KINETICS AND MECHANISM OF THE REACTION BETWEEN THYMOL BLUE AND BROMATE IN ACIDIC MEDIUM

The reaction between Thymol Blue (TB) and bromate in the presence of s ulfuric acid has been studied by monitoring the depletion kinetics of TB. The reaction exhibits complex kinetic behaviour. Under excess brom ate and acid conditions, the reaction was initially fast and was follo wed by a kinetically controlled slow reaction. After an induction peri od, the reductant again underwent a rapid depletion. The kinetic data were analysed by determining the reaction rates under the two rapid de pletion conditions and by estimating the induction time between the tw o rapid depletion stages. Under the initial conditions the rate-limiti ng step was found to be first order with respect to H+, bromate and TB . The overall potential of the reaction increased continuously with th e progress of the reaction. The concentration of bromide dropped initi ally and built up slowly reaching a maximum before decreasing rapidly with corresponding fast depletion of Thymol Blue. At low bromate conce ntration TB reacted with bromate with a stoichiometric ratio of 3:2, b ut with excess bromate the ratio increased to 4:5. The kinetics of the reaction beween Thymol Blue and aqueous bromine were monitored by a s topped-flow technique. The probable reaction mechanism with bromide ac ting as an autocatalyst is discussed.