A kinetic study of water-soluble colloidal MnO2 formed by the reduction of permanganate by thiourea

A conventional spectrophotometric technique was used to study the oxidation of thiourea by permanganate in a dilute perchloric acid solution. Preliminary observations indicate that the reduction of permanganate by thiourea proceeds through the fast formation of colloidal MnO2 as an intermediate. Various experiments were performed to confirm the nature of intermediate(s) formed during the reduction of permanganate by thiourea. The kinetic and spectroscopic data are consistent with the formation of water-soluble colloidal MnO2. The stoichiometry was found to be 1:2 (MnO2:thiourea). The stability of water-soluble colloidal MnO2 depends upon the [H+]. The effect of total [MnO4−], [thiourea] and [H+] on the reaction rate was determined. On the basis of various observations, a mechanism is proposed for the oxidation of thiourea by permanganate.