Kinetic data for the superoxide radical (HO2⇄O−2 +H+, pK=4.8) in aqueous solution have been critically assessed. Rate constants for reactions of O−2 and HO2 with more than 300 organic and inorganic ions, molecules and other transient species have been tabulated.

Publication costs assisted by Brookhaven National LabcratoryThe reduction of nitro blue tetrazolium (NBT2+) by COf and Of has been studied by the stopped-flow method and the pulse-radiolysis technique. Both reductants form the tetrazolinyl radical, and the rate constants ares-1 and h N B p + + 02-= (5.88 f 0.12) X lo4 M-I s-'. In the absence of other reactants, the tetrazolinyl radicals (NBT',) disappear by a pH-dependent second-order disproportionation reaction to produce monoformazan (MF') and nitro blue tetrazolium (NBT2+) ions. The tetrazolinyl radical (NBT+.) has an absorption maximum at 405 nm with a pH-independent molar extinction coefficient ~4 0 5~~= 15 000 f 350 M-' cm-l. Monoformazan (MF+) has an absorption maximum at 530 nm; its molar extinction coefficient varies from E~~~~~ = 25 400 f 1200 M-' cm-l at pH 9.5-11.0 to €53onm = 12 800 f 640 M-l cm-' at pH 5.7-6.7. Mechanisms for the overall reaction are discussed.

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Abstract— The spectra and molar absorbances of the HO2 and O2‐ free radicals have been redetermined in aqueous formate solutions by pulse and stopped‐flow radiolysis as well as by 60Co 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 HO2 and O2‐: KHo2+HO2= (8.60 ± 0.62) × 105M‐1 s‐1; KHo2+O2‐= (1.02 ± 0.49) × 108M‐1 s‐1; KHo2+O2‐ < 0.35 M‐1 s‐1. For the equilibrium HO2→ O2‐+ H+ the dissociation constant is KHo2= (2.05 ± 0.39) × 10‐5M or pKHO2= 4.69 ± 0.08. G(O2‐) has been evaluated as a function of formate concentration.