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

The complexation of FeIII in homogeneous aqueous solution by hydrogen peroxide (H2O2) 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 H2O2 into a solution of FeIII 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 [FeIII]3+, (HO2−) (or [FeIII(HO2)]2+; I1) and [FeIII(OH)]2+, (HO2−) (or [FeIII(OH)(HO2)]+; I2), which are in acid–base equilibrium (KI1/I2=1.8×10−4 mol l−1). Equilibrium constants for the formation of I1 and I2 and the spectrum of each ferric hydroperoxy complex at various wavelengths have also been determined in this study.