Abstract

The redox kinetic and mechanistic studies of the reactions of malachite green (MG⁺) and MnO₄^– were carried out in aqueous acidic medium, at a temperature of 32.5 ± 0.5°C; ionic strength, m = 0.50 mol dm^–3 (Na₂SO₄), [H⁺] = 5.00 x 10^–3 mol dm^–3 (H₂SO₄). In the stoichiometry, one mole of malachite green was consumed by one mole of MnO₄^–. The reaction is first order in both [MG⁺] and the [MnO₄^–]. The rates of redox reaction showed dependence on acid concentrations (in the acid range used). Rate equation for the reaction has been proposed as: –d[MG⁺]/ dt =  (a + b[H⁺])[MG⁺][MnO₄^–]. At [H⁺] = 5.00 x 10^–3 mol dm^–3,    the second order rate constant for the  malachite green – MnO₄^– reaction was found to be  (11.96 ± 0.13) x 10^–3 dm³ mol^–1 s^–1. The rates of reaction displayed negative salt effect. Log k₁ versus 1/D gave positive slope for the reaction. Added anions and cationscatalyzed the malachite green – MnO₄^–   reaction. Results of the Michaelis- Menten analysis gave no evidence of intermediate complex formation. Based on the results obtained experimentally, the outer sphere mechanism is proposed for the malachite green – MnO₄^– reaction.

Key words: Kinetics, mechanism, catalysis, salt effect.