Reevaluation of the Spectral and Kinetic Properties of Ho2 and O2‐ Free Radicals

Abstract— The spectra and molar absorbances of the HO 2 and O 2 ‐ free radicals have been redetermined in aqueous formate solutions by pulse and stopped‐flow radiolysis as well as by 60 Co gamma‐ray studies. The extinction coefficients at the corresponding maxima and 23°C are 225 = 1400 ± 80 M ‐1 cm ‐1 and 225 = 2350 ± 120 M ‐1 cm ‐1 respectively. Reevaluation of earlier published rate data in terms of the new extinction coefficients yielded the following rate constants for the spontaneous decay of HO 2 and O 2 ‐ : K Ho2+HO2 = (8.60 ± 0.62) × 10 5 M ‐1 s ‐1 ; K Ho2+O2‐ = (1.02 ± 0.49) × 10 8 M ‐1 s ‐1 ; K Ho2+O2‐ < 0.35 M ‐1 s ‐1 . For the equilibrium HO 2 → O 2 ‐ + H + the dissociation constant is K Ho2 = (2.05 ± 0.39) × 10 ‐5 M or p K HO2 = 4.69 ± 0.08. G (O 2 ‐ ) has been evaluated as a function of formate concentration.