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