Spectrophotometric study of the formation of iron(III)-hydroperoxy complexes in homogeneous aqueous solutions

Abstract

The complexation of Fe III in homogeneous aqueous solution by hydrogen peroxide (H 2O 2) was investigated spectrophotometrically. Experiments were carried out at 25°C between pH 1 and 3, with ferric perchlorate in perchloric acid/sodium perchlorate solution (ionic strength: 0.1 M). Spectrophotometric analyses showed that the addition of H 2O 2 into a solution of Fe III immediately led to the formation of species absorbing in the region 300–600 nm. Calculations made from the spectral data gave evidence of the formation of two ferric hydroperoxy complexes, formulated as [Fe III] 3+, (HO 2 −) (or [Fe III(HO 2)] 2+; I 1) and [Fe III(OH)] 2+, (HO 2 −) (or [Fe III(OH)(HO 2)] +; I 2), which are in acid–base equilibrium ( K I 1/I 2 =1.8×10 −4 mol l −1). Equilibrium constants for the formation of I 1 and I 2 and the spectrum of each ferric hydroperoxy complex at various wavelengths have also been determined in this study.