Errors in the Kinetics of Disproportionation of Aqueous Superoxide

Many publications have mechanisms that include rate constants for the disproportionation of aqueous superoxide that are far from correct. The well-established rate law is rate = k_{HO2•,HO2•}[HO₂^•]² + k_{HO2•,O2•-}[HO₂^•][O₂^•-] with k_{HO2•,HO2•} = 8.3 × 10⁵ M^-1 s^-1 and k_{HO2•,O2•-} = 9.4 × 10⁷ M^-1 s^-1 at 25 °C. The relative concentrations of HO₂^• and O₂^•- are determined by the pK_a of HO₂^•, which is 4.8. At high pH, the rate of disproportionation becomes so slow that its measurement is unreliable because of catalysis by impurity metal ions. Thus, only an upper limit of 0.3 M^-1 s^-1 can be assigned to the rate constant for disproportionation via two O₂^•- radicals. However, there are many publications that use a value of 8.3 × 10⁹ M^-1 s^-1 for k_{HO2•,HO2•}. There are also several publications that use k_{HO2•,HO2•} = 1.7 × 10⁷ M^-1 s^-1. Kinetic simulations performed with these incorrect k_{HO2•,HO2•} values can produce significantly incorrect predictions of the time dependence of [HO₂^•]. Production of singlet O₂ is also erroneously invoked in several publications.