard from 80 nmol/mol to 75 nmol/mol. The move proved controversial, since the Agency's own scientists and advisory board had recommended lowering the standard to 60 nmol/mol. [ 108 ] Many public health and environmental groups also supported the 60 nmol/mol standard, [ 115 ] and the World Health Organization recommends 100 μg/m 3 (51 nmol/mol). [ 116 ]
On January 7, 2010, the U.S. Environmental Protection Agency (EPA) announced proposed revisions to the National Ambient Air Quality Standard (NAAQS) for the pollutant ozone, the principal component of smog:
... EPA proposes that the level of the 8-hour primary standard, which was set at 0.075 μmol/mol in the 2008 final rule, should instead be set at a lower level within the range of 0.060 to 0.070 μmol/mol, to provide increased protection for children and other at risk populations against an array of O 3 – related adverse health effects that range from decreased lung function and increased respiratory symptoms to serious indicators of respiratory morbidity including emergency department visits and hospital admissions for respiratory causes, and possibly cardiovascular-related morbidity as well as total non- accidental and cardiopulmonary mortality ... [ 117 ]
On October 26, 2015, the EPA published a final rule with an effective date of December 28, 2015, that revised the 8-hour primary NAAQS from 0.075 ppm to 0.070 ppm. [ 118 ]
The EPA has developed an air quality index (AQI) to help explain air pollution levels to the general public. Under the current standards, eight-hour average ozone mole fractions of 85 to 104 nmol/mol are described as "unhealthy for sensitive groups", 105 nmol/mol to 124 nmol/mol as "unhealthy", and 125 nmol/mol to 404 nmol/mol as "very unhealthy". [ 119 ]
Ozone can also be present in indoor air pollution , partly as a result of electronic equipment such as photocopiers. A connection has also been known to exist between the increased pollen, fungal spores, and ozone caused by thunderstorms and hospital admissions of asthma sufferers. [ 120 ]
In the Victorian era , one British folk myth held that the smell of the sea was caused by ozone. In fact, the characteristic "smell of the sea" is caused by dimethyl sulfide , a chemical generated by phytoplankton . Victorian Britons considered the resulting smell "bracing". [ 121 ]
An investigation to assess the joint mortality effects of ozone and heat during the European heat waves in 2003, concluded that these appear to be additive. [ 122 ]
Ozone, along with reactive forms of oxygen such as superoxide , singlet oxygen , hydrogen peroxide , and hypochlorite ions, is produced by white blood cells and other biological systems (such as the roots of marigolds ) as a means of destroying foreign bodies. Ozone reacts directly with organic double bonds. Also, when ozone breaks down to dioxygen it gives rise to oxygen free radicals , which are highly reactive and capable of damaging many organic molecules . Moreover, it is believed that the powerful oxidizing properties of ozone may be a contributing factor of inflammation . The cause-and-effect relationship of how the ozone is created in the body and what it does is still under consideration and still subject to various interpretations, since other body chemical processes can trigger some of the same reactions. There is evidence linking the antibody-catalyzed water-oxidation pathway of the human immune response to the production of ozone. In this system, ozone is produced by antibody-catalyzed production of trioxidane from water and neutrophil-produced singlet oxygen. [ 123 ]
When inhaled, ozone reacts with compounds lining the lungs to form specific, cholesterol-derived metabolites that are thought to facilitate the build-up and pathogenesis of atherosclerotic plaques (a form of heart disease ). These metabolites have been confirmed as naturally occurring in human atherosclerotic arteries and are categorized into a class of secosterols termed atheronals , generated by ozonolysis of cholesterol's double bond to form a 5,6 secosterol [ 124 ] as well as a secondary condensation product via aldolization. [ 125 ]
Ozone has been implicated to have an adverse effect on plant growth: "... ozone reduced total chlorophylls, carotenoid and carbohydrate concentration, and increased 1-aminocyclopropane-1-carboxylic acid (ACC) content and ethylene production. In treated plants, the ascorbate leaf pool was decreased, while lipid peroxidation and solute leakage were significantly higher than in ozone-free controls. The data indicated that ozone triggered protective mechanisms against oxidative stress in citrus." [ 126 ] Studies that have used pepper plants as a model have shown that ozone decreased fruit yield and changed fruit quality. [ 127 ] [ 128 ] Furthermore, it was also observed a decrease in chlorophylls levels and antioxidant defences on the leaves, as well as increased the reactive oxygen species (ROS) levels and lipid and protein damages. [ 127 ] [ 128 ]
A 2022 study concludes that East Asia loses 63 billion dollars in crops per year due to ozone pollution, a byproduct of fossil fuel combustion. China loses about one-third of its potential wheat production and one-fourth of its rice production. [ 129 ] [ 130 ]
Because of the strongly oxidizing properties of ozone, ozone is a primary irritant, affecting especially the eyes and respiratory systems and can be hazardous at even low concentrations. The Canadian Centre for Occupation Safety and Health reports that:
Even very low concentrations of ozone can be harmful to the upper respiratory tract and the lungs. The severity of injury depends on both the concentration of ozone and the duration of exposure. Severe and permanent lung injury or death could result from even a very short-term exposure to relatively low concentrations." [ 131 ]
To protect workers potentially exposed to ozone, U.S. Occupational Safety and Health Administration has established a permissible exposure limit (PEL) of 0.1 μmol/mol (29 CFR 1910.1000 table Z-1), calculated as an 8-hour time weighted average. Higher concentrations are especially hazardous and NIOSH has established an Immediately Dangerous to Life and Health Limit (IDLH) of 5 μmol/mol. [ 132 ] Work environments where ozone is used or where it is likely to be produced should have adequate ventilation and it is prudent to have a monitor for ozone that will alarm if the concentration exceeds the OSHA PEL. Continuous monitors for ozone are available from several suppliers.
Elevated ozone exposure can occur on passenger aircraft , with levels depending on altitude and atmospheric turbulence. [ 133 ] U.S. Federal Aviation Administration regulations set a limit of 250 nmol/mol with a maximum four-hour average of 100 nmol/mol. [ 134 ] Some planes are equipped with ozone converters in the ventilation system to reduce passenger exposure. [ 133 ]
Ozone generators , or ozonators , [ 135 ] are used to produce ozone for cleaning air or removing smoke odours in unoccupied rooms. These ozone generators can produce over 3 g of ozone per hour. Ozone often forms in nature under conditions where O 2 will not react. [ 29 ] Ozone used in industry is measured in μmol/mol (ppm, parts per million), nmol/mol (ppb, parts per billion), μg/m 3 , mg/h (milligrams per hour) or weight percent. The regime of applied concentrations ranges from 1% to 5% (in air) and from 6% to 14% (in oxygen) for older generation methods. New electrolytic methods can achieve up 20% to 30% dissolved ozone concentrations in output water.
Temperature and humidity play a large role in how much ozone is being produced using traditional generation methods (such as corona discharge and ultraviolet light). Old generation methods will produce less than 50% of nominal capacity if operated with humid ambient air, as opposed to very dry air. New generators, using electrolytic methods, can achieve higher purity and dissolution through using water molecules as the source of ozone production.
This is the most common type of ozone generator for most industrial and personal uses. While variations of the "hot spark" coronal discharge method of ozone production exist, including medical grade and industrial grade ozone generators, these units usually work by means of a corona discharge tube or ozone plate. [ 136 ] [ 137 ] They are typically cost-effective and do not require an oxygen source other than the ambient air to produce ozone concentrations of 3–6%. Fluctuations in ambient air, due to weather or other environmental conditions, cause variability in ozone production. However, they also produce nitrogen oxides as a by-product. Use of an air dryer can reduce or eliminate nitric acid formation by removing water vapor and increase ozone production. At room temperature, nitric acid will form into a vapour that is hazardous if inhaled. Symptoms can include chest pain, shortness of breath, headaches and a dry nose and throat causing a burning sensation. Use of an oxygen concentrator can further increase the ozone production and further reduce the risk of nitric acid formation by removing not only the water vapor, but also the bulk of the nitrogen.
UV ozone generators, or vacuum-ultraviolet (VUV) ozone generators, employ a light source that generates a narrow-band ultraviolet light, a subset of that produced by the Sun. The Sun's UV sustains the ozone layer in the stratosphere of Earth. [ 138 ]
UV ozone generators use ambient air for ozone production, no air prep systems are used (air dryer or oxygen concentrator), therefore these generators tend to be less expensive. However, UV ozone generators usually produce ozone with a concentration of about 0.5% or lower which limits the potential ozone production rate. Another disadvantage of this method is that it requires the ambient air (oxygen) to be exposed to the UV source for a longer amount of time, and any gas that is not exposed to the UV source will not be treated. This makes UV generators impractical for use in situations that deal with rapidly moving air or water streams (in-duct air sterilization , for example). Production of ozone is one of the potential dangers of ultraviolet germicidal irradiation . VUV ozone generators are used in swimming pools and spa applications ranging to millions of gallons of water. VUV ozone generators, unlike corona discharge generators, do not produce harmful nitrogen by-products and also unlike corona discharge systems, VUV ozone generators work extremely well in humid air environments. There is also not normally a need for expensive off-gas mechanisms, and no need for air driers or oxygen concentrators which require extra costs and maintenance.
In the cold plasma method, pure oxygen gas is exposed to a plasma created by DBD . The diatomic oxygen is split into single atoms, which then recombine in triplets to form ozone.
It is common in the industry to mislabel some DBD ozone generators as CD Corona Discharge generators. Typically all solid flat metal electrode ozone generators produce ozone using the dielectric barrier discharge method. Cold plasma machines use pure oxygen as the input source and produce a maximum concentration of about 24% ozone. They produce far greater quantities of ozone in a given time compared to ultraviolet production that has about 2% efficiency. The discharges manifest as filamentary transfer of electrons (micro discharges) in a gap between two electrodes. In order to evenly distribute the micro discharges, a dielectric insulator must be used to separate the metallic electrodes and to prevent arcing.
Electrolytic ozone generation (EOG) splits water molecules into H 2 , O 2 , and O 3 .
In most EOG methods, the hydrogen gas will be removed to leave oxygen and ozone as the only reaction products. Therefore, EOG can achieve higher dissolution in water without other competing gases found in corona discharge method, such as nitrogen gases present in ambient air. This method of generation can achieve concentrations of 20–30% and is independent of air quality because water is used as the source material. Production of ozone electrolytically is typically unfavorable because of the high overpotential required to produce ozone as compared to oxygen. This is why ozone is not produced during typical water electrolysis. However, it is possible to increase the overpotential of oxygen by careful catalyst selection such that ozone is preferentially produced under electrolysis. Catalysts typically chosen for this approach are lead dioxide [ 139 ] or boron-doped diamond. [ 140 ]
The ozone-to-oxygen ratio is improved by increasing current density at the anode, cooling the electrolyte around the anode close to 0 °C, using an acidic electrolyte (such as dilute sulfuric acid) instead of a basic solution, and by applying pulsed current instead of DC. [ 141 ]
Ozone cannot be stored and transported like other industrial gases (because it quickly decays into diatomic oxygen) and must therefore be produced on site. Available ozone generators vary in the arrangement and design of the high-voltage electrodes. At production capacities higher than 20 kg per hour, a gas/water tube heat-exchanger may be utilized as ground electrode and assembled with tubular high-voltage electrodes on the gas-side. The regime of typical gas pressures is around 2 bars (200 kPa ) absolute in oxygen and 3 bars (300 kPa) absolute in air. Several megawatts of electrical power may be installed in large facilities, applied as single phase AC current at 50 to 8000 Hz and peak voltages between 3,000 and 20,000 volts. Applied voltage is usually inversely related to the applied frequency.
The dominating parameter influencing ozone generation efficiency is the gas temperature, which is controlled by cooling water temperature and/or gas velocity. The cooler the water, the better the ozone synthesis. The lower the gas velocity, the higher the concentration (but the lower the net ozone produced). At typical industrial conditions, almost 90% of the effective power is dissipated as heat and needs to be removed by a sufficient cooling water flow.
Because of the high reactivity of ozone, only a few materials may be used like stainless steel (quality 316L), titanium , aluminium (as long as no moisture is present), glass , polytetrafluorethylene , or polyvinylidene fluoride . Viton may be used with the restriction of constant mechanical forces and absence of humidity (humidity limitations apply depending on the formulation). Hypalon may be used with the restriction that no water comes in contact with it, except for normal atmospheric levels. Embrittlement or shrinkage is the common mode of failure of elastomers with exposure to ozone. Ozone cracking is the common mode of failure of elastomer seals like O-rings .
Silicone rubbers are usually adequate for use as gaskets in ozone concentrations below 1 wt%, such as in equipment for accelerated aging of rubber samples.
Ozone may be formed from O 2 by electrical discharges and by action of high energy electromagnetic radiation . Unsuppressed arcing in electrical contacts, motor brushes, or mechanical switches breaks down the chemical bonds of the atmospheric oxygen surrounding the contacts [ O 2 -> 2O]. Free radicals of oxygen in and around the arc recombine to create ozone [ O 3 ]. [ 142 ] Certain electrical equipment generate significant levels of ozone. This is especially true of devices using high voltages , such as ionic air purifiers , laser printers , photocopiers , tasers , and arc welders . Electric motors using brushes can generate ozone from repeated sparking inside the unit. Large motors that use brushes, such as those used by elevators or hydraulic pumps, will generate more ozone than smaller motors.
Ozone is similarly formed in the Catatumbo lightning storms phenomenon on the Catatumbo River in Venezuela , though ozone's instability makes it dubious that it has any effect on the ozonosphere. [ 143 ] It is the world's largest single natural generator of ozone, lending calls for it to be designated a UNESCO World Heritage Site . [ 144 ]
In the laboratory, ozone can be produced by electrolysis using a 9 volt battery , a pencil graphite rod cathode , a platinum wire anode , and a 3 molar sulfuric acid electrolyte . [ 145 ] The half cell reactions taking place are:
where E° represents the standard electrode potential .
In the net reaction, three equivalents of water are converted into one equivalent of ozone and three equivalents of hydrogen . Oxygen formation is a competing reaction.
It can also be generated by a high voltage arc . In its simplest form, high voltage AC, such as the output of a neon-sign transformer is connected to two metal rods with the ends placed sufficiently close to each other to allow an arc. The resulting arc will convert atmospheric oxygen to ozone.
It is often desirable to contain the ozone. This can be done with an apparatus consisting of two concentric glass tubes sealed together at the top with gas ports at the top and bottom of the outer tube. The inner core should have a length of metal foil inserted into it connected to one side of the power source. The other side of the power source should be connected to another piece of foil wrapped around the outer tube. A source of dry O 2 is applied to the bottom port. When high voltage is applied to the foil leads, electricity will discharge between the dry dioxygen in the middle and form O 3 and O 2 which will flow out the top port. This is called a Siemen's ozoniser. The reaction can be summarized as follows: [ 29 ]
The largest use of ozone is in the preparation of pharmaceuticals , synthetic lubricants , and many other commercially useful organic compounds , where it is used to sever carbon -carbon bonds. [ 29 ] It can also be used for bleaching substances and for killing microorganisms in air and water sources. [ 146 ] Many municipal drinking water systems kill bacteria with ozone instead of the more common chlorine . [ 147 ] Ozone has a very high oxidation potential . [ 148 ] Ozone does not form organochlorine compounds, nor does it remain in the water after treatment. Ozone can form the suspected carcinogen bromate in source water with high bromide concentrations. The U.S. Safe Drinking Water Act mandates that these systems introduce an amount of chlorine to maintain a minimum of 0.2 μmol/mol residual free chlorine in the pipes, based on results of regular testing. Where electrical power is abundant, ozone is a cost-effective method of treating water, since it is produced on demand and does not require transportation and storage of hazardous chemicals. Once it has decayed, it leaves no taste or odour in drinking water.
Although low levels of ozone have been advertised to be of some disinfectant use in residential homes, the concentration of ozone in dry air required to have a rapid, substantial effect on airborne pathogens exceeds safe levels recommended by the U.S. Occupational Safety and Health Administration and Environmental Protection Agency . Humidity control can vastly improve both the killing power of the ozone and the rate at which it decays back to oxygen (more humidity allows more effectiveness). Spore forms of most pathogens are very tolerant of atmospheric ozone in concentrations at which asthma patients start to have issues.
In 1908 artificial ozonisation of the Central Line of the London Underground was introduced for aerial disinfection. The process was found to be worthwhile, but was phased out by 1956. However the beneficial effect was maintained by the ozone created incidentally from the electrical discharges of the train motors (see above: Incidental production ). [ 149 ]
Ozone generators were made available to schools and universities in Wales for the Autumn term 2021, to disinfect classrooms after COVID-19 outbreaks. [ 150 ]
Industrially, ozone is used to:
Ozone is a reagent in many organic reactions in the laboratory and in industry. Ozonolysis is the cleavage of an alkene to carbonyl compounds.
Many hospitals around the world use large ozone generators to decontaminate operating rooms between surgeries. The rooms are cleaned and then sealed airtight before being filled with ozone which effectively kills or neutralizes all remaining bacteria. [ 156 ]
Ozone is used as an alternative to chlorine or chlorine dioxide in the bleaching of wood pulp . [ 157 ] It is often used in conjunction with oxygen and hydrogen peroxide to eliminate the need for chlorine-containing compounds in the manufacture of high-quality, white paper . [ 158 ]
Ozone can be used to detoxify cyanide wastes (for example from gold and silver mining ) by oxidizing cyanide to cyanate and eventually to carbon dioxide . [ 159 ]
Since the invention of dielectric barrier discharge (DBD) plasma reactors, it has been employed for water treatment with ozone. [ 160 ] However, with cheaper alternative disinfectants like chlorine, such applications of DBD ozone water decontamination have been limited by high power consumption and bulky equipment. [ 161 ] [ 162 ] Despite this, with research revealing the negative impacts of common disinfectants like chlorine with respect to toxic residuals and ineffectiveness in killing certain micro-organisms, [ 163 ] DBD plasma-based ozone decontamination is of interest in current available technologies. Although ozonation of water with a high concentration of bromide does lead to the formation of undesirable brominated disinfection byproducts, unless drinking water is produced by desalination, ozonation can generally be applied without concern for these byproducts. [ 162 ] [ 164 ] [ 165 ] [ 166 ] Advantages of ozone include high thermodynamic oxidation potential, less sensitivity to organic material and better tolerance for pH variations while retaining the ability to kill bacteria, fungi, viruses, as well as spores and cysts. [ 167 ] [ 168 ] [ 169 ] Although, ozone has been widely accepted in Europe for decades, it is sparingly used for decontamination in the U.S. due to limitations of high-power consumption, bulky installation and stigma attached with ozone toxicity. [ 161 ] [ 170 ] Considering this, recent research efforts have been directed toward the study of effective ozone water treatment systems. [ 171 ] Researchers have looked into lightweight and compact low power surface DBD reactors, [ 172 ] [ 173 ] energy efficient volume DBD reactors [ 174 ] and low power micro-scale DBD reactors. [ 175 ] [ 176 ] Such studies can help pave the path to re-acceptance of DBD plasma-based ozone decontamination of water, especially in the U.S.
Ozone levels which are safe for people are ineffective at killing fungi and bacteria. [ 177 ] Some consumer disinfection and cosmetic products emit ozone at levels harmful to human health. [ 177 ]
Devices generating high levels of ozone, some of which use ionization, are used to sanitize and deodorize uninhabited buildings, rooms, ductwork, woodsheds, boats, and other vehicles.
Ozonated water is used to launder clothes and to sanitize food, drinking water, and surfaces in the home. According to the U.S. Food and Drug Administration (FDA), it is "amending the food additive regulations to provide for the safe use of ozone in gaseous and aqueous phases as an antimicrobial agent on food, including meat and poultry." Studies at California Polytechnic University demonstrated that 0.3 μmol/mol levels of ozone dissolved in filtered tapwater can produce a reduction of more than 99.99% in such food-borne microorganisms as salmonella, E. coli 0157:H7 and Campylobacter . This quantity is 20,000 times the WHO -recommended limits stated above. [ 152 ] [ 178 ]
Ozone can be used to remove pesticide residues from fruits and vegetables . [ 179 ] [ 180 ]
Ozone is used in homes and hot tubs to kill bacteria in the water and to reduce the amount of chlorine or bromine required by reactivating them to their free state. Since ozone does not remain in the water long enough, ozone by itself is ineffective at preventing cross-contamination among bathers and must be used in conjunction with halogens . Gaseous ozone created by ultraviolet light or by corona discharge is injected into the water. [ 181 ]
Ozone is also widely used in the treatment of water in aquariums and fishponds. Its use can minimize bacterial growth, control parasites, eliminate transmission of some diseases, and reduce or eliminate "yellowing" of the water. Ozone must not come in contact with fishes' gill structures. Natural saltwater (with life forms) provides enough "instantaneous demand" that controlled amounts of ozone activate bromide ions to hypobromous acid , and the ozone entirely decays in a few seconds to minutes. If oxygen-fed ozone is used, the water will be higher in dissolved oxygen and fishes' gill structures will atrophy, making them dependent on oxygen-enriched water.
Ozonation – a process of infusing water with ozone – can be used in aquaculture to facilitate organic breakdown. Ozone is also added to recirculating systems to reduce nitrite levels [ 182 ] through conversion into nitrate . If nitrite levels in the water are high, nitrites will also accumulate in the blood and tissues of fish, where it interferes with oxygen transport (it causes oxidation of the heme-group of haemoglobin from ferrous ( Fe 2+ ) to ferric ( Fe 3+ ), making haemoglobin unable to bind O 2 ). [ 183 ] Despite these apparent positive effects, ozone use in recirculation systems has been linked to reducing the level of bioavailable iodine in salt water systems, resulting in iodine deficiency symptoms such as goitre and decreased growth in Senegalese sole ( Solea senegalensis ) larvae. [ 184 ]
Ozonate seawater is used for surface disinfection of haddock and Atlantic halibut eggs against nodavirus. Nodavirus is a lethal and vertically transmitted virus which causes severe mortality in fish. Haddock eggs should not be treated with high ozone level as eggs so treated did not hatch and died after 3–4 days. [ 185 ]
Ozone application on freshly cut pineapple and banana shows increase in flavonoids and total phenol contents when exposure is up to 20 minutes. Decrease in ascorbic acid (one form of vitamin C ) content is observed but the positive effect on total phenol content and flavonoids can overcome the negative effect. [ 186 ] Tomatoes upon treatment with ozone show an increase in β-carotene, lutein and lycopene. [ 187 ] However, ozone application on strawberries in pre-harvest period shows decrease in ascorbic acid content. [ 188 ]
Ozone facilitates the extraction of some heavy metals from soil using EDTA . EDTA forms strong, water-soluble coordination compounds with some heavy metals ( Pb and Zn ) thereby making it possible to dissolve them out from contaminated soil. If contaminated soil is pre-treated with ozone, the extraction efficacy of Pb , Am , and Pu increases by 11.0–28.9%, [ 189 ] 43.5% [ 190 ] and 50.7% [ 190 ] respectively.
Crop pollination is an essential part of an ecosystem. Ozone can have detrimental effects on plant-pollinator interactions. [ 191 ] Pollinators carry pollen from one plant to another. This is an essential cycle inside of an ecosystem. Causing changes in certain atmospheric conditions around pollination sites or with xenobiotics could cause unknown changes to the natural cycles of pollinators and flowering plants. In a study conducted in North-Western Europe, crop pollinators were negatively affected more when ozone levels were higher. [ 192 ]
The use of ozone for the treatment of medical conditions is not supported by high quality evidence, and is generally considered alternative medicine . [ 193 ]
Footnotes
Citations
Nascent oxygen O
Dioxygen ( singlet and triplet ) O 2
Trioxygen ( ozone and cyclic ozone ) O 3
Tetraoxygen O 4
Octaoxygen O 8 | https://en.wikipedia.org/wiki/O⚎O⚍O |
Ozone ( / ˈ oʊ z oʊ n / ) (or trioxygen ) is an inorganic molecule with the chemical formula O 3 . It is a pale blue gas with a distinctively pungent smell. It is an allotrope of oxygen that is much less stable than the diatomic allotrope O 2 , breaking down in the lower atmosphere to O 2 ( dioxygen ). Ozone is formed from dioxygen by the action of ultraviolet (UV) light and electrical discharges within the Earth's atmosphere . It is present in very low concentrations throughout the atmosphere, with its highest concentration high in the ozone layer of the stratosphere , which absorbs most of the Sun 's ultraviolet (UV) radiation.
Ozone's odor is reminiscent of chlorine , and detectable by many people at concentrations of as little as 0.1 ppm in air. Ozone's O 3 structure was determined in 1865. The molecule was later proven to have a bent structure and to be weakly diamagnetic . At standard temperature and pressure , ozone is a pale blue gas that condenses at cryogenic temperatures to a dark blue liquid and finally a violet-black solid . Ozone's instability with regard to more common dioxygen is such that both concentrated gas and liquid ozone may decompose explosively at elevated temperatures, physical shock, or fast warming to the boiling point. [ 5 ] [ 6 ] It is therefore used commercially only in low concentrations.
Ozone is a powerful oxidizing agent (far more so than dioxygen) and has many industrial and consumer applications related to oxidation. This same high oxidizing potential, however, causes ozone to damage mucous and respiratory tissues in animals, and also tissues in plants, above concentrations of about 0.1 ppm . While this makes ozone a potent respiratory hazard and pollutant near ground level , a higher concentration in the ozone layer (from two to eight ppm) is beneficial, preventing damaging UV light from reaching the Earth's surface.
The trivial name ozone is the most commonly used and preferred IUPAC name . The systematic names 2λ 4 -trioxidiene [ dubious – discuss ] and catena-trioxygen , valid IUPAC names, are constructed according to the substitutive and additive nomenclatures , respectively. The name ozone derives from ozein (ὄζειν), the Greek neuter present participle for smell, [ 7 ] referring to ozone's distinctive smell.
In appropriate contexts, ozone can be viewed as trioxidane with two hydrogen atoms removed, and as such, trioxidanylidene may be used as a systematic name, according to substitutive nomenclature. By default, these names pay no regard to the radicality of the ozone molecule. In an even more specific context, this can also name the non-radical singlet ground state, whereas the diradical state is named trioxidanediyl .
Trioxidanediyl (or ozonide ) is used, non-systematically, to refer to the substituent group (-OOO-). Care should be taken to avoid confusing the name of the group for the context-specific name for the ozone given above.
In 1785, Dutch chemist Martinus van Marum was conducting experiments involving electrical sparking above water when he noticed an unusual smell, which he attributed to the electrical reactions, failing to realize that he had in fact produced ozone. [ 8 ] [ 9 ]
A half century later, Christian Friedrich Schönbein noticed the same pungent odour and recognized it as the smell often following a bolt of lightning . In 1839, he succeeded in isolating the gaseous chemical and named it "ozone", from the Greek word ozein ( ὄζειν ) meaning "to smell". [ 10 ] [ 11 ] For this reason, Schönbein is generally credited with the discovery of ozone. [ 12 ] [ 13 ] [ 14 ] [ 8 ] He also noted the similarity of ozone smell to the smell of phosphorus, and in 1844 proved that the product of reaction of white phosphorus with air is identical. [ 10 ] A subsequent effort to call ozone "electrified oxygen" he ridiculed by proposing to call the ozone from white phosphorus "phosphorized oxygen". [ 10 ] The chemical formula for ozone, O 3 , was not determined until 1865 by Jacques-Louis Soret [ 15 ] and confirmed by Schönbein in 1867. [ 10 ] [ 16 ]
For much of the second half of the 19th century and well into the 20th, ozone was considered a healthy component of the environment by naturalists and health-seekers. Beaumont, California , had as its official slogan "Beaumont: Zone of Ozone", as evidenced on postcards and Chamber of Commerce letterhead. [ 17 ] Naturalists working outdoors often considered the higher elevations beneficial because of their ozone content which was readily monitored. [ 18 ] "There is quite a different atmosphere [at higher elevation] with enough ozone to sustain the necessary energy [to work]", wrote naturalist Henry Henshaw , working in Hawaii. [ 19 ] Seaside air was considered to be healthy because of its believed ozone content. The smell giving rise to this belief is in fact that of halogenated seaweed metabolites [ 20 ] and dimethyl sulfide . [ 21 ]
Much of ozone's appeal seems to have resulted from its "fresh" smell, which evoked associations with purifying properties. Scientists noted its harmful effects. In 1873 James Dewar and John Gray McKendrick documented that frogs grew sluggish, birds gasped for breath, and rabbits' blood showed decreased levels of oxygen after exposure to "ozonized air", which "exercised a destructive action". [ 22 ] [ 12 ] Schönbein himself reported that chest pains, irritation of the mucous membranes , and difficulty breathing occurred as a result of inhaling ozone, and small mammals died. [ 23 ] In 1911, Leonard Hill and Martin Flack stated in the Proceedings of the Royal Society B that ozone's healthful effects "have, by mere iteration, become part and parcel of common belief; and yet exact physiological evidence in favour of its good effects has been hitherto almost entirely wanting ... The only thoroughly well-ascertained knowledge concerning the physiological effect of ozone, so far attained, is that it causes irritation and œdema of the lungs, and death if inhaled in relatively strong concentration for any time." [ 12 ] [ 24 ]
During World War I , ozone was tested at Queen Alexandra Military Hospital in London as a possible disinfectant for wounds. The gas was applied directly to wounds for as long as 15 minutes. This resulted in damage to both bacterial cells and human tissue. Other sanitizing techniques, such as irrigation with antiseptics , were found preferable. [ 12 ] [ 25 ]
Until the 1920s, it was not certain whether small amounts of oxozone , O 4 , were also present in ozone samples due to the difficulty of applying analytical chemistry techniques to the explosive concentrated chemical. [ 26 ] [ 27 ] In 1923, Georg-Maria Schwab (working for his doctoral thesis under Ernst Hermann Riesenfeld ) was the first to successfully solidify ozone and perform accurate analysis which conclusively refuted the oxozone hypothesis. [ 26 ] [ 27 ] Further hitherto unmeasured physical properties of pure concentrated ozone were determined by the Riesenfeld group in the 1920s. [ 26 ]
Ozone is a colourless or pale blue gas, slightly soluble in water, and much more soluble in inert non-polar solvents such as carbon tetrachloride or fluorocarbons, in which it forms a blue solution. At 161 K (−112 °C; −170 °F), it condenses to form a dark blue liquid . It is dangerous to allow this liquid to warm to its boiling point, because both concentrated gaseous ozone and liquid ozone can detonate. At temperatures below 80 K (−193.2 °C; −315.7 °F), it forms a violet-black solid . [ 28 ]
Ozone has a very specific sharp odour somewhat resembling chlorine bleach . Most people can detect it at the 0.01 μmol/mol level in air. Exposure of 0.1 to 1 μmol/mol produces headaches and burning eyes and irritates the respiratory passages. [ 29 ] Even low concentrations of ozone in air are very destructive to organic materials such as latex, plastics, and animal lung tissue.
The ozone molecule is weakly diamagnetic . [ 30 ]
According to experimental evidence from microwave spectroscopy , ozone is a bent molecule, with C 2v symmetry (similar to the water molecule). [ 31 ] The O–O distances are 127.2 pm (1.272 Å ). The O–O–O angle is 116.78°. [ 32 ] The central atom is sp ² hybridized with one lone pair. Ozone is a polar molecule with a dipole moment of 0.53 D . [ 33 ] The molecule can be represented as a resonance hybrid with two contributing structures, each with a single bond on one side and double bond on the other. The arrangement possesses an overall bond order of 1.5 for both sides. It is isoelectronic with the nitrite anion . Naturally occurring ozone can be composed of substituted isotopes ( 16 O, 17 O, 18 O). A cyclic form has been predicted but not observed.
Ozone is among the most powerful oxidizing agents known, far stronger than O 2 . It is also unstable at high concentrations, decaying into ordinary diatomic oxygen. Its half-life varies with atmospheric conditions such as temperature, humidity, and air movement. Under laboratory conditions, the half-life will average ~1500 minutes (25 hours) in still air at room temperature (24 °C), zero humidity with zero air changes per hour. [ 34 ]
This reaction proceeds more rapidly with increasing temperature. Deflagration of ozone can be triggered by a spark and can occur in ozone concentrations of 10 wt% or higher. [ 35 ]
Ozone can also be produced from oxygen at the anode of an electrochemical cell. This reaction can create smaller quantities of ozone for research purposes. [ 36 ]
This can be observed as an unwanted reaction in a Hoffman apparatus during the electrolysis of water when the voltage is set above the necessary voltage.
Ozone oxidizes most metals (except gold , platinum , and iridium ) into oxides of the metals in their highest oxidation state . For example:
Ozone oxidizes nitric oxide to nitrogen dioxide :
This reaction is accompanied by chemiluminescence .
The NO 2 can be further oxidized to nitrate radical :
The NO 3 formed can react with NO 2 to form dinitrogen pentoxide ( N 2 O 5 ).
Solid nitronium perchlorate can be made from NO 2 , ClO 2 , and O 3 gases:
Ozone does not react with ammonium salts , but it oxidizes ammonia to ammonium nitrate :
Ozone reacts with carbon to form carbon dioxide , even at room temperature:
Ozone oxidizes sulfides to sulfates . For example, lead(II) sulfide is oxidized to lead(II) sulfate :
Sulfuric acid can be produced from ozone, water and either elemental sulfur or sulfur dioxide :
In the gas phase , ozone reacts with hydrogen sulfide to form sulfur dioxide:
In an aqueous solution, however, two competing simultaneous reactions occur, one to produce elemental sulfur, and one to produce sulfuric acid :
Alkenes can be oxidatively cleaved by ozone, in a process called ozonolysis , giving alcohols, aldehydes, ketones, and carboxylic acids, depending on the second step of the workup.
Ozone can also cleave alkynes to form an acid anhydride or diketone product. [ 38 ] If the reaction is performed in the presence of water, the anhydride hydrolyzes to give two carboxylic acids .
Usually ozonolysis is carried out in a solution of dichloromethane , at a temperature of −78 °C. After a sequence of cleavage and rearrangement, an organic ozonide is formed. With reductive workup (e.g. zinc in acetic acid or dimethyl sulfide ), ketones and aldehydes will be formed, with oxidative workup (e.g. aqueous or alcoholic hydrogen peroxide ), carboxylic acids will be formed. [ 39 ]
All three atoms of ozone may also react, as in the reaction of tin(II) chloride with hydrochloric acid and ozone:
Iodine perchlorate can be made by treating iodine dissolved in cold anhydrous perchloric acid with ozone:
Ozone could also react with potassium iodide to give oxygen and iodine gas that can be titrated for quantitative determination: [ 40 ]
Ozone can be used for combustion reactions and combustible gases; ozone provides higher temperatures than burning in dioxygen ( O 2 ). The following is a reaction for the combustion of carbon subnitride which can also cause higher temperatures:
Ozone can react at cryogenic temperatures. At 77 K (−196.2 °C; −321.1 °F), atomic hydrogen reacts with liquid ozone to form a hydrogen superoxide radical , which dimerizes : [ 41 ]
Ozone is a toxic substance, [ 42 ] [ 43 ] commonly found or generated in human environments (aircraft cabins, offices with photocopiers, laser printers, sterilizers, ...).
The catalytic decomposition of ozone is very important to reduce pollution. This type of decomposition is the most widely used, especially with solid catalysts, and it has many advantages such as a higher conversion with a lower temperature. Furthermore, the product and the catalyst can be instantaneously separated, and this way the catalyst can be easily recovered without using any separation operation. The most-used materials in the catalytic decomposition of ozone in the gas phase are manganese dioxide , transition metals such as Mn, Co, Cu, Fe, Ni, or Ag, and noble metals such as Pt, Rh, or Pd.
Free radicals of chlorine (Cl · ), formed by the action of ultraviolet radiation on chlorofluorocarbons (CFCs) and sea salt, are known to catalyze the breakdown of ozone in the atmosphere.
There are two other possibilities for decomposing ozone in the gas phase:
The uncatalyzed process of ozone decomposition in the gas phase is a complex reaction involving two elementary reactions that finally lead to molecular oxygen, [ 45 ] and this means that the reaction order and the rate law cannot be determined by the stoichiometry of the overall reaction.
Overall reaction: 2 O 3 ⟶ 3 O 2 {\displaystyle {\ce {2 O3 -> 3 O2}}}
Rate law (observed): V = K o b s ⋅ [ O 3 ] 2 [ O 2 ] {\displaystyle V={\frac {K_{obs}\cdot [{\ce {O3}}]^{2}}{[{\ce {O2}}]}}}
where K o b s {\displaystyle K_{obs}} is the observed rate constant and V {\displaystyle V} is the reaction rate. From the rate law above it can be determined that the partial order respect to molecular oxygen is −1 and respect to ozone is 2; therefore, the global reaction order is 1.
The first step is a unimolecular reaction wherein one molecule of ozone decomposes into two products (molecular oxygen and oxygen). The oxygen atom from the first step is a reactive intermediate because it participates as a reactant in the second step, which is a bimolecular reaction because there are two different reactants (ozone and oxygen) that give rise to molecular oxygen.
Step 1: Unimolecular reaction O 3 ⟶ O 2 + O {\displaystyle {\ce {O3 -> O2 + O}}}
Step 2: Bimolecular reaction O 3 + O ⟶ 2 O 2 {\displaystyle {\ce {O3 + O -> 2 O2}}}
These two steps have different reaction rates and rate constants. The reaction rate laws for each of these steps are shown below:
The following mechanism allows to explain the rate law of the ozone decomposition observed experimentally, and also it allows to determine the reaction orders with respect to ozone and oxygen, with which the overall reaction order will be determined.
The first step is assumed reversible and faster than the second reaction, which means that the slower rate determining step is the second reaction. This step determines the rate of product formation, and so V = V 2 {\displaystyle V=V_{2}} . However, this equation depends on the concentration of oxygen (intermediate), which does not appear in the observed rate law. Since the first step is a rapid equilibrium, the concentration of the intermediate can be determined as follows:
Then using these equations, the formation rate of molecular oxygen is as shown below:
The mechanism is consistent with the rate law observed experimentally if the rate constant ( K obs ) is given in terms of the individual mechanistic steps' rate constants as follows: [ 46 ]
where K obs = K 2 ⋅ K 1 K − 1 {\displaystyle K_{\text{obs}}={K_{2}\cdot K_{1} \over K_{-1}}}
Reduction of ozone gives the ozonide anion, O − 3 . Derivatives of this anion are explosive and must be stored at cryogenic temperatures. Ozonides for all the alkali metals are known. KO 3 , RbO 3 , and CsO 3 can be prepared from their respective superoxides:
Although KO 3 can be formed as above, it can also be formed from potassium hydroxide and ozone: [ 47 ]
NaO 3 and LiO 3 must be prepared by action of CsO 3 in liquid NH 3 on an ion-exchange resin containing Na + or Li + ions: [ 48 ]
A solution of calcium in ammonia reacts with ozone to give ammonium ozonide and not calcium ozonide: [ 41 ]
Ozone can be used to remove iron and manganese from water , forming a precipitate which can be filtered:
Ozone oxidizes dissolved hydrogen sulfide in water to sulfurous acid :
These three reactions are central in the use of ozone-based well water treatment.
Ozone detoxifies cyanides by converting them to cyanates .
Ozone completely decomposes urea : [ 49 ]
Ozone is a bent triatomic molecule with three vibrational modes: the symmetric stretch (1103.157 cm −1 ), bend (701.42 cm −1 ) and antisymmetric stretch (1042.096 cm −1 ). [ 50 ] The symmetric stretch and bend are weak absorbers, but the antisymmetric stretch is strong and responsible for ozone being an important minor greenhouse gas . This IR band is also used to detect ambient and atmospheric ozone although UV-based measurements are more common. [ 51 ]
The electromagnetic spectrum of ozone is quite complex. An overview can be seen at the MPI Mainz UV/VIS Spectral Atlas of Gaseous Molecules of Atmospheric Interest. [ 52 ]
All of the bands are dissociative, meaning that the molecule falls apart to O + O 2 after absorbing a photon. The most important absorption is the Hartley band, extending from slightly above 300 nm down to slightly above 200 nm. It is this band that is responsible for absorbing UV C in the stratosphere.
On the high wavelength side, the Hartley band transitions to the so-called Huggins band, which falls off rapidly until disappearing by ~360 nm. Above 400 nm, extending well out into the NIR, are the Chappius and Wulf bands. There, unstructured absorption bands are useful for detecting high ambient concentrations of ozone, but are so weak that they do not have much practical effect.
There are additional absorption bands in the far UV, which increase slowly from 200 nm down to reaching a maximum at ~120 nm.
The standard way to express total ozone levels (the amount of ozone in a given vertical column) in the atmosphere is by using Dobson units . Point measurements are reported as mole fractions in nmol/mol (parts per billion, ppb) or as concentrations in μg/m 3 . The study of ozone concentration in the atmosphere started in the 1920s. [ 53 ]
The highest levels of ozone in the atmosphere are in the stratosphere , in a region also known as the ozone layer between about 10 and 50 km above the surface (or between about 6 and 31 miles). However, even in this "layer", the ozone concentrations are only two to eight parts per million, so most of the oxygen there is dioxygen, O 2 , at about 210,000 parts per million by volume. [ 54 ]
Ozone in the stratosphere is mostly produced from short-wave ultraviolet rays between 240 and 160 nm. Oxygen starts to absorb weakly at 240 nm in the Herzberg bands, but most of the oxygen is dissociated by absorption in the strong Schumann–Runge bands between 200 and 160 nm where ozone does not absorb. While shorter wavelength light, extending to even the X-Ray limit, is energetic enough to dissociate molecular oxygen, there is relatively little of it, and, the strong solar emission at Lyman-alpha, 121 nm, falls at a point where molecular oxygen absorption is a minimum. [ 55 ]
The process of ozone creation and destruction is called the Chapman cycle and starts with the photolysis of molecular oxygen
followed by reaction of the oxygen atom with another molecule of oxygen to form ozone.
where "M" denotes the third body that carries off the excess energy of the reaction. The ozone molecule can then absorb a UV-C photon and dissociate
The excess kinetic energy heats the stratosphere when the O atoms and the molecular oxygen fly apart and collide with other molecules. This conversion of UV light into kinetic energy warms the stratosphere. The oxygen atoms produced in the photolysis of ozone then react back with other oxygen molecule as in the previous step to form more ozone. In the clear atmosphere, with only nitrogen and oxygen, ozone can react with the atomic oxygen to form two molecules of O 2 :
An estimate of the rate of this termination step to the cycling of atomic oxygen back to ozone can be found simply by taking the ratios of the concentration of O 2 to O 3 . The termination reaction is catalysed by the presence of certain free radicals, of which the most important are hydroxyl (OH), nitric oxide (NO) and atomic chlorine (Cl) and bromine (Br). In the second half of the 20th century, the amount of ozone in the stratosphere was discovered to be declining , mostly because of increasing concentrations of chlorofluorocarbons (CFC) and similar chlorinated and brominated organic molecules . The concern over the health effects of the decline led to the 1987 Montreal Protocol , the ban on the production of many ozone-depleting chemicals and in the first and second decade of the 21st century the beginning of the recovery of stratospheric ozone concentrations.
Ozone in the ozone layer filters out sunlight wavelengths from about 200 nm UV rays to 315 nm, with ozone peak absorption at about 250 nm. [ 56 ] This ozone UV absorption is important to life, since it extends the absorption of UV by ordinary oxygen and nitrogen in air (which absorb all wavelengths < 200 nm) through the lower UV-C (200–280 nm) and the entire UV-B band (280–315 nm). The small unabsorbed part that remains of UV-B after passage through ozone causes sunburn in humans, and direct DNA damage in living tissues in both plants and animals. Ozone's effect on mid-range UV-B rays is illustrated by its effect on UV-B at 290 nm, which has a radiation intensity 350 million times as powerful at the top of the atmosphere as at the surface. Nevertheless, enough of UV-B radiation at similar frequency reaches the ground to cause some sunburn, and these same wavelengths are also among those responsible for the production of vitamin D in humans.
The ozone layer has little effect on the longer UV wavelengths called UV-A (315–400 nm), but this radiation does not cause sunburn or direct DNA damage. While UV-A probably does cause long-term skin damage in certain humans, it is not as dangerous to plants and to the health of surface-dwelling organisms on Earth in general (see ultraviolet for more information on near ultraviolet).
Ground-level ozone (or tropospheric ozone) is an atmospheric pollutant. [ 57 ] It is not emitted directly by car engines or by industrial operations, but formed by the reaction of sunlight on air containing hydrocarbons and nitrogen oxides that react to form ozone directly at the source of the pollution or many kilometers downwind.
Ozone reacts directly with some hydrocarbons such as aldehydes and thus begins their removal from the air, but the products are themselves key components of smog . Ozone photolysis by UV light leads to production of the hydroxyl radical HO• and this plays a part in the removal of hydrocarbons from the air, but is also the first step in the creation of components of smog such as peroxyacyl nitrates , which can be powerful eye irritants. The atmospheric lifetime of tropospheric ozone is about 22 days; its main removal mechanisms are being deposited to the ground, the above-mentioned reaction giving HO•, and by reactions with OH and the peroxy radical HO 2 •. [ 58 ]
There is evidence of significant reduction in agricultural yields because of increased ground-level ozone and pollution which interferes with photosynthesis and stunts overall growth of some plant species. [ 59 ] [ 60 ] The United States Environmental Protection Agency (EPA) has proposed a secondary regulation to reduce crop damage, in addition to the primary regulation designed for the protection of human health.
Certain examples of cities with elevated ozone readings are Denver, Colorado ; Houston, Texas ; and Mexico City , Mexico . Houston has a reading of around 41 nmol/mol, while Mexico City is far more hazardous, with a reading of about 125 nmol/mol. [ 60 ]
Ground-level ozone, or tropospheric ozone, is the most concerning type of ozone pollution in urban areas and is increasing in general. [ 61 ] Ozone pollution in urban areas affects denser populations, and is worsened by high populations of vehicles, which emit pollutants NO 2 and VOCs , the main contributors to problematic ozone levels. [ 62 ] Ozone pollution in urban areas is especially concerning with increasing temperatures, raising heat-related mortality during heat waves . [ 63 ] During heat waves in urban areas, ground level ozone pollution can be 20% higher than usual. [ 64 ] Ozone pollution in urban areas reaches higher levels of exceedance in the summer and autumn, which may be explained by weather patterns and traffic patterns. [ 62 ] People experiencing poverty are more affected by pollution in general, even though these populations are less likely to be contributing to pollution levels. [ 65 ]
As mentioned above, Denver, Colorado, is one of the many cities in the U.S. that have high amounts of ozone. According to the American Lung Association , the Denver–Aurora area is the 14th most ozone-polluted area in the U.S. [ 66 ] The problem of high ozone levels is not new to this area. In 2004, the EPA allotted the Denver Metro /North Front Range [ b ] as non-attainment areas per 1997's 8-hour ozone standard, [ 67 ] but later deferred this status until 2007. The non-attainment standard indicates that an area does not meet the EPA's air quality standards. The Colorado Ozone Action Plan was created in response, and numerous changes were implemented from this plan. The first major change was that car emission testing was expanded across the state to more counties that did not previously mandate emissions testing, like areas of Larimer and Weld County. There have also been changes made to decrease Nitrogen Oxides (NOx) and Volatile Organic Compound (VOC) emissions, which should help lower ozone levels.
One large contributor to high ozone levels in the area is the oil and natural gas industry situated in the Denver-Julesburg Basin (DJB) which overlaps with a majority of Colorado's metropolitan areas. Ozone is produced naturally in the Earth's stratosphere, but is also produced in the troposphere from human efforts. Briefly mentioned above, NOx and VOCs react with sunlight to create ozone through a process called photochemistry. One hour elevated ozone events (<75 ppb) "occur during June–August indicating that elevated ozone levels are driven by regional photochemistry". [ 68 ] According to an article from the University of Colorado-Boulder, "Oil and natural gas VOC emission have a major role in ozone production and bear the potential to contribute to elevated O 3 levels in the Northern Colorado Front Range (NCFR)". [ 68 ] Using complex analyses to research wind patterns and emissions from large oil and natural gas operations, the authors concluded that "elevated O 3 levels in the NCFR are predominantly correlated with air transport from N– ESE, which are the upwind sectors where the O&NG operations in the Wattenberg Field area of the DJB are located". [ 68 ]
Contained in the Colorado Ozone Action Plan, created in 2008, plans exist to evaluate "emission controls for large industrial sources of NOx" and "statewide control requirements for new oil and gas condensate tanks and pneumatic valves". [ 69 ] In 2011, the Regional Haze Plan was released that included a more specific plan to help decrease NOx emissions. These efforts are increasingly difficult to implement and take many years to come to pass. Of course there are also other reasons that ozone levels remain high. These include: a growing population meaning more car emissions, and the mountains along the NCFR that can trap emissions. If interested, daily air quality readings can be found at the Colorado Department of Public Health and Environment's website. [ 70 ] As noted earlier, Denver continues to experience high levels of ozone to this day. It will take many years and a systems-thinking approach to combat this issue of high ozone levels in the Front Range of Colorado.
Ozone gas attacks any polymer possessing olefinic or double bonds within its chain structure, such as natural rubber , nitrile rubber , and styrene-butadiene rubber. Products made using these polymers are especially susceptible to attack, which causes cracks to grow longer and deeper with time, the rate of crack growth depending on the load carried by the rubber component and the concentration of ozone in the atmosphere. Such materials can be protected by adding antiozonants , such as waxes, which bond to the surface to create a protective film or blend with the material and provide long term protection. Ozone cracking used to be a serious problem in car tires, [ 71 ] for example, but it is not an issue with modern tires. On the other hand, many critical products, like gaskets and O-rings , may be attacked by ozone produced within compressed air systems. Fuel lines made of reinforced rubber are also susceptible to attack, especially within the engine compartment, where some ozone is produced by electrical components. Storing rubber products in close proximity to a DC electric motor can accelerate ozone cracking. The commutator of the motor generates sparks which in turn produce ozone.
Although ozone was present at ground level before the Industrial Revolution , peak concentrations are now far higher than the pre-industrial levels, and even background concentrations well away from sources of pollution are substantially higher. [ 73 ] [ 74 ] Ozone acts as a greenhouse gas , absorbing some of the infrared energy emitted by the earth. Quantifying the greenhouse gas potency of ozone is difficult because it is not present in uniform concentrations across the globe. However, the most widely accepted scientific assessments relating to climate change (e.g. the Intergovernmental Panel on Climate Change Third Assessment Report ) [ 75 ] suggest that the radiative forcing of tropospheric ozone is about 25% that of carbon dioxide .
The annual global warming potential of tropospheric ozone is between 918 and 1022 tons carbon dioxide equivalent /tons tropospheric ozone. This means on a per-molecule basis, ozone in the troposphere has a radiative forcing effect roughly 1,000 times as strong as carbon dioxide . However, tropospheric ozone is a short-lived greenhouse gas, which decays in the atmosphere much more quickly than carbon dioxide . This means that over a 20-year span, the global warming potential of tropospheric ozone is much less, roughly 62 to 69 tons carbon dioxide equivalent / ton tropospheric ozone. [ 76 ]
Because of its short-lived nature, tropospheric ozone does not have strong global effects, but has very strong radiative forcing effects on regional scales. In fact, there are regions of the world where tropospheric ozone has a radiative forcing up to 150% of carbon dioxide . [ 77 ] For example, ozone increase in the troposphere is shown to be responsible for ~30% of upper Southern Ocean interior warming between 1955 and 2000. [ 78 ]
Filters containing an adsorbent or catalyst such as charcoal (carbon) may be used to remove odors and gaseous pollutants such as volatile organic compounds or ozone. [ 79 ]
For the last few decades, scientists studied the effects of acute and chronic ozone exposure on human health. Hundreds of studies suggest that ozone is harmful to people at levels currently found in urban areas. [ 80 ] [ 81 ] Ozone has been shown to affect the respiratory, cardiovascular and central nervous system. Early death and problems in reproductive health and development are also shown to be associated with ozone exposure. [ 82 ]
The American Lung Association has identified five populations who are especially vulnerable to the effects of breathing ozone: [ 83 ]
Additional evidence suggests that women, those with obesity and low-income populations may also face higher risk from ozone, although more research is needed. [ 83 ]
Acute ozone exposure ranges from hours to a few days. Because ozone is a gas, it directly affects the lungs and the entire respiratory system. Inhaled ozone causes inflammation and acute—but reversible—changes in lung function, as well as airway hyperresponsiveness. [ 84 ] These changes lead to shortness of breath, wheezing, and coughing which may exacerbate lung diseases, like asthma or chronic obstructive pulmonary disease (COPD) resulting in the need to receive medical treatment. [ 85 ] [ 86 ] Acute and chronic exposure to ozone has been shown to cause an increased risk of respiratory infections, due to the following mechanism. [ 87 ]
Multiple studies have been conducted to determine the mechanism behind ozone's harmful effects, particularly in the lungs. These studies have shown that exposure to ozone causes changes in the immune response within the lung tissue, resulting in disruption of both the innate and adaptive immune response, as well as altering the protective function of lung epithelial cells. [ 88 ] It is thought that these changes in immune response and the related inflammatory response are factors that likely contribute to the increased risk of lung infections, and worsening or triggering of asthma and reactive airways after exposure to ground-level ozone pollution. [ 88 ] [ 89 ]
The innate (cellular) immune system consists of various chemical signals and cell types that work broadly and against multiple pathogen types, typically bacteria or foreign bodies/substances in the host. [ 89 ] [ 90 ] The cells of the innate system include phagocytes, neutrophils, [ 90 ] both thought to contribute to the mechanism of ozone pathology in the lungs, as the functioning of these cell types have been shown to change after exposure to ozone. [ 89 ] Macrophages, cells that serve the purpose of eliminating pathogens or foreign material through the process of "phagocytosis", [ 90 ] have been shown to change the level of inflammatory signals they release in response to ozone, either up-regulating and resulting in an inflammatory response in the lung, or down-regulating and reducing immune protection. [ 88 ] Neutrophils, another important cell type of the innate immune system that primarily targets bacterial pathogens, [ 90 ] are found to be present in the airways within 6 hours of exposure to high ozone levels. Despite high levels in the lung tissues, however, their ability to clear bacteria appears impaired by exposure to ozone. [ 88 ]
The adaptive immune system is the branch of immunity that provides long-term protection via the development of antibodies targeting specific pathogens and is also impacted by high ozone exposure. [ 89 ] [ 90 ] Lymphocytes, a cellular component of the adaptive immune response, produce an increased amount of inflammatory chemicals called "cytokines" after exposure to ozone, which may contribute to airway hyperreactivity and worsening asthma symptoms. [ 88 ]
The airway epithelial cells also play an important role in protecting individuals from pathogens. In normal tissue, the epithelial layer forms a protective barrier, and also contains specialized ciliary structures that work to clear foreign bodies, mucus and pathogens from the lungs. When exposed to ozone, the cilia become damaged and mucociliary clearance of pathogens is reduced. Furthermore, the epithelial barrier becomes weakened, allowing pathogens to cross the barrier, proliferate and spread into deeper tissues. Together, these changes in the epithelial barrier help make individuals more susceptible to pulmonary infections. [ 88 ]
Inhaling ozone not only affects the immune system and lungs, but it may also affect the heart as well. Ozone causes short-term autonomic imbalance leading to changes in heart rate and reduction in heart rate variability; [ 91 ] and high levels exposure for as little as one-hour results in a supraventricular arrhythmia in the elderly, [ 92 ] both increase the risk of premature death and stroke. Ozone may also lead to vasoconstriction resulting in increased systemic arterial pressure contributing to increased risk of cardiac morbidity and mortality in patients with pre-existing cardiac diseases. [ 93 ] [ 94 ]
Breathing ozone for periods longer than eight hours at a time for weeks, months or years defines chronic exposure. Numerous studies suggest a serious impact on the health of various populations from this exposure.
One study finds significant positive associations between chronic ozone and all-cause, circulatory, and respiratory mortality with 2%, 3%, and 12% increases in risk per 10 ppb [ 95 ] and report an association (95% CI) of annual ozone and all-cause mortality with a hazard ratio of 1.02 (1.01–1.04), and with cardiovascular mortality of 1.03 (1.01–1.05). A similar study finds similar associations with all-cause mortality and even larger effects for cardiovascular mortality. [ 96 ] An increased risk of mortality from respiratory causes is associated with long-term chronic exposure to ozone. [ 97 ]
Chronic ozone has detrimental effects on children, especially those with asthma. The risk for hospitalization in children with asthma increases with chronic exposure to ozone; younger children and those with low-income status are even at greater risk. [ 98 ]
Adults suffering from respiratory diseases (asthma, [ 99 ] COPD, [ 100 ] lung cancer [ 101 ] ) are at a higher risk of mortality and morbidity and critically ill patients have an increased risk of developing acute respiratory distress syndrome with chronic ozone exposure as well. [ 102 ]
Ozone generators sold as air cleaners intentionally produce the gas ozone. [ 43 ] These are often marketed to control indoor air pollution , and use misleading terms to describe ozone. Some examples are describing it as "energized oxygen" or "pure air", suggesting that ozone is a healthy or "better" kind of oxygen. [ 43 ] However, according to the EPA , "There is evidence to show that at concentrations that do not exceed public health standards, ozone is not effective at removing many odor-causing chemicals", and "If used at concentrations that do not exceed public health standards, ozone applied to indoor air does not effectively remove viruses, bacteria, mold, or other biological pollutants.". [ 43 ] Furthermore, another report states that "results of some controlled studies show that concentrations of ozone considerably higher than these [human safety] standards are possible even when a user follows the manufacturer's operating instructions". [ 103 ]
The California Air Resources Board has a page listing air cleaners (many with ionizers ) meeting their indoor ozone limit of 0.050 parts per million. [ 104 ] From that article:
All portable indoor air cleaning devices sold in California must be certified by the California Air Resources Board (CARB). To be certified, air cleaners must be tested for electrical safety and ozone emissions, and meet an ozone emission concentration limit of 0.050 parts per million. For more information about the regulation, visit the air cleaner regulation .
Ozone precursors are a group of pollutants, predominantly those emitted during the combustion of fossil fuels . Ground-level ozone pollution (tropospheric ozone) is produced near the Earth's surface by the action of daylight UV rays on these precursors. The ozone at ground level is primarily from fossil fuel precursors, but methane is a natural precursor, and the very low natural background level of ozone at ground level is considered safe. This section examines the health impacts of fossil fuel burning, which raises ground level ozone far above background levels.
There is a great deal of evidence to show that ground-level ozone can harm lung function and irritate the respiratory system . [ 57 ] [ 106 ] Exposure to ozone (and the pollutants that produce it) is linked to premature death , asthma , bronchitis , heart attack , and other cardiopulmonary problems. [ 107 ] [ 108 ]
Long-term exposure to ozone has been shown to increase risk of death from respiratory illness . [ 43 ] A study of 450,000 people living in U.S. cities saw a significant correlation between ozone levels and respiratory illness over the 18-year follow-up period. The study revealed that people living in cities with high ozone levels, such as Houston or Los Angeles, had an over 30% increased risk of dying from lung disease. [ 109 ] [ 110 ]
Air quality guidelines such as those from the World Health Organization , the U.S. Environmental Protection Agency (EPA), and the European Union are based on detailed studies designed to identify the levels that can cause measurable ill health effects .
According to scientists with the EPA, susceptible people can be adversely affected by ozone levels as low as 40 nmol/mol. [ 108 ] [ 111 ] [ 112 ] In the EU, the current target value for ozone concentrations is 120 μg/m 3 which is about 60 nmol/mol. This target applies to all member states in accordance with Directive 2008/50/EC. [ 113 ] Ozone concentration is measured as a maximum daily mean of 8 hour averages and the target should not be exceeded on more than 25 calendar days per year, starting from January 2010. While the directive requires in the future a strict compliance with 120 μg/m 3 limit (i.e. mean ozone concentration not to be exceeded on any day of the year), there is no date set for this requirement and this is treated as a long-term objective. [ 114 ]
In the US, the Clean Air Act directs the EPA to set National Ambient Air Quality Standards for several pollutants, including ground-level ozone, and counties out of compliance with these standards are required to take steps to reduce their levels. In May 2008, under a court order, the EPA lowered its ozone standard from 80 nmol/mol to 75 nmol/mol. The move proved controversial, since the Agency's own scientists and advisory board had recommended lowering the standard to 60 nmol/mol. [ 108 ] Many public health and environmental groups also supported the 60 nmol/mol standard, [ 115 ] and the World Health Organization recommends 100 μg/m 3 (51 nmol/mol). [ 116 ]
On January 7, 2010, the U.S. Environmental Protection Agency (EPA) announced proposed revisions to the National Ambient Air Quality Standard (NAAQS) for the pollutant ozone, the principal component of smog:
... EPA proposes that the level of the 8-hour primary standard, which was set at 0.075 μmol/mol in the 2008 final rule, should instead be set at a lower level within the range of 0.060 to 0.070 μmol/mol, to provide increased protection for children and other at risk populations against an array of O 3 – related adverse health effects that range from decreased lung function and increased respiratory symptoms to serious indicators of respiratory morbidity including emergency department visits and hospital admissions for respiratory causes, and possibly cardiovascular-related morbidity as well as total non- accidental and cardiopulmonary mortality ... [ 117 ]
On October 26, 2015, the EPA published a final rule with an effective date of December 28, 2015, that revised the 8-hour primary NAAQS from 0.075 ppm to 0.070 ppm. [ 118 ]
The EPA has developed an air quality index (AQI) to help explain air pollution levels to the general public. Under the current standards, eight-hour average ozone mole fractions of 85 to 104 nmol/mol are described as "unhealthy for sensitive groups", 105 nmol/mol to 124 nmol/mol as "unhealthy", and 125 nmol/mol to 404 nmol/mol as "very unhealthy". [ 119 ]
Ozone can also be present in indoor air pollution , partly as a result of electronic equipment such as photocopiers. A connection has also been known to exist between the increased pollen, fungal spores, and ozone caused by thunderstorms and hospital admissions of asthma sufferers. [ 120 ]
In the Victorian era , one British folk myth held that the smell of the sea was caused by ozone. In fact, the characteristic "smell of the sea" is caused by dimethyl sulfide , a chemical generated by phytoplankton . Victorian Britons considered the resulting smell "bracing". [ 121 ]
An investigation to assess the joint mortality effects of ozone and heat during the European heat waves in 2003, concluded that these appear to be additive. [ 122 ]
Ozone, along with reactive forms of oxygen such as superoxide , singlet oxygen , hydrogen peroxide , and hypochlorite ions, is produced by white blood cells and other biological systems (such as the roots of marigolds ) as a means of destroying foreign bodies. Ozone reacts directly with organic double bonds. Also, when ozone breaks down to dioxygen it gives rise to oxygen free radicals , which are highly reactive and capable of damaging many organic molecules . Moreover, it is believed that the powerful oxidizing properties of ozone may be a contributing factor of inflammation . The cause-and-effect relationship of how the ozone is created in the body and what it does is still under consideration and still subject to various interpretations, since other body chemical processes can trigger some of the same reactions. There is evidence linking the antibody-catalyzed water-oxidation pathway of the human immune response to the production of ozone. In this system, ozone is produced by antibody-catalyzed production of trioxidane from water and neutrophil-produced singlet oxygen. [ 123 ]
When inhaled, ozone reacts with compounds lining the lungs to form specific, cholesterol-derived metabolites that are thought to facilitate the build-up and pathogenesis of atherosclerotic plaques (a form of heart disease ). These metabolites have been confirmed as naturally occurring in human atherosclerotic arteries and are categorized into a class of secosterols termed atheronals , generated by ozonolysis of cholesterol's double bond to form a 5,6 secosterol [ 124 ] as well as a secondary condensation product via aldolization. [ 125 ]
Ozone has been implicated to have an adverse effect on plant growth: "... ozone reduced total chlorophylls, carotenoid and carbohydrate concentration, and increased 1-aminocyclopropane-1-carboxylic acid (ACC) content and ethylene production. In treated plants, the ascorbate leaf pool was decreased, while lipid peroxidation and solute leakage were significantly higher than in ozone-free controls. The data indicated that ozone triggered protective mechanisms against oxidative stress in citrus." [ 126 ] Studies that have used pepper plants as a model have shown that ozone decreased fruit yield and changed fruit quality. [ 127 ] [ 128 ] Furthermore, it was also observed a decrease in chlorophylls levels and antioxidant defences on the leaves, as well as increased the reactive oxygen species (ROS) levels and lipid and protein damages. [ 127 ] [ 128 ]
A 2022 study concludes that East Asia loses 63 billion dollars in crops per year due to ozone pollution, a byproduct of fossil fuel combustion. China loses about one-third of its potential wheat production and one-fourth of its rice production. [ 129 ] [ 130 ]
Because of the strongly oxidizing properties of ozone, ozone is a primary irritant, affecting especially the eyes and respiratory systems and can be hazardous at even low concentrations. The Canadian Centre for Occupation Safety and Health reports that:
Even very low concentrations of ozone can be harmful to the upper respiratory tract and the lungs. The severity of injury depends on both the concentration of ozone and the duration of exposure. Severe and permanent lung injury or death could result from even a very short-term exposure to relatively low concentrations." [ 131 ]
To protect workers potentially exposed to ozone, U.S. Occupational Safety and Health Administration has established a permissible exposure limit (PEL) of 0.1 μmol/mol (29 CFR 1910.1000 table Z-1), calculated as an 8-hour time weighted average. Higher concentrations are especially hazardous and NIOSH has established an Immediately Dangerous to Life and Health Limit (IDLH) of 5 μmol/mol. [ 132 ] Work environments where ozone is used or where it is likely to be produced should have adequate ventilation and it is prudent to have a monitor for ozone that will alarm if the concentration exceeds the OSHA PEL. Continuous monitors for ozone are available from several suppliers.
Elevated ozone exposure can occur on passenger aircraft , with levels depending on altitude and atmospheric turbulence. [ 133 ] U.S. Federal Aviation Administration regulations set a limit of 250 nmol/mol with a maximum four-hour average of 100 nmol/mol. [ 134 ] Some planes are equipped with ozone converters in the ventilation system to reduce passenger exposure. [ 133 ]
Ozone generators , or ozonators , [ 135 ] are used to produce ozone for cleaning air or removing smoke odours in unoccupied rooms. These ozone generators can produce over 3 g of ozone per hour. Ozone often forms in nature under conditions where O 2 will not react. [ 29 ] Ozone used in industry is measured in μmol/mol (ppm, parts per million), nmol/mol (ppb, parts per billion), μg/m 3 , mg/h (milligrams per hour) or weight percent. The regime of applied concentrations ranges from 1% to 5% (in air) and from 6% to 14% (in oxygen) for older generation methods. New electrolytic methods can achieve up 20% to 30% dissolved ozone concentrations in output water.
Temperature and humidity play a large role in how much ozone is being produced using traditional generation methods (such as corona discharge and ultraviolet light). Old generation methods will produce less than 50% of nominal capacity if operated with humid ambient air, as opposed to very dry air. New generators, using electrolytic methods, can achieve higher purity and dissolution through using water molecules as the source of ozone production.
This is the most common type of ozone generator for most industrial and personal uses. While variations of the "hot spark" coronal discharge method of ozone production exist, including medical grade and industrial grade ozone generators, these units usually work by means of a corona discharge tube or ozone plate. [ 136 ] [ 137 ] They are typically cost-effective and do not require an oxygen source other than the ambient air to produce ozone concentrations of 3–6%. Fluctuations in ambient air, due to weather or other environmental conditions, cause variability in ozone production. However, they also produce nitrogen oxides as a by-product. Use of an air dryer can reduce or eliminate nitric acid formation by removing water vapor and increase ozone production. At room temperature, nitric acid will form into a vapour that is hazardous if inhaled. Symptoms can include chest pain, shortness of breath, headaches and a dry nose and throat causing a burning sensation. Use of an oxygen concentrator can further increase the ozone production and further reduce the risk of nitric acid formation by removing not only the water vapor, but also the bulk of the nitrogen.
UV ozone generators, or vacuum-ultraviolet (VUV) ozone generators, employ a light source that generates a narrow-band ultraviolet light, a subset of that produced by the Sun. The Sun's UV sustains the ozone layer in the stratosphere of Earth. [ 138 ]
UV ozone generators use ambient air for ozone production, no air prep systems are used (air dryer or oxygen concentrator), therefore these generators tend to be less expensive. However, UV ozone generators usually produce ozone with a concentration of about 0.5% or lower which limits the potential ozone production rate. Another disadvantage of this method is that it requires the ambient air (oxygen) to be exposed to the UV source for a longer amount of time, and any gas that is not exposed to the UV source will not be treated. This makes UV generators impractical for use in situations that deal with rapidly moving air or water streams (in-duct air sterilization , for example). Production of ozone is one of the potential dangers of ultraviolet germicidal irradiation . VUV ozone generators are used in swimming pools and spa applications ranging to millions of gallons of water. VUV ozone generators, unlike corona discharge generators, do not produce harmful nitrogen by-products and also unlike corona discharge systems, VUV ozone generators work extremely well in humid air environments. There is also not normally a need for expensive off-gas mechanisms, and no need for air driers or oxygen concentrators which require extra costs and maintenance.
In the cold plasma method, pure oxygen gas is exposed to a plasma created by DBD . The diatomic oxygen is split into single atoms, which then recombine in triplets to form ozone.
It is common in the industry to mislabel some DBD ozone generators as CD Corona Discharge generators. Typically all solid flat metal electrode ozone generators produce ozone using the dielectric barrier discharge method. Cold plasma machines use pure oxygen as the input source and produce a maximum concentration of about 24% ozone. They produce far greater quantities of ozone in a given time compared to ultraviolet production that has about 2% efficiency. The discharges manifest as filamentary transfer of electrons (micro discharges) in a gap between two electrodes. In order to evenly distribute the micro discharges, a dielectric insulator must be used to separate the metallic electrodes and to prevent arcing.
Electrolytic ozone generation (EOG) splits water molecules into H 2 , O 2 , and O 3 .
In most EOG methods, the hydrogen gas will be removed to leave oxygen and ozone as the only reaction products. Therefore, EOG can achieve higher dissolution in water without other competing gases found in corona discharge method, such as nitrogen gases present in ambient air. This method of generation can achieve concentrations of 20–30% and is independent of air quality because water is used as the source material. Production of ozone electrolytically is typically unfavorable because of the high overpotential required to produce ozone as compared to oxygen. This is why ozone is not produced during typical water electrolysis. However, it is possible to increase the overpotential of oxygen by careful catalyst selection such that ozone is preferentially produced under electrolysis. Catalysts typically chosen for this approach are lead dioxide [ 139 ] or boron-doped diamond. [ 140 ]
The ozone-to-oxygen ratio is improved by increasing current density at the anode, cooling the electrolyte around the anode close to 0 °C, using an acidic electrolyte (such as dilute sulfuric acid) instead of a basic solution, and by applying pulsed current instead of DC. [ 141 ]
Ozone cannot be stored and transported like other industrial gases (because it quickly decays into diatomic oxygen) and must therefore be produced on site. Available ozone generators vary in the arrangement and design of the high-voltage electrodes. At production capacities higher than 20 kg per hour, a gas/water tube heat-exchanger may be utilized as ground electrode and assembled with tubular high-voltage electrodes on the gas-side. The regime of typical gas pressures is around 2 bars (200 kPa ) absolute in oxygen and 3 bars (300 kPa) absolute in air. Several megawatts of electrical power may be installed in large facilities, applied as single phase AC current at 50 to 8000 Hz and peak voltages between 3,000 and 20,000 volts. Applied voltage is usually inversely related to the applied frequency.
The dominating parameter influencing ozone generation efficiency is the gas temperature, which is controlled by cooling water temperature and/or gas velocity. The cooler the water, the better the ozone synthesis. The lower the gas velocity, the higher the concentration (but the lower the net ozone produced). At typical industrial conditions, almost 90% of the effective power is dissipated as heat and needs to be removed by a sufficient cooling water flow.
Because of the high reactivity of ozone, only a few materials may be used like stainless steel (quality 316L), titanium , aluminium (as long as no moisture is present), glass , polytetrafluorethylene , or polyvinylidene fluoride . Viton may be used with the restriction of constant mechanical forces and absence of humidity (humidity limitations apply depending on the formulation). Hypalon may be used with the restriction that no water comes in contact with it, except for normal atmospheric levels. Embrittlement or shrinkage is the common mode of failure of elastomers with exposure to ozone. Ozone cracking is the common mode of failure of elastomer seals like O-rings .
Silicone rubbers are usually adequate for use as gaskets in ozone concentrations below 1 wt%, such as in equipment for accelerated aging of rubber samples.
Ozone may be formed from O 2 by electrical discharges and by action of high energy electromagnetic radiation . Unsuppressed arcing in electrical contacts, motor brushes, or mechanical switches breaks down the chemical bonds of the atmospheric oxygen surrounding the contacts [ O 2 -> 2O]. Free radicals of oxygen in and around the arc recombine to create ozone [ O 3 ]. [ 142 ] Certain electrical equipment generate significant levels of ozone. This is especially true of devices using high voltages , such as ionic air purifiers , laser printers , photocopiers , tasers , and arc welders . Electric motors using brushes can generate ozone from repeated sparking inside the unit. Large motors that use brushes, such as those used by elevators or hydraulic pumps, will generate more ozone than smaller motors.
Ozone is similarly formed in the Catatumbo lightning storms phenomenon on the Catatumbo River in Venezuela , though ozone's instability makes it dubious that it has any effect on the ozonosphere. [ 143 ] It is the world's largest single natural generator of ozone, lending calls for it to be designated a UNESCO World Heritage Site . [ 144 ]
In the laboratory, ozone can be produced by electrolysis using a 9 volt battery , a pencil graphite rod cathode , a platinum wire anode , and a 3 molar sulfuric acid electrolyte . [ 145 ] The half cell reactions taking place are:
where E° represents the standard electrode potential .
In the net reaction, three equivalents of water are converted into one equivalent of ozone and three equivalents of hydrogen . Oxygen formation is a competing reaction.
It can also be generated by a high voltage arc . In its simplest form, high voltage AC, such as the output of a neon-sign transformer is connected to two metal rods with the ends placed sufficiently close to each other to allow an arc. The resulting arc will convert atmospheric oxygen to ozone.
It is often desirable to contain the ozone. This can be done with an apparatus consisting of two concentric glass tubes sealed together at the top with gas ports at the top and bottom of the outer tube. The inner core should have a length of metal foil inserted into it connected to one side of the power source. The other side of the power source should be connected to another piece of foil wrapped around the outer tube. A source of dry O 2 is applied to the bottom port. When high voltage is applied to the foil leads, electricity will discharge between the dry dioxygen in the middle and form O 3 and O 2 which will flow out the top port. This is called a Siemen's ozoniser. The reaction can be summarized as follows: [ 29 ]
The largest use of ozone is in the preparation of pharmaceuticals , synthetic lubricants , and many other commercially useful organic compounds , where it is used to sever carbon -carbon bonds. [ 29 ] It can also be used for bleaching substances and for killing microorganisms in air and water sources. [ 146 ] Many municipal drinking water systems kill bacteria with ozone instead of the more common chlorine . [ 147 ] Ozone has a very high oxidation potential . [ 148 ] Ozone does not form organochlorine compounds, nor does it remain in the water after treatment. Ozone can form the suspected carcinogen bromate in source water with high bromide concentrations. The U.S. Safe Drinking Water Act mandates that these systems introduce an amount of chlorine to maintain a minimum of 0.2 μmol/mol residual free chlorine in the pipes, based on results of regular testing. Where electrical power is abundant, ozone is a cost-effective method of treating water, since it is produced on demand and does not require transportation and storage of hazardous chemicals. Once it has decayed, it leaves no taste or odour in drinking water.
Although low levels of ozone have been advertised to be of some disinfectant use in residential homes, the concentration of ozone in dry air required to have a rapid, substantial effect on airborne pathogens exceeds safe levels recommended by the U.S. Occupational Safety and Health Administration and Environmental Protection Agency . Humidity control can vastly improve both the killing power of the ozone and the rate at which it decays back to oxygen (more humidity allows more effectiveness). Spore forms of most pathogens are very tolerant of atmospheric ozone in concentrations at which asthma patients start to have issues.
In 1908 artificial ozonisation of the Central Line of the London Underground was introduced for aerial disinfection. The process was found to be worthwhile, but was phased out by 1956. However the beneficial effect was maintained by the ozone created incidentally from the electrical discharges of the train motors (see above: Incidental production ). [ 149 ]
Ozone generators were made available to schools and universities in Wales for the Autumn term 2021, to disinfect classrooms after COVID-19 outbreaks. [ 150 ]
Industrially, ozone is used to:
Ozone is a reagent in many organic reactions in the laboratory and in industry. Ozonolysis is the cleavage of an alkene to carbonyl compounds.
Many hospitals around the world use large ozone generators to decontaminate operating rooms between surgeries. The rooms are cleaned and then sealed airtight before being filled with ozone which effectively kills or neutralizes all remaining bacteria. [ 156 ]
Ozone is used as an alternative to chlorine or chlorine dioxide in the bleaching of wood pulp . [ 157 ] It is often used in conjunction with oxygen and hydrogen peroxide to eliminate the need for chlorine-containing compounds in the manufacture of high-quality, white paper . [ 158 ]
Ozone can be used to detoxify cyanide wastes (for example from gold and silver mining ) by oxidizing cyanide to cyanate and eventually to carbon dioxide . [ 159 ]
Since the invention of dielectric barrier discharge (DBD) plasma reactors, it has been employed for water treatment with ozone. [ 160 ] However, with cheaper alternative disinfectants like chlorine, such applications of DBD ozone water decontamination have been limited by high power consumption and bulky equipment. [ 161 ] [ 162 ] Despite this, with research revealing the negative impacts of common disinfectants like chlorine with respect to toxic residuals and ineffectiveness in killing certain micro-organisms, [ 163 ] DBD plasma-based ozone decontamination is of interest in current available technologies. Although ozonation of water with a high concentration of bromide does lead to the formation of undesirable brominated disinfection byproducts, unless drinking water is produced by desalination, ozonation can generally be applied without concern for these byproducts. [ 162 ] [ 164 ] [ 165 ] [ 166 ] Advantages of ozone include high thermodynamic oxidation potential, less sensitivity to organic material and better tolerance for pH variations while retaining the ability to kill bacteria, fungi, viruses, as well as spores and cysts. [ 167 ] [ 168 ] [ 169 ] Although, ozone has been widely accepted in Europe for decades, it is sparingly used for decontamination in the U.S. due to limitations of high-power consumption, bulky installation and stigma attached with ozone toxicity. [ 161 ] [ 170 ] Considering this, recent research efforts have been directed toward the study of effective ozone water treatment systems. [ 171 ] Researchers have looked into lightweight and compact low power surface DBD reactors, [ 172 ] [ 173 ] energy efficient volume DBD reactors [ 174 ] and low power micro-scale DBD reactors. [ 175 ] [ 176 ] Such studies can help pave the path to re-acceptance of DBD plasma-based ozone decontamination of water, especially in the U.S.
Ozone levels which are safe for people are ineffective at killing fungi and bacteria. [ 177 ] Some consumer disinfection and cosmetic products emit ozone at levels harmful to human health. [ 177 ]
Devices generating high levels of ozone, some of which use ionization, are used to sanitize and deodorize uninhabited buildings, rooms, ductwork, woodsheds, boats, and other vehicles.
Ozonated water is used to launder clothes and to sanitize food, drinking water, and surfaces in the home. According to the U.S. Food and Drug Administration (FDA), it is "amending the food additive regulations to provide for the safe use of ozone in gaseous and aqueous phases as an antimicrobial agent on food, including meat and poultry." Studies at California Polytechnic University demonstrated that 0.3 μmol/mol levels of ozone dissolved in filtered tapwater can produce a reduction of more than 99.99% in such food-borne microorganisms as salmonella, E. coli 0157:H7 and Campylobacter . This quantity is 20,000 times the WHO -recommended limits stated above. [ 152 ] [ 178 ]
Ozone can be used to remove pesticide residues from fruits and vegetables . [ 179 ] [ 180 ]
Ozone is used in homes and hot tubs to kill bacteria in the water and to reduce the amount of chlorine or bromine required by reactivating them to their free state. Since ozone does not remain in the water long enough, ozone by itself is ineffective at preventing cross-contamination among bathers and must be used in conjunction with halogens . Gaseous ozone created by ultraviolet light or by corona discharge is injected into the water. [ 181 ]
Ozone is also widely used in the treatment of water in aquariums and fishponds. Its use can minimize bacterial growth, control parasites, eliminate transmission of some diseases, and reduce or eliminate "yellowing" of the water. Ozone must not come in contact with fishes' gill structures. Natural saltwater (with life forms) provides enough "instantaneous demand" that controlled amounts of ozone activate bromide ions to hypobromous acid , and the ozone entirely decays in a few seconds to minutes. If oxygen-fed ozone is used, the water will be higher in dissolved oxygen and fishes' gill structures will atrophy, making them dependent on oxygen-enriched water.
Ozonation – a process of infusing water with ozone – can be used in aquaculture to facilitate organic breakdown. Ozone is also added to recirculating systems to reduce nitrite levels [ 182 ] through conversion into nitrate . If nitrite levels in the water are high, nitrites will also accumulate in the blood and tissues of fish, where it interferes with oxygen transport (it causes oxidation of the heme-group of haemoglobin from ferrous ( Fe 2+ ) to ferric ( Fe 3+ ), making haemoglobin unable to bind O 2 ). [ 183 ] Despite these apparent positive effects, ozone use in recirculation systems has been linked to reducing the level of bioavailable iodine in salt water systems, resulting in iodine deficiency symptoms such as goitre and decreased growth in Senegalese sole ( Solea senegalensis ) larvae. [ 184 ]
Ozonate seawater is used for surface disinfection of haddock and Atlantic halibut eggs against nodavirus. Nodavirus is a lethal and vertically transmitted virus which causes severe mortality in fish. Haddock eggs should not be treated with high ozone level as eggs so treated did not hatch and died after 3–4 days. [ 185 ]
Ozone application on freshly cut pineapple and banana shows increase in flavonoids and total phenol contents when exposure is up to 20 minutes. Decrease in ascorbic acid (one form of vitamin C ) content is observed but the positive effect on total phenol content and flavonoids can overcome the negative effect. [ 186 ] Tomatoes upon treatment with ozone show an increase in β-carotene, lutein and lycopene. [ 187 ] However, ozone application on strawberries in pre-harvest period shows decrease in ascorbic acid content. [ 188 ]
Ozone facilitates the extraction of some heavy metals from soil using EDTA . EDTA forms strong, water-soluble coordination compounds with some heavy metals ( Pb and Zn ) thereby making it possible to dissolve them out from contaminated soil. If contaminated soil is pre-treated with ozone, the extraction efficacy of Pb , Am , and Pu increases by 11.0–28.9%, [ 189 ] 43.5% [ 190 ] and 50.7% [ 190 ] respectively.
Crop pollination is an essential part of an ecosystem. Ozone can have detrimental effects on plant-pollinator interactions. [ 191 ] Pollinators carry pollen from one plant to another. This is an essential cycle inside of an ecosystem. Causing changes in certain atmospheric conditions around pollination sites or with xenobiotics could cause unknown changes to the natural cycles of pollinators and flowering plants. In a study conducted in North-Western Europe, crop pollinators were negatively affected more when ozone levels were higher. [ 192 ]
The use of ozone for the treatment of medical conditions is not supported by high quality evidence, and is generally considered alternative medicine . [ 193 ]
Footnotes
Citations
Nascent oxygen O
Dioxygen ( singlet and triplet ) O 2
Trioxygen ( ozone and cyclic ozone ) O 3
Tetraoxygen O 4
Octaoxygen O 8 | https://en.wikipedia.org/wiki/O⚎O⚎O |
P(doom) is a term in AI safety that refers to the probability of existentially catastrophic outcomes (or "doom") as a result of artificial intelligence . [ 1 ] [ 2 ] The exact outcomes in question differ from one prediction to another, but generally allude to the existential risk from artificial general intelligence . [ 3 ]
Originating as an inside joke among AI researchers, the term came to prominence in 2023 following the release of GPT-4 , as high-profile figures such as Geoffrey Hinton [ 4 ] and Yoshua Bengio [ 5 ] began to warn of the risks of AI. [ 6 ] In a 2023 survey, AI researchers were asked to estimate the probability that future AI advancements could lead to human extinction or similarly severe and permanent disempowerment within the next 100 years. The mean value from the responses was 14.4%, with a median value of 5%. [ 7 ] [ 8 ]
There has been some debate about the usefulness of P(doom) as a term, in part due to the lack of clarity about whether or not a given prediction is conditional on the existence of artificial general intelligence , the time frame, and the precise meaning of "doom". [ 6 ] [ 20 ] | https://en.wikipedia.org/wiki/P(doom) |
P -Chiral phosphines are organophosphorus compounds of the formula PRR′R″, where R, R′, R″ = H, alkyl, aryl, etc. They are a subset of chiral phosphines, a broader class of compounds where the stereogenic center can reside at sites other than phosphorus. P-chirality exploits the high barrier for inversion of phosphines, which ensures that enantiomers of PRR'R" do not racemize readily. The inversion barrier is relatively insensitive to substituents for triorganophosphines. [ 2 ] By contrast, most amines of the type NRR′R″ undergo rapid pyramidal inversion .
Most chiral phosphines are C 2 -symmetric diphosphines . Famous examples are DIPAMP and BINAP . These chelating ligands support catalysts used in asymmetric hydrogenation and related reactions. DIPAMP is prepared by coupling the P -chiral methylphenylanisylphosphine.
P -Chiral phosphines are of particular interest in asymmetric catalysis . P -Chiral phosphines have been investigated for two main applications, as ligands for asymmetric homogeneous catalysts and as nucleophiles in organocatalysis . [ 1 ]
This stereochemistry article is a stub . You can help Wikipedia by expanding it .
This catalysis article is a stub . You can help Wikipedia by expanding it . | https://en.wikipedia.org/wiki/P-Chiral_phosphine |
p -Dimethylaminocinnamaldehyde ( DMACA ) is an aromatic hydrocarbon. It is used in an acidic solution to detect indoles.
The DMACA is any of a number of acidified DMACA solutions:
It is primarily used as a histological dye used to detect indoles , particularly for production in cells. It is used for the rapid identification of bacteria containing tryptophanase enzyme systems. [ citation needed ] It is also particularly useful for localization of proanthocyanidins compounds in plants, resulting in a blue staining. It has been used for grapevine fruit [ 4 ] or for legumes foliage [ 5 ] histology.
A colorimetric assay based upon the reaction of A-rings [ clarification needed ] with the chromogen. p -Dimethylaminocinnamaldehyde has been developed for flavanoids in beer that can be compared with the vanillin procedure. [ 6 ] The DMACA reagent may be superior to the vanillin procedure for the detection of catechins . [ 7 ]
The DMACA reagent changes color over several days when exposed to air but when refrigerated can be stored for up to two weeks. [ 8 ]
The DMACA reagent may also be referred to as the Renz and Loew reagent. [ 3 ] [ 9 ] | https://en.wikipedia.org/wiki/P-Dimethylaminocinnamaldehyde |
8.5 ( acetonitrile ) [ 4 ]
Para -Toluenesulfonic acid ( PTSA , pTSA , or p TsOH ) or tosylic acid ( TsOH ) is an organic compound with the formula CH 3 C 6 H 4 SO 3 H . It is a white extremely hygroscopic solid that is soluble in water, alcohols , and other polar organic solvents. [ 6 ] The CH 3 C 6 H 4 SO 2 group is known as the tosyl group and is often abbreviated as Ts or Tos. Most often, TsOH refers to the monohydrate , TsOH . H 2 O. [ 6 ]
As with other aryl sulfonic acids , TsOH is a strong organic acid . It is about one million times stronger than benzoic acid . [ 6 ] It is one of the few strong acids that is solid and therefore is conveniently weighed and stored.
TsOH is prepared on an industrial scale by the sulfonation of toluene . Common impurities include benzenesulfonic acid and sulfuric acid. TsOH is most often supplied as the monohydrate, and it may be necessary to remove the complexed water before use. Impurities can be removed by recrystallization from its concentrated aqueous solution followed by azeotropic drying with toluene. [ 2 ]
TsOH finds use in organic synthesis as an "organic-soluble" strong acid. Examples of uses include:
Alkyl tosylates are alkylating agents because tosylate is electron-withdrawing as well as a good leaving group . Tosylate is a pseudohalide . Toluenesulfonate esters undergo nucleophilic attack or elimination . Reduction of tosylate esters gives the hydrocarbon. Thus, tosylation followed by reduction allows for the deoxygenation of alcohols.
In a famous and illustrative use of tosylate as a leaving group, the 2-norbornyl cation was formed by an elimination reaction of 7-norbornenyl tosylate. The elimination occurs 10 11 times faster than the solvolysis of anti -7-norbornyl Para -toluenesulfonate. [ 10 ]
Tosylates are also protecting groups for alcohols . They are prepared by combining the alcohol with 4-toluenesulfonyl chloride in the presence of a base. These reactions are usually performed in an aprotic solvent , often pyridine , which additionally acts as a base. [ 11 ]
This reaction is general for aryl sulfonic acids . [ 13 ] [ 14 ] | https://en.wikipedia.org/wiki/P-Toluenesulfonic_acid |
This page provides supplementary chemical data on p -xylene .
The handling of this chemical may incur notable safety precautions. It is highly recommend that you seek the Material Safety Datasheet ( MSDS ) for this chemical from a reliable source and follow its directions.
Table data obtained from CRC Handbook of Chemistry and Physics 44th ed. | https://en.wikipedia.org/wiki/P-Xylene_(data_page) |
In mathematics , p -adic Teichmüller theory describes the "uniformization" of p -adic curves and their moduli , generalizing the usual Teichmüller theory that describes the uniformization of Riemann surfaces and their moduli. It was introduced and developed by Shinichi Mochizuki ( 1996 , 1999 ).
The first problem is to reformulate the Fuchsian uniformization of a complex Riemann surface (an isomorphism from the upper half plane to a universal covering space of the surface) in a way that makes sense for p -adic curves. The existence of a Fuchsian uniformization is equivalent to the existence of a canonical indigenous bundle over the Riemann surface: the unique indigenous bundle that is invariant under complex conjugation and whose monodromy representation is quasi-Fuchsian. For p -adic curves, the analogue of complex conjugation is the Frobenius endomorphism , and the analogue of the quasi-Fuchsian condition is an integrality condition on the indigenous line bundle. So in p -adic Teichmüller theory, the p -adic analogue the Fuchsian uniformization of Teichmüller theory, is the study of integral Frobenius invariant indigenous bundles.
This number theory -related article is a stub . You can help Wikipedia by expanding it . | https://en.wikipedia.org/wiki/P-adic_Teichmüller_theory |
In mathematics , p -adic analysis is a branch of number theory that studies functions of p -adic numbers . Along with the more classical fields of real and complex analysis , which deal, respectively, with functions on the real and complex numbers, it belongs to the discipline of mathematical analysis .
The theory of complex-valued numerical functions on the p -adic numbers is part of the theory of locally compact groups ( abstract harmonic analysis ). The usual meaning taken for p -adic analysis is the theory of p -adic-valued functions on spaces of interest.
Applications of p -adic analysis have mainly been in number theory , where it has a significant role in diophantine geometry and diophantine approximation . Some applications have required the development of p -adic functional analysis and spectral theory . In many ways p -adic analysis is less subtle than classical analysis , since the ultrametric inequality means, for example, that convergence of infinite series of p -adic numbers is much simpler. Topological vector spaces over p -adic fields show distinctive features; for example aspects relating to convexity and the Hahn–Banach theorem are different.
Ostrowski's theorem, due to Alexander Ostrowski (1916), states that every non-trivial absolute value on the rational numbers Q is equivalent to either the usual real absolute value or a p -adic absolute value. [ 1 ]
Mahler's theorem , introduced by Kurt Mahler , [ 2 ] expresses continuous p -adic functions in terms of polynomials.
In any field of characteristic 0, one has the following result. Let
be the forward difference operator . Then for polynomial functions f we have the Newton series :
where
is the k th binomial coefficient polynomial.
Over the field of real numbers, the assumption that the function f is a polynomial can be weakened, but it cannot be weakened all the way down to mere continuity .
Mahler proved the following result:
Mahler's theorem : If f is a continuous p -adic -valued function on the p -adic integers then the same identity holds.
Hensel's lemma, also known as Hensel's lifting lemma, named after Kurt Hensel , is a result in modular arithmetic , stating that if a polynomial equation has a simple root modulo a prime number p , then this root corresponds to a unique root of the same equation modulo any higher power of p , which can be found by iteratively " lifting " the solution modulo successive powers of p . More generally it is used as a generic name for analogues for complete commutative rings (including p -adic fields in particular) of the Newton method for solving equations. Since p -adic analysis is in some ways simpler than real analysis , there are relatively easy criteria guaranteeing a root of a polynomial.
To state the result, let f ( x ) {\displaystyle f(x)} be a polynomial with integer (or p -adic integer) coefficients, and let m , k be positive integers such that m ≤ k . If r is an integer such that
then there exists an integer s such that
Furthermore, this s is unique modulo p k +m , and can be computed explicitly as
Helmut Hasse 's local–global principle, also known as the Hasse principle, is the idea that one can find an integer solution to an equation by using the Chinese remainder theorem to piece together solutions modulo powers of each different prime number . This is handled by examining the equation in the completions of the rational numbers : the real numbers and the p -adic numbers . A more formal version of the Hasse principle states that certain types of equations have a rational solution if and only if they have a solution in the real numbers and in the p -adic numbers for each prime p . | https://en.wikipedia.org/wiki/P-adic_analysis |
In mathematics, a p-adic distribution is an analogue of ordinary distributions (i.e. generalized functions) that takes values in a ring of p -adic numbers .
If X is a topological space , a distribution on X with values in an abelian group G is a finitely additive function from the compact open subsets of X to G . Equivalently, if we define the space of test functions to be the locally constant and compactly supported integer-valued functions, then a distribution is an additive map from test functions to G . This is formally similar to the usual definition of distributions, which are continuous linear maps from a space of test functions on a manifold to the real numbers.
A p -adic measure is a special case of a p -adic distribution, analogous to a measure on a measurable space. A p -adic distribution taking values in a normed space is called a p -adic measure if the values on compact open subsets are bounded. | https://en.wikipedia.org/wiki/P-adic_distribution |
In mathematics , the p -adic gamma function Γ p is a function of a p -adic variable analogous to the gamma function . It was first explicitly defined by Morita (1975) , though Boyarsky (1980) pointed out that Dwork (1964) implicitly used the same function. Diamond (1977) defined a p -adic analog G p of log Γ. Overholtzer (1952) had previously given a definition of a different p -adic analogue of the gamma function, but his function does not have satisfactory properties and is not used much.
The p -adic gamma function is the unique continuous function of a p -adic integer x (with values in Z p {\displaystyle \mathbb {Z} _{p}} ) such that
for positive integers x , where the product is restricted to integers i not divisible by p . As the positive integers are dense with respect to the p -adic topology in Z p {\displaystyle \mathbb {Z} _{p}} , Γ p ( x ) {\displaystyle \Gamma _{p}(x)} can be extended uniquely to the whole of Z p {\displaystyle \mathbb {Z} _{p}} . Here Z p {\displaystyle \mathbb {Z} _{p}} is the ring of p -adic integers . It follows from the definition that the values of Γ p ( Z ) {\displaystyle \Gamma _{p}(\mathbb {Z} )} are invertible in Z p {\displaystyle \mathbb {Z} _{p}} ; this is because these values are products of integers not divisible by p , and this property holds after the continuous extension to Z p {\displaystyle \mathbb {Z} _{p}} . Thus Γ p : Z p → Z p × {\displaystyle \Gamma _{p}:\mathbb {Z} _{p}\to \mathbb {Z} _{p}^{\times }} . Here Z p × {\displaystyle \mathbb {Z} _{p}^{\times }} is the set of invertible p -adic integers.
The classical gamma function satisfies the functional equation Γ ( x + 1 ) = x Γ ( x ) {\displaystyle \Gamma (x+1)=x\Gamma (x)} for any x ∈ C ∖ Z ≤ 0 {\displaystyle x\in \mathbb {C} \setminus \mathbb {Z} _{\leq 0}} . This has an analogue with respect to the Morita gamma function:
The Euler's reflection formula Γ ( x ) Γ ( 1 − x ) = π sin ( π x ) {\displaystyle \Gamma (x)\Gamma (1-x)={\frac {\pi }{\sin {(\pi x)}}}} has its following simple counterpart in the p -adic case:
where x 0 {\displaystyle x_{0}} is the first digit in the p -adic expansion of x , unless x ∈ p Z p {\displaystyle x\in p\mathbb {Z} _{p}} , in which case x 0 = p {\displaystyle x_{0}=p} rather than 0.
and, in general,
At x = 1 2 {\displaystyle x={\frac {1}{2}}} the Morita gamma function is related to the Legendre symbol ( a p ) {\displaystyle \left({\frac {a}{p}}\right)} :
It can also be seen, that Γ p ( p n ) ≡ 1 ( mod p n ) , {\displaystyle \Gamma _{p}(p^{n})\equiv 1{\pmod {p^{n}}},} hence Γ p ( p n ) → 1 {\displaystyle \Gamma _{p}(p^{n})\to 1} as n → ∞ {\displaystyle n\to \infty } . [ 1 ] : 369
Other interesting special values come from the Gross–Koblitz formula , which was first proved by cohomological tools, and later was proved using more elementary methods. [ 2 ] For example,
where − 1 ∈ Z 5 {\displaystyle {\sqrt {-1}}\in \mathbb {Z} _{5}} denotes the square root with first digit 3, and − 3 ∈ Z 7 {\displaystyle {\sqrt {-3}}\in \mathbb {Z} _{7}} denotes the square root with first digit 2. (Such specifications must always be done if we talk about roots.)
Another example is
where − 2 {\displaystyle {\sqrt {-2}}} is the square root of − 2 {\displaystyle -2} in Q 3 {\displaystyle \mathbb {Q} _{3}} congruent to 1 modulo 3. [ 3 ]
The Raabe-formula for the classical Gamma function says that
This has an analogue for the Iwasawa logarithm of the Morita gamma function: [ 4 ]
The ceiling function to be understood as the p -adic limit lim n → ∞ ⌈ x n p ⌉ {\displaystyle \lim _{n\to \infty }\left\lceil {\frac {x_{n}}{p}}\right\rceil } such that x n → x {\displaystyle x_{n}\to x} through rational integers.
The Mahler expansion is similarly important for p -adic functions as the Taylor expansion in classical analysis. The Mahler expansion of the p -adic gamma function is the following: [ 1 ] : 374
where the sequence a k {\displaystyle a_{k}} is defined by the following identity: | https://en.wikipedia.org/wiki/P-adic_gamma_function |
p -adic quantum mechanics is a collection of related research efforts in quantum physics that replace real numbers with p -adic numbers . Historically, this research was inspired by the discovery that the Veneziano amplitude of the open bosonic string , which is calculated using an integral over the real numbers, can be generalized to the p -adic numbers. [ 1 ] This observation initiated the study of p -adic string theory . [ 2 ] [ 3 ] [ 4 ] Another approach considers particles in a p -adic potential well, with the goal of finding solutions with smoothly varying complex-valued wave functions . Alternatively, one can consider particles in p -adic potential wells and seek p -adic valued wave functions, in which case the problem of the probabilistic interpretation of the p -adic valued wave function arises. [ 5 ] As there does not exist a suitable p -adic Schrödinger equation , [ 6 ] [ 7 ] path integrals are employed instead. Some one-dimensional systems have been studied by means of the path integral formulation, including the free particle , [ 8 ] the particle in a constant field, [ 9 ] and the harmonic oscillator . [ 10 ] | https://en.wikipedia.org/wiki/P-adic_quantum_mechanics |
In cellular biology , P-bodies , or processing bodies , are distinct foci formed by phase separation within the cytoplasm of a eukaryotic cell consisting of many enzymes involved in mRNA turnover . [ 1 ] P-bodies are highly conserved structures and have been observed in somatic cells originating from vertebrates and invertebrates , plants and yeast . To date, P-bodies have been demonstrated to play fundamental roles in general mRNA decay , nonsense-mediated mRNA decay , adenylate-uridylate-rich element mediated mRNA decay, and microRNA (miRNA) induced mRNA silencing . [ 2 ] Not all mRNAs which enter P-bodies are degraded, as it has been demonstrated that some mRNAs can exit P-bodies and re-initiate translation . [ 3 ] [ 4 ] Purification and sequencing of the mRNA from purified processing bodies showed that these mRNAs are largely translationally repressed upstream of translation initiation and are protected from 5' mRNA decay. [ 5 ]
P-bodies were originally proposed to be the sites of mRNA degradation in the cell and involved in decapping and digestion of mRNAs earmarked for destruction. [ 6 ] [ 7 ] Later work called this into question suggesting P bodies store mRNA until needed for translation. [ 8 ] [ 5 ] [ 9 ]
In neurons , P-bodies are moved by motor proteins in response to stimulation. This is likely tied to local translation in dendrites . [ 10 ]
P-bodies were first described in the scientific literature by Bashkirov et al. [ 11 ] in 1997, in which they describe "small granules… discrete, prominent foci" as the cytoplasmic location of the mouse exoribonuclease mXrn1p. It wasn’t until 2002 that a glimpse into the nature and importance of these cytoplasmic foci was published., [ 12 ] [ 13 ] [ 14 ] when researchers demonstrated that multiple proteins involved with mRNA degradation localize to the foci. Their importance was recognized after experimental evidence was obtained pointing to P-bodies as the sites of mRNA degradation in the cell. [ 7 ] The researchers named these structures processing bodies or "P bodies". During this time, many descriptive names were used also to identify the processing bodies, including "GW-bodies" and "decapping-bodies"; however "P-bodies" was the term chosen and is now widely used and accepted in the scientific literature. [ 7 ] Recently evidence has been presented suggesting that GW-bodies and P-bodies may in fact be different cellular components. [ 15 ] The evidence being that GW182 and Ago2, both associated with miRNA gene silencing, are found exclusively in multivesicular bodies or GW-bodies and are not localized to P-bodies. Also of note, P-bodies are not equivalent to stress granules and they contain largely non-overlapping proteins. [ 5 ] The two structures support overlapping cellular functions but generally occur under different stimuli. Hoyle et al. suggests a novel site termed EGP bodies, or stress granules, may be responsible for mRNA storage as these sites lack the decapping enzyme. [ 16 ]
microRNA mediated repression occurs in two ways, either by translational repression or stimulating mRNA decay. miRNA recruit the RISC complex to the mRNA to which they are bound. The link to P-bodies comes by the fact that many, if not most, of the proteins necessary for miRNA gene silencing are localized to P-bodies, as reviewed by Kulkarni et al. (2010). [ 2 ] [ 17 ] [ 18 ] [ 19 ] [ 20 ] These proteins include, but are not limited to, the scaffold protein GW182, Argonaute (Ago), decapping enzymes and RNA helicases .
The current evidence points toward P-bodies as being scaffolding centers of miRNA function, especially due to the evidence that a knock down of GW182 disrupts P-body formation. However, there remain many unanswered questions about P-bodies and their relationship to miRNA activity. Specifically, it is unknown whether there is a context dependent (stress state versus normal) specificity to the P-body's mechanism of action. Based on the evidence that P-bodies sometimes are the site of mRNA decay and sometimes the mRNA can exit the P-bodies and re-initiate translation, the question remains of what controls this switch. Another ambiguous point to be addressed is whether the proteins that localize to P-bodies are actively functioning in the miRNA gene silencing process or whether they are merely on standby.
In 2017, a new method to purify processing bodies was published. [ 5 ] Hubstenberger et al. used fluorescence-activated particle sorting (a method based on the ideas of fluorescence-activated cell sorting ) to purify processing bodies from human epithelial cells. From these purified processing bodies they were able to use mass spectrometry and RNA sequencing to determine which proteins and RNAs are found in processing bodies, respectively. This study identified 125 proteins that are significantly associated with processing bodies. [ 5 ] Notably this work provided the most compelling evidence up to this date that P-bodies might not be the sites of degradation in the cell and instead used for storage of translationally repressed mRNA. This observation was further supported by single molecule imaging of mRNA by the Chao group in 2017. [ 9 ]
In 2018, Youn et al. took a proximity labeling approach called BioID to identify and predict the processing body proteome. [ 21 ] They engineered cells to express several processing body-localized proteins as fusion proteins with the BirA* enzyme. When the cells are incubated with biotin , BirA* will biotinylate proteins that are nearby, thus tagging the proteins within processing bodies with a biotin tag. Streptavidin was then used to isolate the tagged proteins and mass spectrometry to identify them. Using this approach, Youn et al. identified 42 proteins that localize to processing bodies. [ 21 ] | https://en.wikipedia.org/wiki/P-bodies |
In mathematics , in particular algebraic topology , a p - compact group is a homotopical version of a compact Lie group , but with all the local structure concentrated at a single prime p . This concept was introduced in Dwyer & Wilkerson (1994) , making precise earlier notions of a mod p finite loop space. A p-compact group has many Lie-like properties like maximal tori and Weyl groups , which are defined purely homotopically in terms of the classifying space, but with the important difference that the Weyl group , rather than being a finite reflection group over the integers, is now a finite p -adic reflection group. They admit a classification in terms of root data, which mirrors the classification of compact Lie groups, but with the integers replaced by the p -adic integers .
A p - compact group is a pointed space BG , which is local with respect to mod p homology , and such the pointed loop space G = ΩBG has finite mod p homology. One sometimes also refer to the p -compact group by G , but then one needs to keep in mind that the loop space structure is part of the data (which then allows one to recover BG ).
A p -compact group is said to be connected if G is a connected space (in general the group of components of G will be a finite p-group). The rank of a p -compact group is the rank of its maximal torus.
The classification of p -compact groups from Andersen & Grodal (2009) states that there is a 1-1 correspondence between connected p -compact groups, up to homotopy equivalence, and root data over the p -adic integers , up to isomorphism. This is analogous to the classical classification of connected compact Lie groups, with the p -adic integers replacing the rational integers .
It follows from the classification that any p -compact group can be written as BG = BH × BK where BH is the p -completion of a compact connected Lie group and BK is finite direct product of simple exotic p-compact groups i.e., simple p-compact groups whose Weyl group group is not a Z {\displaystyle \mathbb {Z} } -reflection groups. Simple exotic p-compact groups are again in 1-1-correspondence with irreducible complex reflection groups whose character field can be embedded in Q p {\displaystyle \mathbb {Q} _{p}} , but is not Q {\displaystyle \mathbb {Q} } .
For instance, when p=2 this implies that every connected 2-compact group can be written BG = BH × BDI(4) s , where BH is the 2-completion of the classifying space of a connected compact Lie group, and BDI(4) s denotes s copies of the " Dwyer -Wilkerson 2-compact group" BDI(4) of rank 3, constructed in Dwyer & Wilkerson (1993) with Weyl group corresponding to group number 24 in the Shepard - Todd enumeration of complex reflection groups . For p=3 a similar statement holds but the new exotic 3-compact group is now group number 12 on the Shepard-Todd list, of rank 2. For primes greater than 3, family 2 on the Shepard-Todd list will contain infinitely many exotic p-compact groups.
A finite loop space is a pointed space BG such that the loop space ΩBG is homotopy equivalent to a finite CW-complex. The classification of connected p-compact groups implies a classification of connected finite loop spaces : Given a connected p-compact group for each prime, all with the same rational type, there is an explicit double coset space of possible connected finite loop spaces with p-completion the give p-compact groups. As connected p-compact groups are classified combinatorially, this implies a classification of connected loop spaces as well.
Using the classification, one can identify the compact Lie groups inside finite loop spaces, giving a homotopical characterisation of compact connected Lie groups : They are exactly those finite loop spaces that admit an integral maximal torus; this was the so-called maximal torus conjecture . (See Andersen & Grodal (2009) and Grodal (2010) .)
The classification also implies a classification of which graded polynomial rings can occur as the cohomology ring of a space, the so-called Steenrod problem . (See Andersen & Grodal (2008) .) | https://en.wikipedia.org/wiki/P-compact_group |
The p-i concept refers to the pharmacological interaction of drugs with immune receptors. It explains a form of drug hypersensitivity , namely T cell stimulation, which can lead to various acute inflammatory manifestations such as exanthems , eosinophilia and systemic symptoms , Stevens–Johnson syndrome , toxic epidermal nercrolysis , and complications upon withdrawing the drug.
The p-i concept links pharmacology with immunology : It implies that drugs bind directly, as an off-target activity to immune receptors which results in various forms of T cell stimulations. P-i thus starts with an off-target pharmacological activity of the drug followed by a cascade of immunological events which always starts with T cell activation, even if the drug did not bind to the T cell itself but to an antigen presenting cell (APC).
The drug bindings occur by non-covalent bonds (e.g. Hydrogen bonds , electrostatic interactions , van der Waals forces ) to some of the highly polymorphic T cell receptors for antigen (TCR) and / or human leukocyte antigens (HLA). The binding occurs mostly on the cell surface and is labile, reversible and transient. It interacts with the crucial molecules of antigen dependent T cell activation, which may alter the self-HLA to make it look like an allo-HLA-allele, to which T cells strongly react; Some drug binding to TCR itself may – together with HLA-peptide interaction – elicit TCR-CDR signalling or alter the TCR conformation, thereby enhancing its interaction with HLA-peptide (allogeneic effect). Certain drugs may not only interact with the immune receptors on the surface but also inside the cell ( endoplasmic reticulum e.g. Abacavir to HLA-B*57:01). This may cause a change of presented peptides (altered peptide model).
The polymorphism of the immune receptors explains to a large extent the notoriously unpredictable “ idiosyncrasy ” of drug hypersensitivity reactions (DHR), as some of the individually distinct protein sequences may bind the drug better than others. Thereby only those individuals react to the drug which express the fitting protein sequence, e.g. a certain HLA-allele.
Until now, only αβ TCR and HLA-class I and II proteins were described as target structures for p-i mediated drug hypersensitivity, but it is likely that other immune receptors ( γδ-TCR , HLA-Class Ib, etc.) are also possible off target structures.
Originally, the immune stimulation by drugs were exclusively explained by the hapten concept, which was investigated in the early 1930s: [ 1 ] [ 2 ] [ 3 ] [ 4 ] In these early studies it was found that drugs were too small to represent an antigen inherently. Only if they or their metabolites are haptens, thereby forming larger and stable drug-protein complexes, were they seen as new antigens. Such a formation of a complete antigen (drug-protein complexes, so-called adducts ) were considered a necessary step to stimulate an immune response, since the drug alone remained unnoticed by the immune cells.
One classical clinical model for a hapten reaction is contact dermatitis . This is a skin disease based on a localized immune reaction in the skin to a chemically reactive, topical applied compound, which binds by covalent bonds to a carrier protein; many of the contact sensitizers also have a toxic effect, which may be important for the costimulation of the immune system. Extensive in vitro and in vivo data support the hapten concept in contact dermatitis.
It was tempting to use the hapten model to explain generalized DH as well: The drug class most often involved in generalized DHR are penicillins . When applied to the skin, they elicit contact dermatitis. Penicillins are classical haptens and penicillin modified proteins like albumin have been repeatedly found in patients after therapy. Thus it was assumed that the generalized DHR upon parenteral or oral application after penicillins is also due to the hapten-feature of this drug class.
The “hapten-concept” was soon extended to explain all immune mediated (adverse drug reactions): Only if the drug or a metabolite could act as hapten and bind covalently to proteins, then was the drug considered to be able to elicit immune reactions including generalized exanthema , drug induced hepatitis , DRESS , SJS / TEN etc. Consequently, during preclinical risk assessment of a new drug, the potential drug candidate may be screened carefully for hapten-features, and if the candidate drugs caused some adverse immune mediated reaction, it was linked to their hapten feature or, if the parent compound lacked hapten characteristics, a hapten feature of a drug metabolite as cause for the DHR was postulated. Importantly, the hapten theory as explanation for all generalized immune reactions was often disputed and hard to reconcile with many experimental or clinical findings.
The p-i concept represents an alternative explanation of immune stimulation by drugs in DHR. It implies that no formation of a new antigen (hapten-protein complex) is needed to elicit an immune activation: T cells are stimulated by the drug binding to immune receptors directly, which leads to conformational changes of HLA and/or TCR as well as to signalling by the TCR-CD3 complex : The p-i concept was created and formulated by Prof. emeritus Werner J. Pichler in early 2000, based on studies with drug specific T cell clones, derived from patients with DHR. The underlying investigations were performed by various PhD students in Pichler's research group at the Inselspital / University of Bern in Bern , Switzerland . The essential, initial finding was that T cells from patients with DH showed a specific reaction to the incriminated drug in vitro ( proliferation , cytotoxicity , cytokine release), and that this drug dependent stimulation of immune cells relied on labile (=non-covalent) binding of drugs to cell surface proteins, namely on antigen presenting cells and T cells, which were present in the cell culture.
That a non-covalent, and drug binding was sufficient for T cell stimulation was shown by three main findings underlying the lability of drug bindings: washing the cells in the cell culture (APCs, T cells) effectively removed the drugs, and the cells could not be stimulated any longer; This means that the drug was binding in a labile way, which was possible with the inert, parent compound; Transformation to a reactive metabolite was not needed to elicit a reaction: blocking processing or metabolism by drugs did not interfere with reactivity and even fixing the antigen presenting cells by glutaraldehyde failed to eliminate T cell reactivity; And lastly by the speed of reaction: the reaction occurred within minutes, before metabolism could happen; addition of the drug to the cell culture containing drug reactive T cells resulted in a Ca 2+ influx in drug specific T cells within less than a minute.
Over the years this p-i concept could be confirmed by many functional and structural studies including crystallography which localized the precise region of the immune receptor (HLA-B*57:01, TCRVβ20, etc.), to which a particular drug binds. Since crucial and highly sensitive molecules of T cell activation are targeted, complex and highly variable immunological consequences can develop: different types of T cells are activated to a variable degree, leading in inflammatory consequences with a highly polymorphic clinical picture of acute symptoms, followed by different late appearing complications.
The modification of the self-HLA, of TCR, or of the TCR-peptide-HLA complex by non-covalent, bindings of drugs is a reversible, transient process, whose effect is highly dependent on the affinity of drug-protein interactions. The drug can bind first to the HLA-peptide complex (p-i HLA) or the TCR complex (p-i TCR). Sometimes the drug may be trapped in between TCR and HLA.
P-i HLA is often linked with a striking HLA-association of the DHR (shown for abacavir, carbamazepine, allopurinol, dapson, vancomycin etc.), since the drug binds to a certain HLA-allele with higher affinity than to other HLA-molecules. P-i TCR is less investigated. Drug binding to certain parts of the TCR Vβ chains may be sufficient for full activation, if interaction with HLA-peptide complexes is possible; other p-i TCR bindings may require additional T cell activation (e.g. by viral infection) to lead to clinical symptoms.
The in vitro analysis of p-i using drug specific T cell clones (TCC) or TCR-transfected cell lines generated from patients with DH revealed a strong stimulation: A high level of T cell mediated cytotoxicity , a broad spectrum of secreted cytokines and polyclonality was observed; The p-i stimulation was unorthodox: some CD4+ T cells were uncharacteristically reacting to HLA-class I or CD8+ T cells to HLA-class II drug presentation or did not show strict HLA restriction, and some TCC were polyspecific (the reactive T cell clones reacted with various peptides): [ 5 ] [ 6 ] [ 7 ] Altogether, the picture emerged that p-i induced T cell stimulations have features of allo-like immune stimulations (allo-stimulation).
The two main clinical outcomes of acute p-i reactions are MPE/DRESS on one hand, and SJS/TEN on the other hand. In MPE/DRESS patients, high numbers of circulating, atypical (activated) lymphocytes and high levels of various cytokines can be found in the circulation. [ 8 ] [ 9 ] In vitro drug stimulation reveals a proliferating, high cytokine secreting, cytotoxic CD4 and CD8 T cell reaction to the incriminated drug, which sometimes can be detected for many years.
Quite in contrast, patients with SJS/TEN may show lymphopenia, just the blister fluids are full of mainly CD8/NK+, cytotoxic T cells, which are able to kill keratinocytes. [ 10 ] [ 11 ] [ 12 ] Cytotoxic molecules ( granzyme B , perforin , granulysin ) can be detected in vivo in blood and the blister fluid in the first few days of the disease. [ 12 ] [ 13 ] During the acute disease (1–2 weeks), the T cells still react in in vitro assays, but after 3–4 weeks, the CD8 cell compartment, which is considered to be the main responsible cell population for the disease, appears to be exhausted and are refractory to drug stimulation. [ 14 ] [ 15 ] How this CD8 exhaustion is achieved is unknown.
If the p-i stimulation resulted in T cell expansion and activation (MPE, DRESS), some of the p-i-activated T cells might continue to react in the absence of drug: their TCR may be cross-reactive with unmodified, self HLA presenting exogenous peptides (mainly of herpes virus origin) or some self-peptides: this kind of cross-reactivity with exogenous or self peptides and self-HLA explain two late complications after severe DHR, mostly DRESS: one is viral reactivation: herpes viruses are permanently harboured in various cell types after infection (fibroblasts, endothelial cells, hematopoietic cells, brain cells, etc.) [ 16 ] [ 17 ] [ 18 ] and are controlled by T cells. When these herpes-virus specific T cells are activated by p-i, they react with the herpes virus peptide expressing cells and damage them by their cytotoxic potential: [ 12 ] [ 19 ] a consequence is the release virus particles into the circulation [ 16 ] [ 17 ] [ 18 ] and symptoms of viral reactivation (high virus load, possibly increase of liver enzymes and of activated lymphocytes) appear.
A second wave of peptide reactivity may end up in autoimmunity: the abnormal stimulation by p-i includes T cells from the naïve and memory T cell pool. It may include self-peptide reactive T cells, which, if the corresponding self-peptides are presented and are encountered, release cytokines and exert cytotoxicity - autoimmunity may arise. [ 20 ] [ 21 ] As such self-peptide reactive T cells are present in relatively low amounts, they need >6–8 weeks to expand and appear after the virus-reactivations. Autoimmunity occurs in a minority of patients (<20%), and may is often manifested as autoimmune poly-endocrine syndrome. [ 20 ] [ 21 ] [ 22 ]
Multiple drug hypersensitivity (MDH): a further consequence of p-i stimulations like DRESS or severe MPE is MDH; such patients develop an additional DHR to a structurally different drug, with the same or different clinical manifestations. [ 23 ] MDH occurs in ca. 20% of patients with DRESS, and can occur any time, from the start of DRESS (often to a combination therapy), during the initial activation, and can even appear years after the first DHR [ 23 ] [ 24 ]
A main difficulty of DHR research and weak point of the p-i concept is the fact that it is cumbersome to demonstrate a p-i reactivity – namely that the T cell stimulation occurred due to non-covalent drug binding to immune receptors. In principle, a T cell mediated DHR was explained by p-i if the drug binding to the immune receptors was found to be labile.
To demonstrate the lability of drug binding, drug specific T cell lines, T cell clones and TCR hybridoma cells were required. [ 25 ] [ 26 ] [ 27 ] Washing of the cell mix of drug, APC and T cells abrogated p-i reactivity, while T cell reaction to haptens persisted. The presentation of peptides by HLA on APC takes > 4hr pulsing (uptake of hapten modified protein, processing and presentation), and may also require metabolism, if the stimulating drug was chemically inert and not a hapten. Therefore, an immediate reactivity of T cells (e.g. measured by rapid Ca 2+ influx) as well as reactivity to the drug in the presence of protein and metabolism inhibitors or by using glutaraldehyde-fixed antigen presenting cells was interpreted as p-i reaction. [ 25 ] [ 26 ] [ 27 ] P-i reactivity was demonstrated for a number of drugs ( SMX , lidocain , lamotrigine , carbamazepine , various radio contrast media , quinolones , vancomycin , dapsone , etc.), including some drugs which can act via p-i or as hapten ( piperacillin , flucloxacillin , amoxicillin , cephalosporins , monobactams ). In these cases the p-i reactivity was responsible for more severe reactions like hepatitis in flucloxacillin/B*57:01 carriers or DRESS with amoxicillin and piperacillin [ 27 ]
DRESS/SJS/TEN (severe DHR) and HLA-linked DHR are p-i. Initially it was thought that p-i reactions were the exception while the hapten mechanism represented the main cause of systemic T cell mediated DHR. Conversely it seems to be the opposite as p-i appears to be the main mechanism in T cell mediated DHR: Whenever the mechanism was investigated how drugs cause severe DHR (DRESS, SJS/TEN), it was always found to be due to p-i. [ 25 ] [ 26 ]
It is unclear whether the majority of severe MPE is due to p-i. As the in vitro analysis of amoxicillin induced MPE (analysis of >150 amoxicillin induced MPE) regularly reveals high secretions of IL-5, IL-13, IFNg, granzyme B and granulysin upon drug exposure (very similar to DRESS cases), the in vitro drug stimulation in MPE is often strong and includes the secretion of Th1, Th2, and cytotoxic cytokines simultaneously. It is actually often stronger and broader than the cytokine secretion upon tetanus control, and is reminiscent of an in vitro mixed leukocyte reaction (MLR). [ 27 ] Further work is needed, but most MPE appear to be mediated by p-i.
Importantly, all drugs which develop DHR and have a strong HLA-linkage (e.g. allopurinol/oxypurinol and B*58:01) stimulate via p-i. Protein reactions are not HLA restricted: A protein is large and is processed into various small peptides. These peptides (including the hapten-modified peptides) fit into different HLA alleles and not in only one HLA like observed with drugs. Thus, the DHR-HLA linkage seen with certain drugs can only be explained by direct drug binding to an allele-typic region of the HLA-molecule. This was also confirmed in structural and computational studies. Importantly, if a drug can be stimulatory by p-i or by hapten mechanism (SMX/SMX-NO, beta-lactams), the severe T cell mediated symptoms are mediated by p-i and are HLA-allele restricted, [ 28 ] while the hapten-reactions are not. Thus, the list of drugs acting via p-i and causing DHR is now longer than the list of hapten-like drugs (table).
First hapten, then non covalent drug binding: A strong argument for a hapten-mechanism underlying DHR was the ability of the drug to cause all, namely IgG, IgE and T cell mediated DHR, since these different immune reactions required the immunogenic presentation of the drug in various ways. This is best achieved using hapten (and thus antigenic) features of a drug. Indeed, the classical hapten-drugs beta-lactam antibiotics, SMX-NO, or PPI are able to induce all forms of Gell and Coombs immune stimulations, while e.g. classical and exclusive p-i drugs like carbamazepine or abacavir induce only T cell reactions, but never anaphylaxis. [ 29 ] Importantly, an ability to act as hapten does not rule out that non-covalent binding like in p-i plays a role in DHR. Actually, during a DHR the type of drug-protein binding may change: A drug may act as hapten in the induction phase causing asymptomatic immunity, but the effector mechanism of immunoglobulin-reactions and some severe T-cell mediated DHR may actually be due to non-covalent drug bindings. Thus, beta-lactam antibiotics – the classical hapten-drugs - are the main elicitor for "fake antigen" reactions, drug induced immune thrombocytopenia (DITP) and p-i stimulations, which are all based on non-covalent drug-protein interactions and are not antigen induced.
The polyclonal T cells response stemming from the memory T cell pool includes T cells which are primed by prior immune responses. An important role play herpes viruses (HHV6, CMV, EBV, Herpes simplex I), where a relatively large amount of T cells are involved in the control of these herpes viruses. Indeed, herpes virus reactivation is so common, that it is part of the Japanese definition of DRESS. [ 30 ] Since the precursor frequency of such herpes virus specific T cells is high (up to 10% of the CD8+ T cells in the elderly can be devoted to herpes virus control, [ 30 ] [ 31 ] symptoms due to such T cells appear already after ca. 2–6 weeks. | https://en.wikipedia.org/wiki/P-i_mechanism |
p-nuclei ( p stands for proton -rich) are certain proton-rich, naturally occurring isotopes of some elements between selenium and mercury inclusive which cannot be produced in either the s- or the r-process .
The classical, ground-breaking works of Burbidge, Burbidge, Fowler and Hoyle (1957) [ 1 ] and of A. G. W. Cameron (1957) [ 2 ] showed how the majority of naturally occurring nuclides beyond the element iron can be made in two kinds of neutron capture processes, the s- and the r-process. Some proton-rich nuclides found in nature are not reached in these processes and therefore at least one additional process is required to synthesize them. These nuclei are called p-nuclei .
Since the definition of the p-nuclei depends on the current knowledge of the s- and r-process (see also nucleosynthesis ), the original list of 35 p-nuclei may be modified over the years, as indicated in the Table below.
For example, it is recognized today that the abundances of 152 Gd and 164 Er contain at least strong contributions from the s-process . [ 3 ] This also seems to apply to those of 113 In and 115 Sn, which additionally could be made in the r-process in small amounts. [ 4 ]
The long-lived radionuclides 92 Nb, 97 Tc, 98 Tc, 146 Sm, 150 Gd, and 154 Dy [ 5 ] are not among the classically defined p-nuclei as they no longer occur naturally on Earth. By the above definition, however, they are also p-nuclei because they cannot be made in either the s- or the r-process. From the discovery of their decay products in presolar grains it can be inferred that at least 92 Nb and 146 Sm were present in the solar nebula . This offers the possibility to estimate the time since the last production of these p-nuclei before the formation of the Solar System . [ 6 ]
p-nuclei are very rare. Those isotopes of an element which are p-nuclei are less abundant typically by factors of ten to one thousand than the other isotopes of the same element. The abundances of p-nuclei can only be determined in geochemical investigations and by analysis of meteoritic material and presolar grains . They cannot be identified in stellar spectra . Therefore, the knowledge of p-abundances is restricted to those of the Solar System and it is unknown whether the solar abundances of p-nuclei are typical for the Milky Way . [ 7 ]
The astrophysical production of p-nuclei is not completely understood yet. The favored γ-process (see below) in core-collapse supernovae cannot produce
all p-nuclei in sufficient amounts, according to current computer simulations . This is why additional production mechanisms and astrophysical sites are under investigation, as outlined below. It is also conceivable that there is not just a single process responsible for all p-nuclei but that different processes in a number of astrophysical sites produce certain ranges of p-nuclei. [ 13 ]
In the search for the relevant processes creating p-nuclei, the usual way is to identify the possible production mechanisms (processes) and then to investigate their possible realization in various astrophysical sites. The same logic is applied in the discussion below.
In principle, there are two ways to produce proton-rich nuclides : by successively adding protons to a nuclide (these are nuclear reactions of type (p,γ)) or by removing neutrons from a nucleus through sequences of photodisintegrations of type (γ,n). [ 7 ] [ 13 ]
Under conditions encountered in astrophysical environments it is difficult to obtain p-nuclei through proton captures because the Coulomb barrier of a nucleus increases with increasing proton number . A proton requires more energy to be incorporated ( captured ) into an atomic nucleus when the Coulomb barrier is higher. The available average energy of the protons is determined by the temperature of the stellar plasma . Increasing the temperature, however, also speeds up the (γ,p) photodisintegrations which counteract the (p,γ) captures. The only alternative avoiding this would be to have a very large number of protons available so that the effective number of captures per second is large even at low temperature. In extreme cases (as discussed below) this leads to the synthesis of extremely short-lived radionuclides which decay to stable nuclides only after the captures cease. [ 7 ] [ 13 ]
Appropriate combinations of temperature and proton density of a stellar plasma have to be explored in the search of possible production mechanisms for p-nuclei. Further parameters are the time available for the nuclear processes, and number and type of initially present nuclides ( seed nuclei ).
In a p-process it is suggested that p-nuclei were made through a few proton captures on stable nuclides. The seed nuclei originate from the s- and r-process and are already present in the stellar plasma. As outlined above, there are serious difficulties explaining all p-nuclei through such a process although it was originally suggested to achieve exactly this. [ 1 ] [ 2 ] [ 7 ] It was shown later that the required conditions are not reached in stars or stellar explosions. [ 14 ]
Based on its historical meaning, the term p-process is sometimes used for any process synthesizing p-nuclei, even when no proton captures are involved, but this usage is discouraged.
p-nuclei can also be obtained by photodisintegration of s -process and r -process nuclei. At temperatures around 2–3 gigakelvins (GK) and short process time of a few seconds (this requires an explosive process) photodisintegration of the pre-existing nuclei will remain small, just enough to produce the required tiny abundances of p-nuclei. [ 7 ] [ 15 ] This is called the γ-process (gamma process) because the photodisintegration proceeds by nuclear reactions of the types (γ,n), (γ,α) and (γ,p), which are caused by highly energetic photons ( gamma rays ). [ 15 ]
If a sufficiently intensive source of neutrinos is available, nuclear reactions can directly produce certain nuclides, for example 7 Li, 11 B, 19 F, 138 La in core-collapse supernovae . [ 16 ]
In a p-process protons are added to stable or weakly radioactive atomic nuclei .
If there is a high proton density in the stellar plasma, even short-lived radionuclides can capture one or more protons before they beta decay . This quickly moves the nucleosynthesis path from the region of stable nuclei to the very proton-rich side of the chart of nuclides . This is called rapid proton capture . [ 13 ]
Here, a series of (p,γ) reactions proceeds until either the beta decay of a nucleus is faster than a further proton capture, or the proton drip line is reached. Both cases lead to one or several sequential beta decays until a nucleus is produced which again can capture protons before it beta decays. Then the proton capture sequences continue.
It is possible to cover the region of the lightest nuclei up to 56 Ni within a second because both proton captures and beta decays are fast. Starting with 56 Ni, however, a number of waiting points are encountered in the reaction path. These are nuclides which both have relatively long half-lives (compared to the process timescale) and can only slowly add another proton (that is, their cross section for (p,γ) reactions is small). Examples for such waiting points are: 56 Ni, 60 Zn, 64 Ge, 68 Se. Further waiting points may be important, depending on the detailed conditions and location of the reaction path. It is typical for such waiting points to show half-lives of minutes to days. Thus, they considerably increase the time required to continue the reaction sequences. If the conditions required for this rapid proton capture are only present for a short time (the timescale of explosive astrophysical events is of the order of seconds), the waiting points limit or hamper the continuation of the reactions to heavier nuclei. [ 17 ]
In order to produce p-nuclei, the process path has to encompass nuclides bearing the same mass number (but usually containing more protons) as the desired p-nuclei. These nuclides are then converted into p-nuclei through sequences of beta decays after the rapid proton captures ceased.
Variations of the main category rapid proton captures are the rp-, pn-, and νp-processes, which will be briefly outlined below.
The so-called rp-process ( rp is for rapid proton capture ) is the purest form of the rapid proton capture process described above. At proton densities of more than 10 28 protons/cm 3 and temperatures around 2 × 10 9 K , the reaction path is close to the proton drip line . [ 17 ] The waiting points can be bridged provided that the process time is 10–600 s. Waiting-point nuclides are produced with larger abundances while the production of nuclei "behind" each waiting point is increasingly suppressed.
A definitive endpoint is reached close to 104 Te because the reaction path runs into a region of nuclides which decay preferably by alpha decay and thus loop the path back onto itself. [ 18 ] Therefore, an rp-process would only be able to produce p-nuclei with mass numbers less than or equal to 104.
The waiting points in rapid proton capture processes can be avoided by (n,p) reactions which are much faster than proton captures on or beta decays of waiting points nuclei. This results in a considerable reduction of the time required to build heavy elements and allows an efficient production within seconds. [ 7 ] This requires, however, a (small) supply of free neutrons which are usually not present in such proton-rich plasmas. One way to obtain them is to release them through other reactions occurring simultaneously as the rapid proton captures. This is called neutron-rich rapid proton capture or pn-process . [ 19 ]
Another possibility to obtain the neutrons required for the accelerating (n,p) reactions in proton-rich environments is to use the anti-neutrino capture on protons ( ν e + p → e + + n ), turning a proton and an anti-neutrino into a positron and a neutron. Since (anti-)neutrinos interact only very weakly with protons, a high flux of anti-neutrinos has to act on a plasma with high proton density. This is called νp -process (nu p process). [ 20 ]
Massive stars end their life in a core-collapse supernova . In such a supernova, a shockfront from an explosion runs from the center of the star through its outer layers and ejects these. When the shockfront reaches the O/Ne-shell of the star (see also stellar evolution ), the conditions for a 𝛾-process are reached for 1-2 s.
Although the majority of p-nuclei can be made in this way, some mass regions of p-nuclei turn out to be problematic in model calculations. It has been known already for decades that p-nuclei with mass numbers A < 100 cannot be produced in a 𝛾-process. [ 7 ] [ 15 ] Modern simulations also show problems in the range 150 ≤ A ≤ 165 . [ 13 ] [ 21 ]
The p-nucleus 138 La is not produced in the 𝛾-process but it can be made in a ν -process. A hot neutron star is made in the center of such a core-collapse supernova and it radiates neutrinos with high intensity. The neutrinos interact also with the outer layers of the exploding star and cause nuclear reactions which create 138 La, among other nuclei. [ 16 ] [ 21 ] Also 180m Ta may receive a contribution from this ν -process.
It was suggested [ 20 ] to supplement the γ-process in the outer layers of the star by another process, occurring in the deepest layers of the star, close to the neutron star but still being ejected instead of falling onto the neutron star surface. Due to the initially high flow of neutrinos from the forming neutron star, these layers become extremely proton-rich through the reaction ν e + n → e − + p . Although the anti-neutrino flux is initially weaker a few neutrons will be created, nevertheless, because of the large number of protons. This allows a ν p {\displaystyle \nu \mathrm {p} } -process in these deep layers. Because of the short timescale of the explosion and the high Coulomb barrier of the heavier nuclei, such a νp-process could possibly only produce the lightest p-nuclei. Which nuclei are made and how much of them depends sensitively on many details in the simulations and also on the actual explosion mechanism of a core-collapse supernova, which still is not completely understood. [ 20 ] [ 22 ]
A thermonuclear supernova is the explosion of a white dwarf in a binary star system, triggered by thermonuclear reactions in matter from a companion star accreted on the surface of the white dwarf. The accreted matter is rich in hydrogen (protons) and helium ( α particles ) and becomes hot enough to allow nuclear reactions .
A number of models for such explosions are discussed in literature, of which two were explored regarding the prospect of producing p-nuclei. None of these explosions release neutrinos, therefore rendering ν- and νp-process impossible. Conditions required for the rp-process are also not attained.
Details of the possible production of p-nuclei in such supernovae depend sensitively on the composition of the matter accreted from the companion star (the seed nuclei for all subsequent processes). Since this can change considerably from star to star, all statements and models of p-production in thermonuclear supernovae are prone to large uncertainties. [ 7 ]
The consensus model of thermonuclear supernovae postulates that the white dwarf explodes after exceeding the Chandrasekhar limit by the accretion of matter because the contraction and heating ignites explosive carbon burning under degenerate conditions. A nuclear burning front runs through the white dwarf from the inside out and tears it apart. Then the outermost layers closely beneath the surface of the white dwarf (containing 0.05 solar masses of matter) exhibit the right conditions for a γ-process. [ 23 ]
The p-nuclei are made in the same way as in the γ-process in core-collapse supernovae and also the same difficulties are encountered. In addition, 138 La and 180m Ta are not produced. A variation of the seed abundances by assuming increased s-process abundances only scales the abundances of the resulting p-nuclei without curing the problems of relative underproduction in the nuclear mass ranges given above. [ 7 ]
In a subclass of type Ia supernovae , the so-called subChandrasekhar supernova , the white dwarf may explode long before it reaches the Chandrasekhar limit because nuclear reactions in the accreted matter can already heat the white dwarf during its accretion phase and trigger explosive carbon burning prematurely. Helium-rich accretion favors this type of explosion. Helium burning ignites degeneratively on the bottom of the accreted helium layer and causes two shockfronts. The one running inwards ignites the carbon explosion. The outwards moving front heats the outer layers of the white dwarf and ejects them. Again, these outer layers are site to a γ-process at temperatures of 2-3 GK. Due to the presence of α particles (helium nuclei), however, additional nuclear reactions become possible. Among those are such which release a large number of neutrons, such as 18 O(α,n) 21 Ne, 22 Ne(α,n) 25 Mg, and 26 Mg(α,n) 29 Si. This allows a pn-process in that part of the outer layers which experiences temperatures above 3 GK. [ 7 ] [ 19 ]
Those light p-nuclei which are underproduced in the γ-process can be so efficiently made in the pn-process that they even show much larger abundances than the other p-nuclei. To obtain the observed solar relative abundances, a strongly enhanced s-process seed (by factors of 100-1000 or more) has to be assumed which increases the yield of heavy p-nuclei from the γ-process. [ 7 ] [ 19 ]
A neutron star in a binary star system can also accrete matter from the companion star on its surface. Combined hydrogen and helium burning ignites when the accreted layer of degenerate matter reaches a density of 10 5 – 10 6 g/cm 3 and a temperature exceeding 0.2 GK . This leads to thermonuclear burning comparable to what happens in the outwards moving shockfront of subChandrasekhar supernovae. The neutron star itself is not affected by the explosion and therefore the nuclear reactions in the accreted layer can proceed longer than in an explosion. This allows to establish an rp-process. It will continue until either all free protons are used up or the burning layer has expanded due to the increase in temperature and its density falls below the one required for the nuclear reactions. [ 17 ]
It was shown that the properties of X-ray bursts in the Milky Way can be explained by an rp-process on the surface of accreting neutron stars. [ 24 ] It remains unclear, yet, whether matter (and if, how much matter) can be ejected and escape the gravitational field of the neutron star. Only if this is the case can such objects be considered as possible sources of p-nuclei. Even if this is corroborated, the demonstrated endpoint of the rp-process limits the production to the light p-nuclei (which are underproduced in core-collapse supernovae). [ 18 ] | https://en.wikipedia.org/wiki/P-nuclei |
The term p-process ( p for proton ) is used in two ways in the scientific literature concerning the astrophysical origin of the elements ( nucleosynthesis ). Originally it referred to a proton capture process which was proposed to be the source of certain, naturally occurring, neutron-deficient isotopes of the elements from selenium to mercury . [ 1 ] [ 2 ] These nuclides are called p-nuclei and their origin is still not completely understood. Although it was shown that the originally suggested process cannot produce the p-nuclei, later on the term p-process was sometimes used to generally refer to any nucleosynthesis process supposed to be responsible for the p-nuclei. [ 3 ]
Often, the two meanings are confused. Recent scientific literature therefore suggests to use the term p-process only for the actual proton capture process, as it is customary with other nucleosynthesis processes in astrophysics. [ 4 ]
Proton-rich nuclides can be produced by sequentially adding one or more protons to an atomic nucleus . Such a nuclear reaction of type (p,γ) is called proton capture reaction . By adding a proton to a nucleus, the element is changed because the chemical element is defined by the proton number of a nucleus. At the same time the ratio of protons to neutrons is changed, resulting in a more neutron-deficient isotope of the next element. This led to the original idea for the production of p-nuclei: free protons (the nuclei of hydrogen atoms are present in stellar plasmas ) should be captured on heavy nuclei ( seed nuclei ) also already present in the stellar plasma (previously produced in the s -process and/or r -process ). [ 1 ] [ 2 ]
Such proton captures on stable nuclides (or nearly stable), however, are not very efficient in producing p-nuclei, especially the heavier ones, because the electric charge increases with each added proton, leading to an increased repulsion of the next proton to be added, according to Coulomb's law . In the context of nuclear reactions this is called a Coulomb barrier . The higher the Coulomb barrier, the more kinetic energy a proton requires to get close to a nucleus and be captured by it. The average energy of the available protons is given by the temperature of the stellar plasma. Even if this temperature could be increased arbitrarily (which is not the case in stellar environments), protons would be removed faster from a nucleus by photodisintegration than they could be captured at high temperature. A possible alternative would be to have a very large number of protons available to increase the effective number of proton captures per second without having to raise the temperature too much. Such conditions, however, are not found in core-collapse supernovae which were supposed to be the site of the p-process. [ 3 ] [ 4 ]
Proton captures at extremely high proton densities are called rapid proton capture processes . They are distinct from the p-process not only by the required high proton density but also by the fact that very short-lived radionuclides are involved and the reaction path is located close to the proton drip line . Rapid proton capture processes are the rp-process , the νp-process , and the pn-process .
The term p-process was originally proposed in the famous B 2 FH paper in 1957. The authors assumed that this process was solely responsible for the p-nuclei and proposed that it occurs in the hydrogen-shell (see also stellar evolution ) of a star exploding as a type II supernova . [ 1 ] It was shown later that the required conditions are not found in such supernovae. [ 5 ]
At the same time as B 2 FH, Alastair Cameron independently realized the necessity to add another nucleosynthesis process to neutron capture nucleosynthesis but simply mentioned proton captures without assigning a special name to the process. He also thought about alternatives, for example photodisintegration (called the γ-process today) or a combination of p-process and photodisintegration. [ 2 ] | https://en.wikipedia.org/wiki/P-process |
In mathematics a P-recursive equation is a linear equation of sequences where the coefficient sequences can be represented as polynomials . P-recursive equations are linear recurrence equations (or linear recurrence relations or linear difference equations) with polynomial coefficients. These equations play an important role in different areas of mathematics, specifically in combinatorics . The sequences which are solutions of these equations are called holonomic , P-recursive or D-finite.
From the late 1980s, the first algorithms were developed to find solutions for these equations. Sergei A. Abramov, Marko Petkovšek and Mark van Hoeij described algorithms to find polynomial, rational, hypergeometric and d'Alembertian solutions.
Let K {\textstyle \mathbb {K} } be a field of characteristic zero (for example K = Q {\textstyle \mathbb {K} =\mathbb {Q} } ), p k ( n ) ∈ K [ n ] {\textstyle p_{k}(n)\in \mathbb {K} [n]} polynomials for k = 0 , … , r {\textstyle k=0,\dots ,r} , f ∈ K N {\textstyle f\in \mathbb {K} ^{\mathbb {N} }} a sequence and y ∈ K N {\textstyle y\in \mathbb {K} ^{\mathbb {N} }} an unknown sequence. The equation ∑ k = 0 r p k ( n ) y ( n + k ) = f ( n ) {\displaystyle \sum _{k=0}^{r}p_{k}(n)\,y(n+k)=f(n)} is called a linear recurrence equation with polynomial coefficients (all recurrence equations in this article are of this form). If p 0 {\textstyle p_{0}} and p r {\textstyle p_{r}} are both nonzero, then r {\textstyle r} is called the order of the equation. If f {\textstyle f} is zero the equation is called homogeneous, otherwise it is called inhomogeneous.
This can also be written as L y = f {\textstyle Ly=f} where L = ∑ k = 0 r p k N k {\textstyle L=\sum _{k=0}^{r}p_{k}N^{k}} is a linear recurrence operator with polynomial coefficients and N {\textstyle N} is the shift operator, i.e. N y ( n ) = y ( n + 1 ) {\textstyle N\,y(n)=y(n+1)} .
Let ∑ k = 0 r p k ( n ) y ( n + k ) = f ( n ) {\textstyle \sum _{k=0}^{r}p_{k}(n)\,y(n+k)=f(n)} or equivalently L y = f {\textstyle Ly=f} be a recurrence equation with polynomial coefficients. There exist several algorithms which compute solutions of this equation. These algorithms can compute polynomial, rational, hypergeometric and d'Alembertian solutions. The solution of a homogeneous equation is given by the kernel of the linear recurrence operator: ker L = { y ∈ K N : L y = 0 } {\textstyle \ker L=\{y\in \mathbb {K} ^{\mathbb {N} }\,:\,Ly=0\}} . As a subspace of the space of sequences this kernel has a basis . [ 1 ] Let { y ( 1 ) , y ( 2 ) , … , y ( m ) } {\textstyle \{y^{(1)},y^{(2)},\dots ,y^{(m)}\}} be a basis of ker L {\textstyle \ker L} , then the formal sum c 1 y ( 1 ) + ⋯ + c m y ( m ) {\textstyle c_{1}y^{(1)}+\dots +c_{m}y^{(m)}} for arbitrary constants c 1 , … , c m ∈ K {\textstyle c_{1},\dots ,c_{m}\in \mathbb {K} } is called the general solution of the homogeneous problem L y = 0 {\textstyle Ly=0} . If y ~ {\textstyle {\tilde {y}}} is a particular solution of L y = f {\textstyle Ly=f} , i.e. L y ~ = f {\textstyle L{\tilde {y}}=f} , then c 1 y ( 1 ) + ⋯ + c m y ( m ) + y ~ {\textstyle c_{1}y^{(1)}+\dots +c_{m}y^{(m)}+{\tilde {y}}} is also a solution of the inhomogeneous problem and it is called the general solution of the inhomogeneous problem.
In the late 1980s Sergei A. Abramov described an algorithm which finds the general polynomial solution of a recurrence equation, i.e. y ( n ) ∈ K [ n ] {\textstyle y(n)\in \mathbb {K} [n]} , with a polynomial right-hand side f ( n ) ∈ K [ n ] {\textstyle f(n)\in \mathbb {K} [n]} . He (and a few years later Marko Petkovšek ) gave a degree bound for polynomial solutions. This way the problem can simply be solved by considering a system of linear equations . [ 2 ] [ 3 ] [ 4 ] In 1995 Abramov, Bronstein and Petkovšek showed that the polynomial case can be solved more efficiently by considering power series solution of the recurrence equation in a specific power basis (i.e. not the ordinary basis ( x n ) n ∈ N {\textstyle (x^{n})_{n\in \mathbb {N} }} ). [ 5 ]
The other algorithms for finding more general solutions (e.g. rational or hypergeometric solutions) also rely on algorithms which compute polynomial solutions.
In 1989 Sergei A. Abramov showed that a general rational solution, i.e. y ( n ) ∈ K ( n ) {\textstyle y(n)\in \mathbb {K} (n)} , with polynomial right-hand side f ( n ) ∈ K [ n ] {\textstyle f(n)\in \mathbb {K} [n]} , can be found by using the notion of a universal denominator. A universal denominator is a polynomial u {\textstyle u} such that the denominator of every rational solution divides u {\textstyle u} . Abramov showed how this universal denominator can be computed by only using the first and the last coefficient polynomial p 0 {\textstyle p_{0}} and p r {\textstyle p_{r}} . Substituting this universal denominator for the unknown denominator of y {\displaystyle y} all rational solutions can be found by computing all polynomial solutions of a transformed equation. [ 6 ]
A sequence y ( n ) {\textstyle y(n)} is called hypergeometric if the ratio of two consecutive terms is a rational function in n {\displaystyle n} , i.e. y ( n + 1 ) / y ( n ) ∈ K ( n ) {\textstyle y(n+1)/y(n)\in \mathbb {K} (n)} . This is the case if and only if the sequence is the solution of a first-order recurrence equation with polynomial coefficients. The set of hypergeometric sequences is not a subspace of the space of sequences as it is not closed under addition.
In 1992 Marko Petkovšek gave an algorithm to get the general hypergeometric solution of a recurrence equation where the right-hand side f {\displaystyle f} is the sum of hypergeometric sequences. The algorithm makes use of the Gosper-Petkovšek normal-form of a rational function. With this specific representation it is again sufficient to consider polynomial solutions of a transformed equation. [ 3 ]
A different and more efficient approach is due to Mark van Hoeij. Considering the roots of the first and the last coefficient polynomial p 0 {\textstyle p_{0}} and p r {\textstyle p_{r}} – called singularities – one can build a solution step by step making use of the fact that every hypergeometric sequence y ( n ) {\textstyle y(n)} has a representation of the form y ( n ) = c r ( n ) z n Γ ( n − ξ 1 ) e 1 Γ ( n − ξ 2 ) e 2 ⋯ Γ ( n − ξ s ) e s {\displaystyle y(n)=c\,r(n)\,z^{n}\,\Gamma (n-\xi _{1})^{e_{1}}\Gamma (n-\xi _{2})^{e_{2}}\cdots \Gamma (n-\xi _{s})^{e_{s}}} for some c ∈ K , z ∈ K ¯ , s ∈ N , r ( n ) ∈ K ¯ ( n ) , ξ 1 , … , ξ s ∈ K ¯ {\textstyle c\in \mathbb {K} ,z\in {\overline {\mathbb {K} }},s\in \mathbb {N} ,r(n)\in {\overline {\mathbb {K} }}(n),\xi _{1},\dots ,\xi _{s}\in {\overline {\mathbb {K} }}} with ξ i − ξ j ∉ Z {\textstyle \xi _{i}-\xi _{j}\notin \mathbb {Z} } for i ≠ j {\textstyle i\neq j} and e 1 , … , e s ∈ Z {\textstyle e_{1},\dots ,e_{s}\in \mathbb {Z} } . Here Γ ( n ) {\textstyle \Gamma (n)} denotes the Gamma function and K ¯ {\textstyle {\overline {\mathbb {K} }}} the algebraic closure of the field K {\textstyle \mathbb {K} } . Then the ξ 1 , … , ξ s {\textstyle \xi _{1},\dots ,\xi _{s}} have to be singularities of the equation (i.e. roots of p 0 {\textstyle p_{0}} or p r {\textstyle p_{r}} ). Furthermore one can compute bounds for the exponents e i {\textstyle e_{i}} . For fixed values ξ 1 , … , ξ s , e 1 , … , e s {\textstyle \xi _{1},\dots ,\xi _{s},e_{1},\dots ,e_{s}} it is possible to make an ansatz which gives candidates for z {\textstyle z} . For a specific z {\textstyle z} one can again make an ansatz to get the rational function r ( n ) {\textstyle r(n)} by Abramov's algorithm. Considering all possibilities one gets the general solution of the recurrence equation. [ 7 ] [ 8 ]
A sequence y {\displaystyle y} is called d'Alembertian if y = h 1 ∑ h 2 ∑ ⋯ ∑ h k {\textstyle y=h_{1}\sum h_{2}\sum \cdots \sum h_{k}} for some hypergeometric sequences h 1 , … , h k {\textstyle h_{1},\dots ,h_{k}} and y = ∑ x {\textstyle y=\sum x} means that Δ y = x {\textstyle \Delta y=x} where Δ {\textstyle \Delta } denotes the difference operator, i.e. Δ y = N y − y = y ( n + 1 ) − y ( n ) {\textstyle \Delta y=Ny-y=y(n+1)-y(n)} . This is the case if and only if there are first-order linear recurrence operators L 1 , … , L k {\textstyle L_{1},\dots ,L_{k}} with rational coefficients such that L k ⋯ L 1 y = 0 {\textstyle L_{k}\cdots L_{1}y=0} . [ 4 ]
1994 Abramov and Petkovšek described an algorithm which computes the general d'Alembertian solution of a recurrence equation. This algorithm computes hypergeometric solutions and reduces the order of the recurrence equation recursively. [ 9 ]
The number of signed permutation matrices of size n × n {\displaystyle n\times n} can be described by the sequence y ( n ) ∈ Q N {\textstyle y(n)\in \mathbb {Q} ^{\mathbb {N} }} . A signed permutation matrix is a square matrix which has exactly one nonzero entry in every row and in every column. The nonzero entries can be ± 1 {\textstyle \pm 1} . The sequence is determined by the linear recurrence equation with polynomial coefficients y ( n ) = 4 ( n − 1 ) 2 y ( n − 2 ) + 2 y ( n − 1 ) {\displaystyle y(n)=4(n-1)^{2}\,y(n-2)+2\,y(n-1)} and the initial values y ( 0 ) = 1 , y ( 1 ) = 2 {\textstyle y(0)=1,y(1)=2} . Applying an algorithm to find hypergeometric solutions one can find the general hypergeometric solution y ( n ) = c 2 n n ! {\displaystyle y(n)=c\,2^{n}n!} for some constant c {\textstyle c} . Also considering the initial values, the sequence y ( n ) = 2 n n ! {\textstyle y(n)=2^{n}n!} describes the number of signed permutation matrices. [ 10 ]
The number of involutions y ( n ) {\textstyle y(n)} of a set with n {\textstyle n} elements is given by the recurrence equation y ( n ) = ( n − 1 ) y ( n − 2 ) + y ( n − 1 ) . {\displaystyle y(n)=(n-1)\,y(n-2)+y(n-1).} Applying for example Petkovšek's algorithm it is possible to see that there is no polynomial, rational or hypergeometric solution for this recurrence equation. [ 4 ]
A function F ( n , k ) {\textstyle F(n,k)} is called hypergeometric if F ( n , k + 1 ) / F ( n , k ) , F ( n + 1 , k ) / F ( n , k ) ∈ K ( n , k ) {\textstyle F(n,k+1)/F(n,k),F(n+1,k)/F(n,k)\in \mathbb {K} (n,k)} where K ( n , k ) {\textstyle \mathbb {K} (n,k)} denotes the rational functions in n {\textstyle n} and k {\textstyle k} . A hypergeometric sum is a finite sum of the form f ( n ) = ∑ k F ( n , k ) {\textstyle f(n)=\sum _{k}F(n,k)} where F ( n , k ) {\textstyle F(n,k)} is hypergeometric. Zeilberger 's creative telescoping algorithm can transform such a hypergeometric sum into a recurrence equation with polynomial coefficients. This equation can then be solved to get for example a linear combination of hypergeometric solutions which is called a closed form solution of f {\textstyle f} . [ 4 ] | https://en.wikipedia.org/wiki/P-recursive_equation |
The P-type ATPases , also known as E 1 -E 2 ATPases , are a large group of evolutionarily related ion and lipid pumps that are found in bacteria , archaea , and eukaryotes . [ 1 ] P-type ATPases are α-helical bundle primary transporters named based upon their ability to catalyze auto- (or self-) phosphorylation (hence P) of a key conserved aspartate residue within the pump and their energy source, adenosine triphosphate (ATP). In addition, they all appear to interconvert between at least two different conformations, denoted by E 1 and E 2 . [ 2 ] P-type ATPases fall under the P-type ATPase (P-ATPase) Superfamily ( TC# 3.A.3 ) which, as of early 2016, includes 20 different protein families.
Most members of this transporter superfamily catalyze cation uptake and/or efflux, however one subfamily, the flippases , ( TC# 3.A.3.8 ) is involved in flipping phospholipids to maintain the asymmetric nature of the biomembrane .
In humans, P-type ATPases serve as a basis for nerve impulses , relaxation of muscles, secretion and absorption in the kidney , absorption of nutrient in the intestine and other physiological processes. Prominent examples of P-type ATPases are the sodium-potassium pump (Na + /K + -ATPase), the proton-potassium pump (H + /K + -ATPase), the calcium pump (Ca 2+ -ATPase) and the plasma membrane proton pump (H + -ATPase) of plants and fungi.
The generalized reaction for P-type ATPases is
nLigand 1 (out) + mLigand 2 (in) + ATP → nLigand 1 (in) + mLigand 2 (out) + ADP + P i .
where the ligand can be either a metal ion or a phospholipid molecule.
The first P-type ATPase discovered was the Na + /K + -ATPase , which Nobel laureate Jens Christian Skou isolated in 1957. [ 3 ] The Na + /K + -ATPase was only the first member of a large and still-growing protein family (see Swiss-Prot Prosite motif PS00154 ).
P-type ATPases have a single catalytic subunit of 70 - 140 kDa. The catalytic subunit hydrolyzes ATP, contains the aspartyl phosphorylation site and binding sites for the transported ligand(s) and catalyzes ion transport. Various subfamilies of P-type ATPases also need additional subunits for proper function. Additional subunits that lack catalytic activity are present in the ATPase complexes of P1A, P2A, P2C and P4 ATPases. E.g. the catalytic alpha subunit of Na + /K + -ATPase consists of two additional subunits, beta and gamma, involved in trafficking, folding, and regulation of these pumps. The first P-type ATPase to be crystallized was SERCA1a , a sarco(endo)plasmic reticulum Ca 2+ -ATPase of fast twitch muscle from adult rabbit . [ 4 ] It is generally acknowledged that the structure of SERCA1a is representative for the superfamily of P-type ATPases. [ 5 ]
The catalytic subunit of P-type ATPases is composed of a cytoplasmic section and a transmembrane section with binding sites for the transported ligand(s). The cytoplasmic section consists of three cytoplasmic domains, designated the P, N, and A domains, containing over half the mass of the protein.
The transmembrane section ( M domain ) typically has ten transmembrane helices (M1-M10), with the binding sites for transported ligand(s) located near the midpoint of the bilayer. While most subfamilies have 10 transmembrane helices, there are some notable exceptions. The P1A ATPases are predicted to have 7, and the large subfamily of heavy metal pumps P1B) is predicted to have 8 transmembrane helices. P5 ATPases appear to have a total of 12 transmembrane helices.
Common for all P-type ATPases is a core of 6 transmembrane-spanning segments (also called the 'transport (T) domain'; M1-M6 in SERCA), that harbors the binding sites for the translocated ligand(s). The ligand(s) enter through a half-channel to the binding site and leave on the other side of the membrane through another half-channel.
Varying among P-type ATPase is the additional number of transmembrane-spanning segments (also called the 'support (S) domain', which between subfamilies ranges from 2 to 6. Extra transmembrane-segments likely provides structural support for the T domain and can also have specialized functions.
The P domain contains the canonical aspartic acid residue phosphorylated (in a conserved DKTGT motif; the 'D' is the one letter abbreviation of the amino acid aspartate) during the reaction cycle. It is composed of two parts widely separated in sequence. These two parts assemble into a seven-strand parallel β-sheet with eight short associated a-helices, forming a Rossmann fold .
The folding pattern and the locations of the critical amino acids for phosphorylation in P-type ATPases has the haloacid dehalogenase fold characteristic of the haloacid dehalogenase (HAD) superfamily , as predicted by sequence homology. The HAD superfamily functions on the common theme of an aspartate ester formation by an S N 2 reaction mechanism. This S N 2 reaction is clearly observed in the solved structure of SERCA with ADP plus AlF 4 − . [ 6 ]
The N domain serves as a built-in protein kinase that functions to phosphorylate the P domain. The N domain is inserted between the two segments of the P domain, and is formed of a seven-strand antiparallel β-sheet between two helix bundles. This domain contains the ATP-binding pocket, pointing out toward the solvent near the P-domain.
The A domain serves as a built-in protein phosphatase that functions to dephosphorylate the phosphorylated P domain. The A domain is the smallest of the three cytoplasmic domains. It consists of a distorted jellyroll structure and two short helices. It is the actuator domain modulating the occlusion of the transported ligand(s) in the transmembrane binding sites, and it is pivot in transposing the energy from the hydrolysis of ATP in the cytoplasmic domains to the vectorial transport of cations in the transmembrane domain. The A domain dephosphorylates the P domain as part of the reaction cycle using a highly conserved TGES motif located at one end of the jellyroll.
Some members of the P-type ATPase family have additional regulatory (R) domains fused to the pump. Heavy metal P1B pumps can have several N- and C-terminal heavy metal-binding domains that have been found to be involved in regulation. The P2B Ca 2+ ATPases have autoinbitory domains in their amino-terminal (plants) or carboxy-terminal (animals) regions, which contain binding sites for calmodulin , which, in the presence of Ca 2+ , activates P2B ATPases by neutralizing the terminal constraint. The P3A plasma membrane proton pumps have a C-terminal regulatory domain, which, when unphosphorylated, inhibits pumping.
All P-type ATPases use the energy derived from ATP to drive transport. They form a high-energy aspartyl-phosphoanhydride intermediate in the reaction cycle, and they interconvert between at least two different conformations, denoted by E 1 and E 2 . The E 1 -E 2 notation stems from the initial studies on this family of enzymes made on the Na + /K + -ATPase, where the sodium form and the potassium form are referred to as E 1 and E 2 , respectively, in the "Post-Albers scheme". The E 1 -E 2 schema has been proven to work, but there exist more than two major conformational states. The E 1 -E 2 notation highlights the selectivity of the enzyme . In E 1 , the pump has high affinity for the exported substrate and low affinity for the imported substrate. In E 2 , it has low affinity of the exported substrate and high affinity for the imported substrate. Four major enzyme states form the cornerstones in the reaction cycle. Several additional reaction intermediates occur interposed. These are termed E 1 ~P, E 2 P, E 2 -P*, and E 1 /E 2 . [ 7 ]
ATP hydrolysis occurs in the cytoplasmic headpiece at the interface between domain N and P. Two Mg-ion sites form part of the active site. ATP hydrolysis is tightly coupled to translocation of the transported ligand(s) through the membrane, more than 40 Å away, by the A domain.
A phylogenetic analysis of 159 sequences made in 1998 by Axelsen and Palmgren suggested that P-type ATPases can be divided into five subfamilies (types; designated as P1-P5), based strictly on a conserved sequence kernel excluding the highly variable N and C terminal regions. [ 8 ] Chan et al. (2010) also analyzed P-type ATPases in all major prokaryotic phyla for which complete genome sequence data were available and compared the results with those for eukaryotic P-type ATPases. [ 9 ] The phylogenetic analysis grouped the proteins independent of the organism from which they are isolated and showed that the diversification of the P-type ATPase family occurred prior to the separation of eubacteria , archaea , and eucaryota . This underlines the significance of this protein family for cell survival under stress conditions. [ 8 ]
P1 ATPases (or Type I ATPases) consists of the transition/heavy metal ATPases. Topological type I (heavy metal) P-type ATPases predominate in prokaryotes (approx. tenfold). [ 10 ]
P1A ATPases (or Type IA) are involved in K + import ( TC# 3.A.3.7 ). They are atypical P-type ATPases because, unlike other P-type ATPases, they function as part of a heterotetrameric complex (called KdpFABC ), where the actual K + transport is mediated by another subcomponent of the complex.
P1B ATPases (or Type IB ATPases) are involved in transport of the soft Lewis acids : Cu + , Ag + , Cu 2+ , Zn 2+ , Cd 2+ , Pb 2+ and Co 2+ (TC#s 3.A.3.5 and 3.A.3.6 ). They are key elements for metal resistance and metal homeostasis in a wide range of organisms.
Metal binding to transmembrane metal-binding sites (TM-MBS) in Cu + -ATPases is required for enzyme phosphorylation and subsequent transport. However, Cu + does not access Cu + -ATPases in a free ( hydrated ) form but is bound to a chaperone protein . The delivery of Cu + by Archaeoglobus fulgidus Cu + -chaperone, CopZ (see TC# 3.A.3.5.7 ), to the corresponding Cu + -ATPase, CopA ( TC# 3.A.3.5.30 ), has been studied. [ 11 ] CopZ interacted with and delivered the metal to the N-terminal metal binding domain(s) of CopA (MBDs). Cu + -loaded MBDs, acting as metal donors, were unable to activate CopA or a truncated CopA lacking MBDs. Conversely, Cu + -loaded CopZ activated the CopA ATPase and CopA constructs in which MBDs were rendered unable to bind Cu + . Furthermore, under nonturnover conditions, CopZ transferred Cu + to the TM-MBS of a CopA lacking MBDs altogether. Thus, MBDs may serve a regulatory function without participating directly in metal transport, and the chaperone delivers Cu + directly to transmembrane transport sites of Cu + -ATPases. [ 11 ] Wu et al. (2008) have determined structures of two constructs of the Cu (CopA) pump from Archaeoglobus fulgidus by cryoelectron microscopy of tubular crystals, which revealed the overall architecture and domain organization of the molecule. They localized its N-terminal MBD within the cytoplasmic domains that use ATP hydrolysis to drive the transport cycle and built a pseudoatomic model by fitting existing crystallographic structures into the cryoelectron microscopy maps for CopA. The results also similarly suggested a Cu-dependent regulatory role for the MBD. [ 12 ]
In the Archaeoglobus fulgidus CopA ( TC# 3.A.3.5.7 ), invariant residues in helixes 6, 7 and 8 form two transmembrane metal binding sites (TM-MBSs). These bind Cu + with high affinity in a trigonal planar geometry. The cytoplasmic Cu + chaperone CopZ transfers the metal directly to the TM-MBSs; however, loading both of the TM-MBSs requires binding of nucleotides to the enzyme. In agreement with the classical transport mechanism of P-type ATPases, occupancy of both transmembrane sites by cytoplasmic Cu + is a requirement for enzyme phosphorylation and subsequent transport into the periplasmic or extracellular milieu. Transport studies have shown that most Cu + -ATPases drive cytoplasmic Cu + efflux, albeit with quite different transport rates in tune with their various physiological roles. Archetypical Cu + -efflux pumps responsible for Cu + tolerance, like the Escherichia coli CopA, have turnover rates ten times higher than those involved in cuproprotein assembly (or alternative functions). This explains the incapability of the latter group to significantly contribute to the metal efflux required for survival in high copper environments. Structural and mechanistic details of copper-transporting P-type ATPase functionhave been described. [ 13 ]
P2 ATPases (or Type II ATPases) are split into four groups. Topological type II ATPases (specific for Na + ,K + , H + Ca 2+ , Mg 2+ and phospholipids) predominate in eukaryotes (approx. twofold). [ 10 ]
P2A ATPases (or Type IIA ATPases) are Ca 2+ ATPases that transport Ca 2+ . P2A ATPases are split into two groups. Members of the first group are called sarco/endoplasmatic reticulum Ca 2+ -ATPases (also referred to as SERCA). These pumps have two Ca 2+ ion binding sites and are often regulated by inhibitory accessory proteins having a single trans-membrane spanning segment (e.g. phospholamban and sarcolipin . In the cell, they are located in the sarcoplasmic or endoplasmatic reticulum. SERCA1a is a type IIA pump. The second group of P2A ATPases is called secretory pathway Ca 2+ -ATPases (also referred to as SPCA). These pumps have a single Ca 2+ ion binding site and are located in secretory vesicles (animals) or the vacuolar membrane (fungi). (TC# 3.A.3.2)
Crystal structures of Sarcoplasimc/endoplasmic reticulum ATP driven calcium pumps can be found in RCSB. [ 14 ]
SERCA1a is composed of a cytoplasmic section and a transmembrane section with two Ca 2+ -binding sites. The cytoplasmic section consists of three cytoplasmic domains, designated the P, N, and A domains, containing over half the mass of the protein. The transmembrane section has ten transmembrane helices (M1-M10), with the two Ca 2+ -binding sites located near the midpoint of the bilayer. The binding sites are formed by side-chains and backbone carbonyls from M4, M5, M6, and M8. M4 is unwound in this region due to a conserved proline (P308). This unwinding of M4 is recognised as a key structural feature of P-type ATPases.
Structures are available for both the E 1 and E 2 states of the Ca 2+ ATPase showing that Ca 2+ binding induces major changes in all three cytoplasmic domains relative to each other. [ 15 ]
In the case of SERCA1a , energy from ATP is used to transport 2 Ca 2+ -ions from the cytoplasmic side to the lumen of the sarcoplasmatic reticulum , and to countertransport 1-3 protons into the cytoplasm . Starting in the E 1 /E 2 state, the reaction cycle begins as the enzyme releases 1-3 protons from the cation-ligating residues, in exchange for cytoplasmic Ca 2+ -ions. This leads to assembly of the phosphorylation site between the ATP-bound N domain and the P domain, while the A domain directs the occlusion of the bound Ca 2+ . In this occluded state, the Ca 2+ ions are buried in a proteinaceous environment with no access to either side of the membrane. The Ca 2 E 1 ~P state becomes formed through a kinase reaction, where the P domain becomes phosphorylated, producing ADP. The cleavage of the β-phosphodiester bond releases the gamma-phosphate from ADP and unleashes the N domain from the P domain.
This then allows the A domain to rotate toward the phosphorylation site, making a firm association with both the P and the N domains. This movement of the A domain exerts a downward push on M3-M4 and a drag on M1-M2, forcing the pump to open at the luminal side and forming the E 2 P state. During this transition, the transmembrane Ca 2+ -binding residues are forced apart, destroying the high-affinity binding site. This is in agreement with the general model form substrate translocation, showing that energy in primary transport is not used to bind the substrate but to release it again from the buried counter ions. At the same time the N domain becomes exposed to the cytosol, ready for ATP exchange at the nucleotide-binding site.
As the Ca 2+ dissociate to the luminal side, the cation binding sites are neutralised by proton binding, which makes a closure of the transmembrane segments favourable. This closure is coupled to a downward rotation of the A domain and a movement of the P domain, which then leads to the E 2 -P* occluded state. Meanwhile, the N domain exchanges ADP for ATP.
The P domain is dephosphorylated by the A domain, and the cycle completes when the phosphate is released from the enzyme, stimulated by the newly bound ATP, while a cytoplasmic pathway opens to exchange the protons for two new Ca 2+ ions. [ 7 ]
Xu et al. proposed how Ca 2+ binding induces conformational changes in TMS 4 and 5 in the membrane domain (M) that in turn induce rotation of the phosphorylation domain (P). [ 15 ] The nucleotide binding (N) and β-sheet (β) domains are highly mobile, with N flexibly linked to P, and β flexibly linked to M. Modeling of the fungal H + ATPase, based on the structures of the Ca 2+ pump, suggested a comparable 70º rotation of N relative to P to deliver ATP to the phosphorylation site. [ 16 ]
One report suggests that this sarcoplasmic reticulum (SR) Ca 2+ ATPase is homodimeric. [ 17 ]
Crystal structures have shown that the conserved TGES loop of the Ca 2+ -ATPase is isolated in the Ca 2 E 1 state but becomes inserted in the catalytic site in E 2 states. [ 18 ] Anthonisen et al. (2006) characterized the kinetics of the partial reaction steps of the transport cycle and the binding of the phosphoryl analogs BeF, AlF, MgF, and vanadate in mutants with alterations to conserved TGES loop residues. The data provide functional evidence supporting a role of Glu 183 in activating the water molecule involved in the E 2 P → E 2 dephosphorylation and suggest a direct participation of the side chains of the TGES loop in the control and facilitation of the insertion of the loop in the catalytic site. The interactions of the TGES loop furthermore seem to facilitate its disengagement from the catalytic site during the E 2 → Ca 2 E 1 transition. [ 18 ]
Crystal Structures of Calcium ATPase are available in RCSB and include: PDB : 4AQR , 2L1W , 2M7E , 2M73 , among others. [ 19 ]
P2B (or Type IIB ATPases) are Ca 2+ ATPases that transport Ca 2+ . These pumps have a single Ca 2+ ion binding site and are regulated by binding of calmodulin to autoinhibitory built-in domains situated at either the carboxy-terminal (animals) or amino-terminal (plants) end of the pump protein. In the cell, they are situated in the plasma membrane (animals and plants) and the internal membranes (plants). Plasma membrane Ca 2+ -ATPase (PMCA) of animals is a P2B ATPase ( TC# 3.A.3.2 )
P2C ATPases (or Type IIC) include the closely related Na + /K + and H + /K + ATPases from animal cells. ( TC# 3.A.3.1 )
The X-ray crystal structure at 3.5 Å resolution of the pig renal Na + /K + -ATPase has been determined with two rubidium ions bound in an occluded state in the transmembrane part of the α-subunit. [ 20 ] Several of the residues forming the cavity for rubidium/potassium occlusion in the Na + /K + -ATPase are homologous to those binding calcium in the Ca 2+ -ATPase of the sarco(endo)plasmic reticulum. The carboxy terminus of the α-subunit is contained within a pocket between transmembrane helices and seems to be a novel regulatory element controlling sodium affinity, possibly influenced by the membrane potential .
Crystal Structures are available in RCSB and include: PDB : 4RES , 4RET , 3WGU , 3WGV , among others. [ 21 ]
P2D ATPases (or Type IID) include a small number of Na + (and K + ) exporting ATPases found in fungi and mosses. (Fungal K + transporters; TC# 3.A.3.9 )
P3 ATPases (or Type III ATPases) are split into two groups.
P3A ATPases (or Type IIIA) contain the plasma membrane H + -ATPases from prokaryotes, protists, plants and fungi.
Plasma membrane H + -ATPase is best characterized in plants and yeast. It maintains the level of intracellular pH and transmembrane potential . [ 22 ] Ten transmembrane helices and three cytoplasmic domains define the functional unit of ATP-coupled proton transport across the plasma membrane, and the structure is locked in a functional state not previously observed in P-type ATPases. The transmembrane domain reveals a large cavity, which is likely to be filled with water, located near the middle of the membrane plane where it is lined by conserved hydrophilic and charged residues. Proton transport against a high membrane potential is readily explained by this structural arrangement. [ 23 ]
P3B ATPases (or Type IIIB) are presumed Mg 2+ -ATPases found in eubacteria and plants. Fungal H + transporters ( TC# 3.A.3.3 ) and Mg 2+ ( TC# 3.A.3.4 )
P4 ATPases (or Type IV ATPases) are flippases involved in the transport of phospholipids , [ 24 ] such as phosphatidylserine , phosphatidylcholine and phosphatidylethanolamine . [ 25 ]
P5 ATPases (or Type V ATPases) have unknown specificity. This large group is found only in eukaryotes and is further divided into two groups.
P5A ATPases (or Type VA) are involved in regulation of homeostasis in the endoplasmic reticulum . [ 26 ]
P5B ATPases (or Type VB) are found in the lysosomal membrane of animals. Mutations in these pumps are linked to a variety of neurological diseases. [ 27 ] [ 28 ]
In addition to the subfamilies of P-type ATPases listed above, several prokaryotic families of unknown function have been identified. [ 29 ] The Transporter Classification Database provides a representative list of members of the P-ATPase superfamily, which as of early 2016 consisting of 20 families. Members of the P-ATPase superfamily are found in bacteria , archaea and eukaryotes . Clustering on the phylogenetic tree is usually in accordance with specificity for the transported ion(s).
In eukaryotes, they are present in the plasma membranes or endoplasmic reticular membranes. In prokaryotes, they are localized to the cytoplasmic membranes.
P-type ATPases from 26 eukaryotic species were analyzed later. [ 10 ] [ 30 ]
Chan et al., (2010) conducted an equivalent but more extensive analysis of the P-type ATPase Superfamily in Prokaryotes and compared them with those from Eukaryotes. While some families are represented in both types of organisms, others are found only in one of the other type. The primary functions of prokaryotic P-type ATPases appear to be protection from environmental stress conditions. Only about half of the P-type ATPase families are functionally characterized. [ 29 ]
Many P-type ATPase families are found exclusively in prokaryotes (e.g. Kdp-type K + uptake ATPases (type III) and all prokaryotic functionally uncharacterized P-type ATPase (FUPA) families), while others are restricted to eukaryotes (e.g. phospholipid flippases and all 13 eukaryotic FUPA families). [ 10 ] Horizontal gene transfer has occurred frequently among bacteria and archaea, which have similar distributions of these enzymes , but rarely between most eukaryotic kingdoms, and even more rarely between eukaryotes and prokaryotes. In some bacterial phyla (e.g. Bacteroidota and Fusobacteriota ), ATPase gene gain and loss as well as horizontal transfer occurred seldom in contrast to most other bacterial phyla. Some families (i.e., Kdp-type ATPases) underwent far less horizontal gene transfer than other prokaryotic families, possibly due to their multisubunit characteristics. Functional motifs are better conserved across family lines than across organismal lines, and these motifs can be family specific, facilitating functional predictions. In some cases, gene fusion events created P-type ATPases covalently linked to regulatory catalytic enzymes. In one family (FUPA Family 24), a type I ATPase gene (N-terminal) is fused to a type II ATPase gene (C-terminal) with retention of function only for the latter. Genome minimalization led to preferential loss of P-type ATPase genes. Chan et al. (2010) suggested that in prokaryotes and some unicellular eukaryotes, the primary function of P-type ATPases is protection from extreme environmental stress conditions. The classification of P-type ATPases of unknown function into phylogenetic families provides guides for future molecular biological studies. [ 9 ]
Human genes encoding P-type ATPases or P-type ATPase-like proteins include: | https://en.wikipedia.org/wiki/P-type_ATPase |
In mathematical analysis , p -variation is a collection of seminorms on functions from an ordered set to a metric space , indexed by a real number p ≥ 1 {\displaystyle p\geq 1} . p -variation is a measure of the regularity or smoothness of a function. Specifically, if f : I → ( M , d ) {\displaystyle f:I\to (M,d)} , where ( M , d ) {\displaystyle (M,d)} is a metric space and I a totally ordered set, its p -variation is:
‖ f ‖ p -var = ( sup D ∑ t k ∈ D d ( f ( t k ) , f ( t k − 1 ) ) p ) 1 / p {\displaystyle \|f\|_{p{\text{-var}}}=\left(\sup _{D}\sum _{t_{k}\in D}d(f(t_{k}),f(t_{k-1}))^{p}\right)^{1/p}}
where D ranges over all finite partitions of the interval I .
The p variation of a function decreases with p . If f has finite p -variation and g is an α -Hölder continuous function, then g ∘ f {\displaystyle g\circ f} has finite p α {\displaystyle {\frac {p}{\alpha }}} -variation.
The case when p is one is called total variation , and functions with a finite 1-variation are called bounded variation functions.
This concept should not be confused with the notion of p-th variation along a sequence of partitions, which is computed as a limit along a given sequence ( D n ) {\displaystyle (D_{n})} of time partitions: [ 1 ]
[ f ] p = ( lim n → ∞ ∑ t k n ∈ D n d ( f ( t k n ) , f ( t k − 1 n ) ) p ) {\displaystyle [f]_{p}=\left(\lim _{n\to \infty }\sum _{t_{k}^{n}\in D_{n}}d(f(t_{k}^{n}),f(t_{k-1}^{n}))^{p}\right)}
For example for p=2, this corresponds to the concept of quadratic variation, which is different from 2-variation.
One can interpret the p -variation as a parameter-independent version of the Hölder norm, which also extends to discontinuous functions.
If f is α – Hölder continuous (i.e. its α–Hölder norm is finite) then its 1 α {\displaystyle {\frac {1}{\alpha }}} -variation is finite. Specifically, on an interval [ a , b ], ‖ f ‖ 1 α -var ≤ ‖ f ‖ α ( b − a ) α {\displaystyle \|f\|_{{\frac {1}{\alpha }}{\text{-var}}}\leq \|f\|_{\alpha }(b-a)^{\alpha }} .
If p is less than q then the space of functions of finite p -variation on a compact set is continuously embedded with norm 1 into those of finite q -variation. I.e. ‖ f ‖ q -var ≤ ‖ f ‖ p -var {\displaystyle \|f\|_{q{\text{-var}}}\leq \|f\|_{p{\text{-var}}}} . However unlike the analogous situation with Hölder spaces the embedding is not compact. For example, consider the real functions on [0,1] given by f n ( x ) = x n {\displaystyle f_{n}(x)=x^{n}} . They are uniformly bounded in 1-variation and converge pointwise to a discontinuous function f but this not only is not a convergence in p -variation for any p but also is not uniform convergence.
If f and g are functions from [ a , b ] to R {\displaystyle \mathbb {R} } with no common discontinuities and with f having finite p -variation and g having finite q -variation, with 1 p + 1 q > 1 {\displaystyle {\frac {1}{p}}+{\frac {1}{q}}>1} then the Riemann–Stieltjes Integral
is well-defined. This integral is known as the Young integral because it comes from Young (1936) . [ 2 ] The value of this definite integral is bounded by the Young-Loève estimate as follows
where C is a constant which only depends on p and q and ξ is any number between a and b . [ 3 ] If f and g are continuous, the indefinite integral F ( w ) = ∫ a w f ( x ) d g ( x ) {\displaystyle F(w)=\int _{a}^{w}f(x)\,dg(x)} is a continuous function with finite q -variation: If a ≤ s ≤ t ≤ b then ‖ F ‖ q -var ; [ s , t ] {\displaystyle \|F\|_{q{\text{-var}};[s,t]}} , its q -variation on [ s , t ], is bounded by C ‖ g ‖ q -var ; [ s , t ] ( ‖ f ‖ p -var ; [ s , t ] + ‖ f ‖ ∞ ; [ s , t ] ) ≤ 2 C ‖ g ‖ q -var ; [ s , t ] ( ‖ f ‖ p -var ; [ a , b ] + f ( a ) ) {\displaystyle C\|g\|_{q{\text{-var}};[s,t]}(\|f\|_{p{\text{-var}};[s,t]}+\|f\|_{\infty ;[s,t]})\leq 2C\|g\|_{q{\text{-var}};[s,t]}(\|f\|_{p{\text{-var}};[a,b]}+f(a))} where C is a constant which only depends on p and q . [ 4 ]
A function from R d {\displaystyle \mathbb {R} ^{d}} to e × d real matrices is called an R e {\displaystyle \mathbb {R} ^{e}} -valued one-form on R d {\displaystyle \mathbb {R} ^{d}} .
If f is a Lipschitz continuous R e {\displaystyle \mathbb {R} ^{e}} -valued one-form on R d {\displaystyle \mathbb {R} ^{d}} , and X is a continuous function from the interval [ a , b ] to R d {\displaystyle \mathbb {R} ^{d}} with finite p -variation with p less than 2, then the integral of f on X , ∫ a b f ( X ( t ) ) d X ( t ) {\displaystyle \int _{a}^{b}f(X(t))\,dX(t)} , can be calculated because each component of f ( X ( t )) will be a path of finite p -variation and the integral is a sum of finitely many Young integrals. It provides the solution to the equation d Y = f ( X ) d X {\displaystyle dY=f(X)\,dX} driven by the path X .
More significantly, if f is a Lipschitz continuous R e {\displaystyle \mathbb {R} ^{e}} -valued one-form on R e {\displaystyle \mathbb {R} ^{e}} , and X is a continuous function from the interval [ a , b ] to R d {\displaystyle \mathbb {R} ^{d}} with finite p -variation with p less than 2, then Young integration is enough to establish the solution of the equation d Y = f ( Y ) d X {\displaystyle dY=f(Y)\,dX} driven by the path X . [ 5 ]
The theory of rough paths generalises the Young integral and Young differential equations and makes heavy use of the concept of p -variation.
p -variation should be contrasted with the quadratic variation which is used in stochastic analysis , which takes one stochastic process to another. In particular the definition of quadratic variation looks a bit like the definition of p -variation, when p has the value 2. Quadratic variation is defined as a limit as the partition gets finer, whereas p -variation is a supremum over all partitions. Thus the quadratic variation of a process could be smaller than its 2-variation. If W t is a standard Brownian motion on [0, T ], then with probability one its p -variation is infinite for p ≤ 2 {\displaystyle p\leq 2} and finite otherwise. The quadratic variation of W is [ W ] T = T {\displaystyle [W]_{T}=T} .
For a discrete time series of observations X 0 ,...,X N it is straightforward to compute its p -variation with complexity of O ( N 2 ). Here is an example C++ code using dynamic programming :
There exist much more efficient, but also more complicated, algorithms for R {\displaystyle \mathbb {R} } -valued processes [ 6 ] [ 7 ] and for processes in arbitrary metric spaces. [ 7 ] | https://en.wikipedia.org/wiki/P-variation |
There are two kinds of seismic body waves in solids, pressure waves (P-waves) and shear waves. In linear elasticity , the P-wave modulus M {\displaystyle M} , also known as the longitudinal modulus , or the constrained modulus , is one of the elastic moduli available to describe isotropic homogeneous materials.
It is defined as the ratio of axial stress to axial strain in a uniaxial strain state. This occurs when expansion in the transverse direction is prevented by the inertia of neighboring material, such as in an earthquake, or underwater seismic blast.
where all the other strains ϵ ∗ ∗ {\displaystyle \epsilon _{**}} are zero.
This is equivalent to stating that
where V P is the velocity of a P-wave and ρ is the density of the material through which the wave is propagating.
There are two valid solutions. The plus sign leads to ν ≥ 0 {\displaystyle \nu \geq 0} .
This article about materials science is a stub . You can help Wikipedia by expanding it . | https://en.wikipedia.org/wiki/P-wave_modulus |
The phosphate/oxygen ratio , or P/O ratio , refers to the amount of ATP produced from the movement of two electrons through a defined electron transport chain , terminated by reduction of an oxygen atom. [ 1 ]
The P/O ratio is dependent on the number of hydrogen ions transported outward across an electrochemical gradient , and the number of protons which return inward through the membrane via an enzyme such as ATP synthase . The ATP synthase works by a rotary mechanism. The ATP generated will be dependent on the amount of ATP produced per rotation of the ATP synthase rotor, and the number of protons necessary to complete a rotation. Every full rotation produces 3 ATPs. According to current understanding of the mechanism of the F 0 part, [ 2 ] the number of protons translocated per rotation is exactly equal to the number of subunits in the c ring. Recent structural studies show that this is not the same for all organisms. For vertebrate mitochondrial ATP synthase, the number of c subunits is 8 [ 3 ] . [ 4 ] The synthase thus requires 8 protons to synthesize three ATP, or 8/3 protons/ATP.
Inward moving protons must not only power rotation of ATP synthase, but may also be used in the transport of products and precursors. Given the net charge differences between ATP and ADP, the enzyme ATP–ADP translocase dissipates the charge equivalent of one hydrogen ion from the gradient when moving ATP (outward) and ADP (inward) across the inner mitochondrial membrane. The electroneutral symport of phosphate ion and H+ results in importing one proton, without its charge, per phosphate. Taken together, import of ADP and Pi and export of the resulting ATP results in one proton imported, subtracting from the number available for use by the ATP synthase directly. Taking this into account, it takes 8/3 +1 or 3.67 protons for vertebrate mitochondria to synthesize one ATP in the cytoplasm from ADP and Pi in the cytoplasm.
Within aerobic respiration , the P/O ratio continues to be debated; however, current figures place it at 2.5 ATP per 1/2(O 2 ) reduced to water, though some claim the ratio is 3. [ 5 ] This figure arises from accepting that 10 H + are transported out of the matrix per 2 e − , and 4 H + are required to move inward to synthesize a molecule of ATP. [ 6 ]
The H+/2e − ratios of the three major respiratory complexes are generally agreed to be 4, 4, and 2 for Complexes I, III, and IV respectively. [ 7 ] The H + /O ratio thus depends whether the substrate electrons enter at the level of NADH (passing through all three for 10 H + /2e − ) or ubiquinol (passing through only complexes III and IV for 6H + /2e − ). The latter is the case when the substrate is succinate or extramitochondrial NADH being oxidized via the glycerol phosphate shuttle ; or other UQH2-linked dehydrogenase. During normal aerobic respiration the ratio would be somewhere between these values, as the TCA cycle produces both NADH and ubiquinol.
The resulting P/O ratio would be the ratio of H/O and H/P; which is 10/3.67 or 2.73 for NADH-linked respiration, and 6/3.67 or 1.64 for UQH2-linked respiration, with actual values being somewhere between. | https://en.wikipedia.org/wiki/P/O_ratio |
A P1-derived artificial chromosome, or PAC, is a DNA construct derived from the DNA of P1 bacteriophages and Bacterial artificial chromosome . It can carry large amounts (about 100–300 kilobases ) of other sequences for a variety of bioengineering purposes in bacteria . It is one type of the efficient cloning vector used to clone DNA fragments (100- to 300-kb insert size; average,150 kb) in Escherichia coli cells. [ 1 ]
The bacteriophage P1 was first isolated by Dr. Giuseppe Bertani . In his study, he noticed that the lysogen produced abnormal non-continuous phages, and later found phage P1 was produced from the Lisbonne lysogen strain, in addition to bacteriophages P2 and P3. P1 has the ability to copy a bacteria's host genome and integrate that DNA information into other bacteria hosts, also known as generalized transduction . [ 2 ] Later on, P1 was developed as a cloning vector by Nat Sternberg and colleagues in the 1990s. It is capable of Cre-Lox recombination . [ 3 ] [ 4 ] The P1 vector system was first developed to carry relatively large DNA fragments in plasmids (95-100kb). [ 4 ]
PAC has 2 loxP sites, which can be used by phage recombinases to form the product from its cre- gene recognition during Cre-Lox recombination . This process circularizes the DNA strand, forming a plasmid , which can then be inserted into bacteria such as Escherichia coli . [ 4 ] The transformation is usually done by electroporation , which uses electricity to allow the plasmids permeate into the cells. If high expression levels are desired, the P1 lytic replicon can be used in constructs. [ 5 ] Electroporation allows for lysogeny of PACs so that they can replicate within cells without disturbing other chromosomes. [ 1 ]
PAC is one of the artificial chromosome vectors. Some other artificial chromosomes include: bacterial artificial chromosome , yeast artificial chromosome and the human artificial chromosome . Compared to other artificial chromosomes, it can carry relatively large DNA fragments, however less so than the yeast artificial chromosome (YAC). Some advantages of PACs compared to YACs includes easier manipulation of bacteria system, easier separation from DNA hosts, higher transformation rate, more stable inserts, and they are non-chimeric which means they do not rearrange and ligate to form new DNA strand, allowing for a user friendly vector choice. [ 1 ]
PAC is commonly used as a large capacity vector which allows propagation of large DNA inserts in Escherichia coli . [ 1 ] This feature has been commonly used for:
Since PAC was derived from phages, PAC and its variants are also useful in the PAC-based phage therapy and antibiotic studies. [ 10 ] | https://en.wikipedia.org/wiki/P1-derived_artificial_chromosome |
2A5E , 1A5E , 1BI7 , 1DC2 ,%%s 1A5E , 1BI7 , 1DC2 , 2A5E
1029
12578
ENSG00000147889
ENSMUSG00000044303
P42771 Q8N726
P51480 Q64364
NM_001363763
NM_001040654 NM_009877
NP_478102.2
NP_001035744 NP_034007 NP_034007.1
p16 (also known as p16 INK4a , cyclin-dependent kinase inhibitor 2A , CDKN2A , multiple tumor suppressor 1 and numerous other synonyms), is a protein that slows cell division by slowing the progression of the cell cycle from the G1 phase to the S phase , thereby acting as a tumor suppressor . It is encoded by the CDKN2A gene . A deletion (the omission of a part of the DNA sequence during replication) in this gene can result in insufficient or non-functional p16, accelerating the cell cycle and resulting in many types of cancer. [ 5 ] [ 6 ] [ 7 ]
p16 can be used as a biomarker to improve the histological diagnostic accuracy of grade 3 cervical intraepithelial neoplasia (CIN). p16 is also implicated in the prevention of melanoma , oropharyngeal squamous cell carcinoma , cervical cancer , vulvar cancer and esophageal cancer .
p16 was discovered in 1993. It is a protein with 148 amino acids and a molecular weight of 16 kDa that comprises four ankyrin repeats . [ 8 ] The name of p16 is derived from its molecular weight , and the alternative name p16 INK4a refers to its role in inhibiting cyclin-dependent kinase CDK4. [ 8 ]
p16 is also known as:
In humans, p16 is encoded by the CDKN2A gene, located on chromosome 9 (9p21.3). This gene generates several transcript variants that differ in their first exons . At least three alternatively spliced variants encoding distinct proteins have been reported, two of which encode structurally related isoforms known to function as inhibitors of CDK4 . The remaining transcript includes an alternate exon 1 located 20 kb upstream of the remainder of the gene; this transcript contains an alternate open reading frame (ARF) that specifies a protein that is structurally unrelated to the products of the other variants. [ 9 ] The ARF product functions as a stabilizer of the tumor suppressor protein p53 , as it can interact with and sequester MDM2 , a protein responsible for the degradation of p53. [ 10 ] [ 11 ] In spite of their structural and functional differences, the CDK inhibitor isoforms and the ARF product encoded by this gene, through the regulatory roles of CDK4 and p53 in cell cycle G1 progression , share a common functionality in controlling the G1 phase of the cell cycle. This gene is frequently mutated or deleted in a wide variety of tumors and is known to be an important tumor suppressor gene. [ 5 ]
When organisms age, the expression of p16 increases to reduce the proliferation of stem cells . [ 12 ] This reduction in the division and production of stem cells protects against cancer while increasing the risks associated with cellular senescence .
p16 is an inhibitor of cyclin-dependent kinases (CDK). It slows down the cell cycle by prohibiting progression from G1 phase to S phase. Otherwise, CDK4/6 binds cyclin D and forms an active protein complex that phosphorylates retinoblastoma protein (pRB). Once phosphorylated, pRB dissociates from the transcription factor E2F1 . This liberates E2F1 from its bound state in the cytoplasm and allows it to enter the nucleus. Once in the nucleus, E2F1 promotes the transcription of target genes that are essential for transition from G1 to S phase. [ 13 ] [ 14 ]
This pathway connects the processes of tumor oncogenesis and senescence, fixing them on opposite ends of a spectrum. On one end, p16 hypermethylation, mutation, or deletion leads to downregulation of the gene and can lead to cancer through the dysregulation of cell cycle progression. Conversely, activation of p16 through reactive oxygen species , DNA damage, or senescence leads to the buildup of p16 in tissues and is implicated in the aging of cells. [ 13 ]
Regulation of p16 is complex and involves the interaction of several transcription factors, as well as several proteins involved in epigenetic modification through methylation and repression of the promoter region. [ 13 ]
PRC1 and PRC2 are two protein complexes that modify the expression of p16 through the interaction of various transcription factors that execute methylation patterns that can repress transcription of p16. These pathways are activated in the cellular response to reduce senescence. [ 15 ] [ 16 ]
Mutations resulting in deletion or reduction of function of the CDKN2A gene are associated with increased risk of a wide range of cancers, and alterations of the gene are frequently seen in cancer cell lines . [ 17 ] [ 18 ] Examples include:
Pancreatic adenocarcinoma is often associated with mutations in the CDKN2A gene. [ 19 ] [ 20 ] [ 21 ]
Carriers of germline mutations in CDKN2A have, besides their high risks of melanoma, also increased risks of pancreatic, lung, laryngeal and oropharyngeal cancers. Tobacco smoking increases the carriers’ susceptibility for such non-melanoma cancers. [ 22 ]
Homozygous deletions of p16 are frequently found in esophageal cancer and gastric cancer cell lines. [ 23 ]
Germline mutations in CDKN2A are associated with an increased susceptibility to develop skin cancer . [ 24 ]
Hypermethylation of tumor suppressor genes has been implicated in various cancers. In 2013, a meta-analysis revealed an increased frequency of DNA methylation of the p16 gene in esophageal cancer. As the degree of tumor differentiation increased, so did the frequency of p16 DNA methylation.
Tissue samples of primary oral squamous cell carcinoma (OSCC) often display hypermethylation in the promoter regions of p16. Cancer cells show a significant increase in the accumulation of methylation in CpG islands in the promoter region of p16. This epigenetic change leads to loss of the tumor suppressor gene function through two possible mechanisms: first, methylation can physically inhibit the transcription of the gene, and second, methylation can lead to the recruitment of transcription factors that repress transcription. Both mechanisms cause the same end result: downregulation of gene expression that leads to decreased levels of the p16 protein. It has been suggested that this process is responsible for the development of various forms of cancer serving as an alternative process to gene deletion or mutation. [ 25 ] [ 26 ] [ 27 ] [ 28 ] [ 29 ] [ 30 ]
p16 positivity has been shown to be favorably prognostic in oropharyngeal squamous cell carcinoma. [ 31 ] In a retrospective trial analysis of patients with Stage III and IV oropharyngeal cancer, HPV status was assessed and it was found that the 3-year rates of overall survival were 82.4% (95% CI, 77.2 to 87.6) in the HPV-positive subgroup and 57.1% (95% CI, 48.1 to 66.1) in the HPV-negative subgroup, and the 3-year rates of progression-free survival were 73.7% (95% CI, 67.7 to 79.8) and 43.4% (95% CI, 34.4 to 52.4), respectively. p16 status is so prognostic that the AJCC staging system has been revised to include p16 status in oropharyngeal squamous cell cancer group staging. [ 32 ] However, some people can have elevated levels of p16 but test negative for HPV and vice versa. This is known as discordant cancer. The 5-year survival for people who test positive for HPV and p16 is 81%, for discordant cancer it is 53–55%, and 40% for those who test negative for p16 and HPV. [ 33 ] [ 34 ]
Expression of p16 is used as a prognostic biomarker for certain types of cancer. The reason for this is different types of cancer can have different effects on p16 expression: cancers that overexpress p16 are usually caused by the human papillomavirus (HPV), whereas cancers in which p16 is downregulated will usually have other causes. For patients with oropharyngeal squamous cell carcinoma, using immunohistochemistry to detect the presence of the p16 biomarker has been shown to be the strongest indicator of disease course. Presence of the biomarker is associated with a more favorable prognosis as measured by cancer-specific survival (CSS), recurrence-free survival (RFS), locoregional control (LRC), as well as other measurements. The appearance of hypermethylation of p16 is also being evaluated as a potential prognostic biomarker for prostate cancer. [ 35 ] [ 36 ] [ 37 ]
p16 deletion detected by FISH in surface epithelial mesothelial proliferations is predictive of underlying invasive mesothelioma . [ 38 ]
As consensus grows regarding the strength of p16 as a biomarker for detecting and determining prognoses of cancer, p16 immunohistochemistry is growing in importance. [ 13 ] [ 35 ] [ 40 ]
p16 is a widely used immunohistochemical marker in gynecologic pathology. Strong and diffuse cytoplasmic and nuclear expression of p16 in squamous cell carcinomas (SCC) of the female genital tract is strongly associated with high-risk human papilloma virus (HPV) infection and neoplasms of cervical origin. The majority of SCCs of uterine cervix express p16. However, p16 can be expressed in other neoplasms and in several normal human tissues. [ 41 ]
More than a third of urinary bladder SCCs express p16. SCCs of urinary bladder express p16 independent of gender. p16 immunohistochemical expression alone cannot be used to discriminate between SCCs arising from uterine cervix versus urinary bladder. [ 41 ]
Concentrations of p16INK4a increase dramatically as tissue ages. p16INK4a, along with senescence-associated beta-galactosidase , is regarded to be a biomarker of cellular senescence . [ 42 ] Therefore, p16INK4a could potentially be used as a blood test that measures how fast the body's tissues are aging at a molecular level. [ 43 ] Notably, a recent survey of cellular senescence induced by multiple treatments to several cell lines does not identify p16 as belonging to a "core signature" of senescence markers. [ 44 ]
It has been used as a target to delay some aging changes in mice. [ 45 ]
Increasing p16INK4a expression during aging is associated with reduced progenitor functions from the subventricular zone, which generates throughout life new neurons migrating to the olfactory bulb, thereby reducing olfactory neurogenesis. [ 46 ] Deletion of p16INK4a does not affect neurogenesis in the other adult neurogenic niche, the dentate gyrus of the hippocampus. [ 46 ] However, recently, it has been demonstrated that p16INK4a protects from depletion after a powerful proneurogenic stimulus—i.e., running— also stem and progenitor cells of the aged dentate gyrus. [ 47 ] In fact, after deletion of p16INK4a, stem cells of the dentate gyrus are greatly activated by running, while, in wild-type p16INK4a dentate gyrus stem cells are not affected by running. [ 47 ] Therefore, p16Ink4a plays a role in the maintenance of dentate gyrus stem cells after stimulus, by keeping a reserve of their self-renewal capacity during aging. Since the dentate gyrus plays a key role in spatial and contextual memory formation, p16INK4a is implicated in the maintenance of cognitive functions during aging.
Researchers Manuel Serrano, Gregory J. Hannon and David Beach discovered p16 in 1993 and correctly characterized the protein as a cyclin-dependent kinase inhibitor.
Since its discovery, p16 has become significant in the field of cancer research. The protein was suspected to be involved in carcinogenesis due to the observation that mutation or deletion in the gene was implicated in human cancer cell lines. The detection of p16 inactivation in familial melanoma supplied further evidence. p16 deletion, mutation, hypermethylation, or overexpression is now associated with various cancers. Whether mutations in p16 can be considered to be driver mutations requires further investigation. [ 17 ]
p16 has been shown to interact with: | https://en.wikipedia.org/wiki/P16 |
P1 is a temperate bacteriophage that infects Escherichia coli and some other bacteria. When undergoing a lysogenic cycle the phage genome exists as a plasmid in the bacterium [ 1 ] unlike other phages (e.g. the lambda phage ) that integrate into the host DNA. P1 has an icosahedral head containing the DNA attached to a contractile tail with six tail fibers.
The P1 phage has gained research interest because it can be used to transfer DNA from one bacterial cell to another in a process known as transduction . As it replicates during its lytic cycle it captures fragments of the host chromosome. If the resulting viral particles are used to infect a different host the captured DNA fragments can be integrated into the new host's genome. This method of in vivo genetic engineering was widely used for many years and is still used today, though to a lesser extent. P1 can also be used to create the P1-derived artificial chromosome cloning vector which can carry relatively large fragments of DNA. P1 encodes a site-specific recombinase, Cre, that is widely used to carry out cell-specific or time-specific DNA recombination by flanking the target DNA with loxP sites (see Cre-Lox recombination ).
The virion is similar in structure to the T4 phage but simpler. [ 1 ] It has an icosahedral head [ 2 ] containing the genome attached at one vertex to the tail. The tail has a tube surrounded by a contractile sheath. It ends in a base plate with six tail fibres. The tail fibres are involved in attaching to the host and providing specificity. [ 3 ]
The genome of the P1 phage is moderately large, around 93kbp [ 1 ] in length (compared to the genomes of e.g. T4 – 169kbp, lambda – 48kbp and Ff – 6.4kbp). In the viral particle it is in the form of a linear double stranded DNA molecule. Once inserted into the host it circularizes and replicates as a plasmid. [ 4 ] [ 5 ]
In the viral particle the DNA molecule is longer (110kbp) than the actual length of the genome. It is created by cutting an appropriately sized fragment from a concatemeric DNA chain having multiple copies of the genome (see the section below on lysis for how this is made). Due to this the ends of the DNA molecule are identical. This is referred to as being terminally redundant. This is important for the DNA to be circularized in the host. Another consequence of the DNA being cut out of a concatemer is that a given linear molecule can start at any location on the circular genome. This is called a cyclic permutation. [ 4 ]
The genome is especially rich in Chi sequences recognized by the bacterial recombinase RecBCD . The genome contains two origins of replication: oriR which replicates it during the lysogenic cycle and oriL which replicates it during the lytic stage. The genome of P1 encodes three tRNAs which are expressed in the lytic stage. [ 1 ]
Proteome . The genome of P1 encodes 112 proteins and 5 untranslated genes and is this about twice the size of bacteriophage lambda . [ 1 ]
The phage particle adsorbs onto the surface of the bacterium using the tail fibers for specificity. The tail sheath contracts and the DNA of the phage is injected into the host cell. The host DNA recombination machinery or the cre enzyme translated from the viral DNA recombine the terminally redundant ends and circularize the genome. Depending on various physiological cues, the phage may immediately proceed to the lytic phase or it may enter a lysogenic state. [ 5 ]
The gene that encodes the tail fibers have a set of sequences that can be targeted by a site specific recombinase Cin . This causes the C terminal end of the protein to switch between two alternate forms at a low frequency. The viral tail fibers are responsible for the specificity of binding to the host receptor. The targets of the viral tail fibers are under a constant pressure to evolve and evade binding. This method of recombinational diversity of the tail allows the virus to keep up with the bacterium. [ 6 ] This system has close sequence homologies to recombinational systems in the tail fibers of unrelated phages like the mu phage and the lambda phage .
The genome of the P1 phage is maintained as a low copy number plasmid in the bacterium. The relatively large size of the plasmid requires [ 1 ] it to keep a low copy number lest it become too large a metabolic burden while it is a lysogen. As there is usually only one copy of the plasmid per bacterial genome, the plasmid stands a high chance of not being passed to both daughter cells. [ 5 ] The P1 plasmid combats this by several methods:
The P1 plasmid has a separate origin of replication (oriL) that is activated during the lytic cycle. Replication begins by a regular bidirectional theta replication at oriL but later in the lytic phase, it switches to a rolling circle method of replication using the host recombination machinery. [ 1 ] [ 11 ] [ 12 ] This results in numerous copies of the genome being present on a single linear DNA molecule called a concatemer. The end of the concatemer is cut a specific site called the pac site or packaging site. [ 13 ] This is followed by the packing of the DNA into the heads till they are full. The rest of the concatemer that does not fit into one head is separated and the machinery begins packing this into a new head. The location of the cut is not sequence specific. Each head holds around 110kbp of DNA [ 13 ] so there is a little more than one complete copy of the genome (~90kbp) in each head, with the ends of the strand in each head being identical. After infecting a new cell this terminal redundancy is used by the host recombination machinery to cyclize the genome if it lacks two copies of the lox locus. [ 1 ] [ 13 ] If two lox sites are present (one in each terminally redundant end) the cyclization is carried out by the Cre recombinase. [ 1 ] [ 14 ]
Once the complete virions are assembled, the host cell is lysed, releasing the viral particles. [ 15 ]
P1 was discovered in 1951 by Giuseppe Bertani in Salvador Luria 's laboratory, but the phage was little studied until Ed Lennox, also in Luria's group, showed in 1954–5 that it could transduce genetic material between host bacteria. This discovery led to the phage being used for genetic exchange and genome mapping in E. coli , and stimulated its further study as a model organism. [ 1 ] [ 16 ] [ 17 ] In the 1960s, Hideo Ikeda and Jun-ichi Tomizawa showed the phage's DNA genome to be linear and double-stranded, with redundancy at the ends. In the 1970s, Nat Sternberg characterised the Cre– lox site-specific recombination system, which allows the linear genome to circularise to form a plasmid after infection. During the 1980s, Sternberg developed P1 as a vector for cloning large pieces of eukaryotic DNA. [ 16 ] A P1 gene map based on a partial DNA sequence was published in 1993 by Michael Yarmolinsky and Małgorzata Łobocka, and the genome was completely sequenced by Łobocka and colleagues in 2004. [ 1 ] [ 17 ] | https://en.wikipedia.org/wiki/P1_phage |
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