Ruthenium(III) Catalyzed Oxidation of Indigo Carmine by Manganese(III) in Sulfuric Acid Medium: A Kinetic and Mechanistic Study

Aims: Reactions of manganese(III)-porphyrins have been reported as possible models for closely related and biologically significant systems. Method: In this study, a stock solution of manganese(III) sulfate was prepared using a standard method of anodic oxidation of manganese(II) in H2SO4. The Indigo carmine (IC)-Mn(III) reaction catalyzed by Ru(III), under pseudo-first-order conditions, has been spectrophotometrically monitored at the IC λmax IC (610 nm) at constant temperature. The experimental rate law for the reaction is: rate = k' [IC][H+]x[Ru(III)]y, where x and y are fractional orders. Objective: The reaction rate shows a zero-order dependence on the concentration of the oxidant, Mn(III), indicating its involvement in fast steps following the slow step. Additionally, the effect on the rate of adding the reduction product, Mn(II), has been found to be negligible. Variations of the ionic strength and the dielectric constant of the reaction medium have negligible effect on the rate. Result: Based on the effect of temperature, activation parameters have been evaluated using Arrhenius and Erying plots. A suitable mechanism has been presented. The rate law derived is consistent with experimental data.