Dioxygen standard redox potentials

Fig. 6.2 The changing availability of some elements with time and as redox potentials increased due to dioxygen pressure increases. Note that it is solubility as well as standard redox potential that controls availability. NB. NH3 — N2, S2 —> S042 and Fe2+ — Fe3+ (ppt).

Dioxygen is a formidable oxidizing agent with relatively high standard redox potential under acidic conditions (2) ... 

The chemical inertness of dioxygen at first seems surprising because the transformation to water is so strongly thermodynamically favorable ( 80 kcal) (Fig. 1) (28, 29). However, on the basis of the standard redox potentials, the simplest reduction step, the one-electron step to... 

The reduction of dioxygen to water may occur via a number of intermediate states having various thermodynamic stabilities. These are compounds with hydrogen and their different ionized (deprotonated) forms. The standard redox potentials relevant to reduction to hydrogen peroxide and subsequently to water are listed in Table II [20,21]. 

TTHE COPPER(II) AND COPPER(I) IONS undergo facile redox interconversions for which the standard reduction potential is highly dependent on the nature of the ligands and coordination geometries observed (1). Thus, copper ion is a useful electron transfer or oxidation catalyst in the presence of dioxygen (02) (2-4). These properties have been put to advantage by nature, where copper-containing proteins (5-11) exist as electron... 

However, based strictly on the standard reduction potentials of the two redox couples involved, the equilibrium constant of the reaction as written would be exceedingly small. To further complicate matters, the rates of disappearance of 02 in the presence of Fe(II)TMPyP, as monitored by RRDE techniques, and the rates of disappearance of 02 in the presence of Fe(III)TMPyP, as measured by spectrophotometric methods [52], yielded very similar values. The most likely resolution of this seeming quandary invokes formation of a macrocycle-dioxygen adduct as a short-lived, albeit yet to be detected, intermediate. 

Free-energy changes in the aqueous redox chemistry of dioxygen. Standard state PlOj) = 1. Electrode potentials are at pH 7 those in parentheses are at unit activity. 

The standard potential of the 02/02 pair is equal to -0.15 V in water and -0.60 V in DMF. Usually, dioxygen easily captures two electrons in the stepwise reaction O2 + e —> O2 , then O2 + e 02 . In DMSO, dioxygen reductions into the superoxide ion and then into the dioxygen dianion are characterized by Ey2 = -0.5 V and Ey = -1.5 V in regard to the saturated calomel electrode (Sawyer and Gibian 1979). The superoxide ion occupies an intermediate position in the following redox triad O2 —> 02 —> In accordance with such a position, the superoxide ion... 

Standard potentials for redox reactions involving oxygen at 25°C. (From Sawyer, D.T. and Nanni, E.J., Jr., Redox chemistry of dioxygen species and their chemical reactivity, in Oxygen and Oxy-Radicals in Chemistry and Biology, Rodgers, M.A.J. and Powers, E.L., Eds., Academic Press, New York, 1981,15 44. With permission.)... 

In spite of the high standard potential of the O2/H2O redox couple ( ° = 1.23 V), oxidation of pollutants by atmospheric dioxygen is usually slow. This unfortunate effect may be explained with two facts ... 

For example, let us consider the interface between a very acidic aqueous electrolyte ipH= 1), containing neither dioxygen nor dihydrogen dissolved species, and a platinum electrode which is inert in this electrolyte. The only electroactive species present in the bulk electrolyte are protons and water. The corresponding redox couples are 02/H20,0H" and H H20/H2. The shape of the current-potential curve (represented in figure 2.25 with a dashed and dotted line) is the sum of the contributions of each couple (grey curves). These couples are supposed to be fast, and the values of the apparent standard potentials are ... 

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