Oxidation of Ferrous Ions by Perhydroxyl Radicals

Abstract

The rate of the reaction between HO2 radicals and Fe2+ is dependent on temperature, the rate constant at 20 C being 9.1 +0.4x 105 M-1 s-1 (1.2±0.05x 106 M-1 s-1 at 25^C) and the activation energy 10.0± 1.0 kcal mol-1 (42 kJ mol-1). The initial reaction product is considered to be an outer-sphere complex between Fe31 and HO2. This hydroperoxide complex appears to react with Fe2+ in a fast reversible reaction to give a bridge compound. The absorption spectra of the two complexes have been measured. The molar extinction coefficients at 450 nm arc 140± 50 M-1 cm-1 and 1240+100 M-1 cm-1, respectively. The equilibrium constant for the formation of the bridge compound varies from 22 + 2 M-1 at 20°C to 37.5 ±0.5 M-1 at 40°C. The hydroperoxide complex dissociates unimolecularly with a rate constant at 20 or 25°C of 1.8 ±0.1 x 103 s-1, activation energy 2.1 ±0.2 kcal mol-1. The bridge compound dissociates unimolecularly with a rate constant at 20°C of 1.8±0.1 x 104 s-1 (2.5±0.1 x 104 s-1 at 25°C), activation energy 11.4± 1.1 kcal mol-1. The final product of the whole reaction is the hydrated ferric ion.