Mechanism of the disproportionation of superoxide radicals

Abstract

New measurements are reported on the rate of the spontaneous uncatalyzed second-order decay of the superoxide radical in aqueous solution at 23°C. By extreme purification of reagents, and addition of small amounts of EDTA to sequester traces of catalytic metal ions, purely second-order kinetics were found up to pH 13.2. The pH effects clearly show that the ionic form O2- never reacts with itself but only with the HO2 in equilibrium with it, even when the ratio HO2/O2- is as small as 2 × 10-8; the rate constant for O2- + O2- is less than 0.3 M-1 s-1. The pK of HO2 was determined as 4.75 ± 0.08, and the bimolecular rate constants were k1 = (7.61 ± 0.55) × 105 M-1 s-1 for HO2 + HO2 and k2 = (8.86 ± 0.43) × 107 M-1 s-1 for HO2 + O2-, all in good agreement with published values. The change in the spontaneous decay of superoxide radicals with pH follows the equation kobsd = [k1 + k2KHO2/(H+)]/[1 + KHO2/(H+)]2. The optical extinction coefficients of hydrogen peroxide were determined over the pH range 6.1-13.8 at wavelengths from 280 to 230 nm.