A kinetic study of the oxidation of indigo carmine with acidic bromate

Abstract

The oxidation kinetics of indigo carmine (disodium 3,3′-dioxobi-indolin-2,2′-ylidene-5,5′-disulphonate) with potassium bromate have been studied in aqueous sulphuric acid, by monitoring the absorbance of indigo carmine (IC) at 610 nm. The reaction involves competitive and consecutive reaction steps—an initial slow step followed by a rapid one for depletion of IC. For the initial stages the reaction order is four- first-order with respect to IC and bromate ion and second order with H⁺ ion. For the fast reaction step the studies are limited to qualitative treatment due to the complex nature of the reaction. The rate of depletion of IC increased with time and with the increase in HOBr concentration. Hypobromous acid, the reaction intermediate, may possibly compete with bromate ion for IC to give an intermediate, which is further oxidised to yield the final product, isatin-5-monosulphonic acid. The stoicheiometric ratio of IC to bromate is 3:2. The dual role of bromine ion as an inhibitor and autocatalyst in the reaction mechanism is discussed. Computer simulations were performed using the proposed mechanism. The results of the computer simulations are similar to the experimental observations.