THE EQUILIBRIUM BETWEEN THE OXIDATION OF HYDROGEN-PEROXIDE BY OXYGEN AND THE DISMUTATION OF PEROXYL OR SUPEROXIDE RADICALS IN AQUEOUS-SOLUTIONS IN CONTACT WITH OXYGEN

The equilibrium O-2 + HO2- + OH- = 2O(2)(.-) + H2O, which plays an imp ortant role in the chemistry of H2O2 in aqueous media, including H2O2 bleaching of wood pulps and other materials, has been overlooked in th e literature. Usually only the backward reaction is considered. We hav e calculated the equilibrium constant of this reaction from literature data on reversible electrochemical cell measurements without a liquid junction. Re-analysis of these data reveals the reason for a long-sta nding discrepancy between experiments and predictions based on thermal and equilibria data of the National Bureau of Standards: since HO2- i s in equilibrium with O-2(.-), the actual HO2- concentration is less t han that assumed. We find that the standard redox potential of the oxy gen/superoxide couple is -0.137 V, different from the widely quoted li terature value of -0.33 V in irradiated solutions. Re-evaluation of pu lse radiolysis equilibria on duroquinone-superoxide yields a redox pot ential for the O-2(g)/O-2(aq)(.-) couple of ca. - 0.13 V. The standard Gibbs energy of formation of the superoxide radical is found to be 13 .18 kJ mol(-1) (3.15 kcal mol(-1)) and that of the peroxyl radical - 1 4.69 kJ mol(-1) (-3.51 kcal mol(-1)).