Formation, characterization and stabilization of water-soluble colloidal MnO₂ in the oxidation of methionine, thiourea and thioacetamide by permanganate

Stabilization and characterisation of water soluble colloidal MnO₂ during the oxidation of sulphur-containing organic reductants “thiourea, thioactamide and methionine” by permanganate in aqueous neutral media are reported for the first time. Upon addition of permanganate to a solution of methionine, a transient species appears within the time of mixing, which is stable for several weeks. On the other hand, the transient species is unstable in the presence of thiourea and thioacetamide, respectively. The nature of manganese (IV) species present in the solution was characterized by spectrophotometric and coagulation measurements. On addition of HClO₄, there is a decrease in the absorbance of the reaction mixture. Under pseudo first-order conditions ([reductants] > [\( {\text{MnO}}^{{\text{ - }}}_{{\text{4}}} \)]), the reduction rate was very fast up to the formation of water soluble colloidal MnO₂. The effect of various parameters, such as hydrogen ion concentration, amount of \( {\text{MnO}}^{{\text{ - }}}_{{\text{4}}} \) and concentration of reductants were investigated. Mechanisms consistent with the observed results have been proposed and discussed.