The Kinetics of the Silver(i)-induced Oxidation of Chromium(iii) by Peroxodisulphate

Abstract

Chromium(III) and chromium(VI) compounds play an important role in natural oxidation processes in terrestrial and atmospheric water. During the oxidation of SO2, peroxodisulphate is formed as an intermediate. In acidic and neutral solutions, peroxodisulphate oxidizes chromium(III) very slowly. This reaction rate is markedly enhanced by silver ions, resulting in a reaction rate that allows the reaction to be studied conveniently under laboratory conditions. The kinetics of the Cr(III)/Ag(I)/S₂O₈^2– reaction system were studied as a function of different Cr(III), Ag(I) and S₂O₈^2– concentrations, temperature and pressure. The formation of Cr(VI) was observed as a first-order process at high [Cr(III)] and as a zero-order process at low [Cr(III)].Aninduction period was observed in both cases. For the first-order process, reaction rates were found to be independent of [Cr(III)], linearly dependent on [Ag⁺] and independent of [S₂O₈^2–]. The activation enthalpy (ΔH^≠) was calculated as 56 ± 5 kJ mol^–1, the activation entropy (ΔS^≠) as –136 ± 16 J K^–1 mol^–1 and the activation volume as –5.8 ± 0.7 cm³ mol^–1. At low [Cr(III)], the reaction rate was independent of [Cr(III)], linearly dependent on [S₂O₈^2–] and non-linearly dependent on [Ag⁺], reaching a limiting value at high [Ag⁺]. The activation enthalpy (ΔH^≠) was calculated as 61±5kJmol^–1, the activation entropy (ΔS^≠) as –119±15 J K^–1 mol^–1 and the activation volume as –1.7±0.1 cm³ mol^–1. A mechanism involving the reversible formation of a silver-peroxodisulphate complex that decomposes into oxidizing intermediates is proposed. The empirical observations can be adequately described by this mechanism.

Keywords: Chromium(III), peroxodisulphate, oxidation, silver.