Absorption spectrum and decay kinetics of O₂^- and HO₂ in aqueous solutions by pulse radiolysis

The absorption spectrum and decay kinetics of O₂^- radical ions have been investigated in H₂O₂ and H₂ + O₂ solutions by the method of pulse radiolysis. The peak absorption at 2400-2450 Å has an extinction coefficient of 1970 M^-1 cm^-1. In the pH range ∼5 to 9.7, optical absorption assigned to O₂^- was observed. The decay of O₂^- absorption is consistent with k_(HO2+O2-) = 7.9 × 10⁷ M^-1 sec^-1. and k_(O2-+O2-) < 10⁵ M^-1 sec^-1. In the pH range below 5, absorption due to HO₂ was also observed. The extinction coefficient of HO₂ at 2400 Å was measured and equals 1150 M^-1 cm^-1. Using both our own data and those of Bielski and Schwarz recently reported results, we could account for both sets of data with k_(HO2+HO2) = 6.7 × 10⁵ M^-1 sec^-1. Both kinetic and spectroscopic results indicate that the pK of HO₂ radicals may be slightly higher than the previously reported value of 4.5, namely 4.8.