Experimental determination of the redox potential of the superoxide radical •O2−

Abstract The redox potential of •O2− was determined based on the dependence of the electron transfer reaction from •O2− upon the known redox potential of various acceptors A (including a range of quinones, dyes and ferricyanide). The efficiencies and the rates of these electron transfer processes were determined, using the technique of pulse radiolysis, by monitoring the formation kinetics of the semiquinone radical anions at the appropriate wavelength. From the percentage efficiency versus Eo′ plot, an Eo′ value of + 0.15 ± 0.01 V at pH 7.0 and 22°C, or Eo = + 0.57 ± 0.01 V, for the • O 2 − O 2 couple was obtained. The rate k(•O2− + A → O2 + •A−) = 9.8 × 108 M−1 sec−1 where A = p-benzoquinone. The Eo value for the •HO2 radical is > 1.0 V. It was also found that hydroquinone can quantitatively reduce •O2− to H2O2, k(•O2− + QH2 → QH• + HO2−) = 1.6 ± 0.1 × 107 M−1 sec−1 at pH 7.0 and 22°C.