A Kinetic and Mechanistic Study of the Reaction Between and Methionine: Evidence for the Formation of Water Soluble Colloidal MnO₂

Abstract

The kinetic results of the oxidation of methionine by in aqueous medium are presented. Upon mixing aqueous solutions of permanganate and methionine, a readily distinguishable brown colour appears. Under pseudo-first order conditions ([methionine] > []), the formation and decomposition of brown colour was very fast. Experiments have been made to confirm the nature of brown colour formed during the reduction of by methionine. The kinetics of both the stages (formation and decomposition of colloidal MnO₂) was investigated spectrophotometrically. Second- and pseudo-first-order conditions were used to determine the reaction rate for the formation and decomposition of colloidal MnO₂, respectively. The changes observed in the direction of the rate constant –[H⁺] profile correspond to the hydrolysis of the colloidal MnO₂. The addition of sodium fluoride has composite effect (catalytic and inhibition). The proposed mechanism and the derived rate law are consistent with the observed kinetics.

Keywords:

• Colloidal MnO₂
• kinetics
• methionine
• oxidation
• permanganate

Notes

^a [] = 1.0 × 10⁻⁴ mol dm⁻³, temp. = 25°C.