Kinetics of stepwise oxidation of manganese(II) by peroxodisulphate in phosphoric acid medium

Summary

The oxidation of Mn^II by S₂O₈ ²⁻ to Mn^VII in phosphoric acid medium proceeds via a stable Mn^III and Mn^IV species. The reaction is catalysed by Ag⁺ and exhibits first order dependence on [S₂O₈ ²⁻], [Ag⁺] and, is independent of [Mn^II]. The [H⁺] has no significant effect on the reaction. It is observed that the PO₄ ³⁻ ion stabilises the transient manganese(III) and manganese(IV) species by forming a stable and soluble phosphato-complexes. The activation parameters for the two stages of oxidation, namely Mn^II →Mn^IV and Mn^IV→Mn^VII at 25° C are E_a=52 ±4 kJ mole⁻¹, ΔS^*=−57±2 JK⁻¹ mole⁻¹ and E_a =56±4 kJ mole⁻¹, ΔS^*=−44±2 JK⁻¹ mole⁻¹, respectively. A mechanism consistent with the experimental observations is proposed.