Kinetics of the oxidation of iron(II) by oxygen and hydrogen peroxide in concentrated chloride solutions - A re-evaluation and comparison with the oxidation of copper(I)

The kinetics of the oxidation of iron(II) in 4 M sodium chloride solutions has been studied in order to resolve differences in the rates and rate equations from previously published studies. It has been confirmed that the rate is second order in iron(II) and first order in dissolved oxygen. The presence of trace concentrations of copper in the iron(II) solutions and catalysis by platinum metal used to monitor the solution potentials in previous studies have been experimentally identified as points of difference. The rates of oxidation of iron(II) and copper(I) by peroxide under similar conditions have been studied and the rate equations shown to be first order in both metal ion and peroxide. It is shown that peroxide is not a detectable intermediate in the reduction of oxygen by iron(II). Comparison with published data for the oxidation of copper(I) and that of iron(II) by oxygen in the presence of strongly complexing anions for iron(III) suggests that both outer-sphere and inner-sphere mechanisms are involved in the oxidation of iron(II) depending on the equilibrium for the transfer of the first electron to oxygen. This interpretation is supported by thermodynamic calculations and published kinetic data for the oxidation of iron(II) and copper(I) by the perhydroxyl radical.