Kinetics and Mechanism of Malachite Green Oxidation by Hypochlorite Ion in Aqueous Acidic Medium

The kinetic investigations of the redox reaction between malachite green, MG+, and hypochlorite ion, ClO–, were carried out in aqueous HClO4 medium at [H+] = 1x10–3 mol dm–3, µ = 0.1 mol dm–3, T = 25±1°C. The reaction showed a stoichiometry of 1:2, which conforms to the equation:

MG+ + 2ClO– Products

Kinetic data revealed a first order dependence on both [MG+] and [ClO-] giving a second order overall. The second order rate law for the reaction conforms to the equation:

−d[MG+]/dt = k2[MG+][ClO-]

where k2 = 115.55± 1.21 dm3 mol-1 s–1

Reaction rate increased with increase in [H+]. Changes in ionic strength and dielectric constant of the reaction medium did not alter the reaction rate. Addition of anions at different concentrations also did not affect reaction rate. The absence of spectroscopic evidence of intermediate complex formation suggests that the reaction proceeds through the outersphere mechanism. This suggestion is further reinforced by kinetic evidence from the Michaelis-Menten plot. A plausible mechanism based on these observations is proposed.