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 ± 0.4 × 10⁵ M^–1 s^–1(1.2 ± 0.05 × 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 Fe³⁺ and HO^–₂. 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 are 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 × 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 × 10⁴ s^–1(2.5 ± 0.1 × 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.