Osmium(VIII)-Catalyzed Kinetics and Mechanism of Indigo Carmine Oxidation by Chloramine-B in Basic Medium

Abstract

Indigo carmine (IC) or sodium indigotin disulfonate is a natural dye that finds applications in clinical diagnosis, chemistry and biology. The osmium(VIII)-catalyzed oxidation of IC by chloramine-B (CAB) in alkaline solutions has been spectrophotometrically monitored at the indigo carmine λmax of 610 nm at 298 K. The reaction stoichiometry has been found to be 1:4 (mol:mol), resulting in the formation of major products that are the sodium salt of sulfonated anthranilic acid (SAA) and benzenesulfonamide (BSA). The reaction shows a first-order dependence of the rate on [IC], a fractional-order dependence each on [Os(VIII)] and [OH−], and a zero-order dependence each on [CAB], [BSA], and [SAA]. The variation of the ionic strength of the reaction medium has a negligible effect on the rate. Based on the effect of temperature in the range 288–313 K, activation parameters are evaluated from Arrhenius and Eyring plots. A mechanism consistent with the observed kinetic and activation data has been proposed and a rate law has been derived.