Electrochemistry of the HO2 and O2– radicals under steady state conditions. Part 2.—The effect of pH over the range 0.4 to 11

Abstract

The investigation of the electrochemistry of the HO₂ and O₂^– radicals has been extended over the pH range 0.4 to 11. The rest potentials are shown to be mixed potentials at each pH value. The radical anion O₂^– is both oxidant and reductant at pH > 8. Evidence is presented that at pH < 3, O₂^– is oxidized and HO₂ reduced at the electrode surface. The Marcus electron transfer theory has been used to establish the conditions under which these mixed potentials may be used to predict the reactivity of HO₂ and O₂^– with electrochemical couples. These conditions are realised for the Cu⁺/Cu²⁺ couple at pH 7, the superoxide dismutase enzyme, and the oxy- and methaemaglobin system; but not for the Ce^III/Ce^IV couple at pH 0.4 in which the chemically reactive species is HO₂, but electrochemically, O₂^– is oxidized and HO₂ reduced.