Kinetics and mechanism of the reaction between Thymol Blue and bromate in acidic medium

Abstract

The reaction between Thymol Blue (TB) and bromate in the presence of sulfuric acid has been studied by monitoring the depletion kinetics of TB. The reaction exhibits complex kinetic behaviour. Under excess bromate and acid conditions, the reaction was initially fast and was followed by a kinetically controlled slow reaction. After an induction period, the reductant again underwent a rapid depletion. The kinetic data were analysed by determining the reaction rates under the two rapid depletion conditions and by estimating the induction time between the two rapid depletion stages. Under the initial conditions the rate-limiting step was found to be first order with respect to H⁺, bromate and TB. The overall potential of the reaction increased continuously with the progress of the reaction. The concentration of bromide dropped initially and built up slowly reaching a maximum before decreasing rapidly with corresponding fast depletion of Thymol Blue. At low bromate concentration TB reacted with bromate with a stoichiometric ratio of 3 : 2, but with excess bromate the ratio increased to 4 : 5. The kinetics of the reaction beween Thymol Blue and aqueous bromine were monitored by a stopped-flow technique. The probable reaction mechanism with bromide acting as an autocatalyst is discussed.