Kinetics of metal oxide dissolution. Oxidative dissolution of chromium(III) oxide by potassium permanganate

Abstract

A detailed study of the oxidative dissolution of chromium(III) oxide (Cr₂O₃) in potassium permanganate is reported. The kinetics of the reaction follow an inverse-cubic rate law under both acid and alkaline conditions; no inhibition by MnO₂ product is observed. In alkaline solution, in the temperature range 43–140 °C, the rate exhibits dependences on permanganate and hydroxide concentrations which are explained in terms of a Langmuirian adsorption mechanism. The rate of dissolution is controlled by electron transfer within a surface complex in which both OH^– and MnO^–₄ are bound. Thermodynamic parameters imply chemical bond formation between permanganate and ions in the oxide surface. A study in dilute HNO₃ shows that adsorption of MnO^–₄ is also a prerequisite for reaction under acid conditions.