Oxygen chemistry reduction potentials

The standard reduction potential of Cr " (Table 2) shows that this ion is a strong reducing agent, and Cr(II) compounds have been used as reagents in analytical chemistry procedures (26). The reduction potential also explains why Cr(II) compounds are unstable in aqueous solutions. In the presence of air, the oxidation to Cr(III) occurs by reaction with oxygen. However, Cr(II) also reacts with water in deoxygenated solutions, depending on acidity and the anion present, to produce H2 and Cr(III) (27,28). 

In addition to simple dissolution, ionic dissociation and solvolysis, two further classes of reaction are of pre-eminent importance in aqueous solution chemistry, namely acid-base reactions (p. 48) and oxidation-reduction reactions. In water, the oxygen atom is in its lowest oxidation state (—2). Standard reduction potentials (p. 435) of oxygen in acid and alkaline solution are listed in Table 14.10- and shown diagramatically in the scheme opposite. It is important to remember that if or OH appear in the electrode half-reaction, then the electrode potential will change markedly with the pH. Thus for the first reaction in Table 14.10 O2 -I-4H+ -I- 4e 2H2O, although E° = 1.229 V,... 

Figure 3, Standard reduction potentials associated with the chemistry of oxygen values in upper and lower halves of diagram refer to pH 0,0 and pH 7,0 conditions,...

The species of components present will also be affected by oxidation-reduction, and pH. For example, iron is primarily in the Fe3+ (oxidized) or the Fe2+ (reduced) state depending on the oxidation-reduction potential of the soil. Speciation, which depends, in part, on the oxygen status of soil, is of environmental concern because some species are more soluble, such as Fe2+, and are thus more biologically available than others. The occurrence of a specific species is related to the chemistry occurring in a soil, which is related to its features. Thus, large features must be taken into consideration when studying soil chemistry and when developing analytical and instrumental methods. 

This review is concerned with the quantitative aspects of metal-catalysed oxyradical reactions. As such one will find discussions of structures of metal complexes, rate constants and reduction potentials, not unlike our review of 1985 [34], Two areas related to the role of transition metals in radical chemistry and biology have been reviewed recently these are the metal-ion-catalysed oxidation of proteins [35] and the role of iron in oxygen-mediated toxicities [36]. These topics will not be discussed in detail in this review. Related to this work is a review on the role of transition metals in autoxidation reactions [37]. Additional information can be obtained from Afanas ev s two volumes on superoxide [38,39], This subject is also treated in a more general and less quantitative manner by Halliwell and Gutteridge [40],... 

Edge R, Land EJ, McGarvey D, Mulroy L and Tmscott TG (1998) Relative one-electron reduction potentials of carotenoid radical cations and the interactions of carotenoids with the vitamin E radical cation. J AmerChem Soc 120 4087-4090 Everett SA, Dennis MF, Patel KB, Maddix S, Kundu SC and Willson RL (1996) Scavengingofnitrogendioxide, thiyl, and sulphonyl free radicals by the nutritional anti-oxidant fi-carotene. J Biol Chem 271 3988-3994 Foote CS and Denny RW (1968) Chemistry of singlet oxygen. 

The quite remarkable chemistry of iron proteins is reflected by the fact that the square planar tetradentate porphyrin (XXXXVI) is coordinated to Fe in many different proteins but these molecules show such diverse behaviour as oxygen transport, electron transport and catalytic reactivity in metalloenzymes. The protein must be responsible, by providing the other ligand(s) and by general environmental (cf. solvent) effects, for the differences in reduction potential and chemical reactivity of complexes of the various iron porphyrin systems. 

Merenyi, G., Lind, J., and Eriksen, T.E., 1984. The equilibrium reaction of the luminol radical with oxygen and the one electron reduction potential of 5-aminophthalazine-l,4-dione. Journal of Physical Chemistry. 88 2320 2323. 

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