Mechanism of the disproportionation of superoxide radicals

New measurements are reported on the rate of the spontaneous uncatalyzed second-order decay of the superoxide radical in aqueous solution at 23/sup 0/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 O/sub 2//sup -/ never reacts with itself but only with the HO/sub 2/ in equilibrium with it, even when the ratio HO/sub 2//O/sub 2//sup -/ is as small as 2 x 1/sup -8/; the rate constant for O/sub 2//sup -/ + O/sub 2//sup -/ is less than 0.3 M/sup -1/ s/sup -1/. The pK of HO/sub 2/ was determined as 4.75 +- 0.08, and the bimolecular rate constants were k/sub 1/ = (7.61 +- 0.55) x 10/sup 5/ M/sup -1/ s/sup -1/ for HO/sub 2/ + HO/sub 2/ and k/sub 2/ = (8.86 +- 0.43) x 10/sup 7/ M/sup -1/ s/sup -1/ for HO/sub 2/ + O/sub 2//sup -/, all in good agreement with published values. The change in the spontaneous decay of superoxide radicals with pH follows the equation k/sub obsd/ = (k/sub 1/ + k/sub 2/K/sub HO/sub 2///(H/sup +/))/(1 + K/sub HO/sub 2///(H/sup +/))/sup 2/. The optical extinction coefficients of hydrogen peroxide were determined over the pH range 6.1 to 13.8 at wavelengths from 280 to 230 nm.