Oxidative dissolution of chromium(III) oxides by metaperiodate ions in perchloric acid

Abstract

The kinetics of oxidative dissolution of a number of different samples of chromium(III) oxide by periodate ions in 1 mol dm^–3 HClO₄ solution have been studied and the results interpreted using the inverse-cubic rate law. The metaperiodate acts as a two-electron oxidant and the overall reaction stoichiometry involves the reaction of 3 mol of periodate with 1 mol of Cr^III oxide. From a detailed study of the kinetics of dissolution the rate-determining step appears to be the reaction between an adsorbed periodate ion and its associated Cr^III oxide surface site, with inhibition by one of the reaction products, iodate, through competitive adsorption. Analysis of the kinetic data generates values for the Langmuir adsorption coefficients for periodate and iodate ions on highly hydrated Cr^III oxide of 84 ± 8 and 2600 ± 370 dm³ mol^–1, respectively. The Cr^III oxide–periodate reaction has a high overall activation energy, 82 ± 6 kJ mol^–1. The kinetics of dissolution of highly hydrated Cr^III oxide under conditions in which the simple inverse-cubic rate law function does not apply can be successfully predicted using a simple kinetic model.