Kinetics and simulations of reaction between safranine‐ O and acidic bromate and role of bromide therein

Safranine‐ O , a dye of the phenazinium class, was found to exhibit intricate kinetics during its reaction with bromate at low pH conditions. Under conditions of excess concentrations of acid and bromate, safranine‐ O (SA + ) initially depleted very slowly ( k = (3.9 ± 0.3) × 10 −4 M −3 s −1 ) but after an induction time, the reaction occurred swiftly. Bromide exhibited a dual role in the reaction mechanism, both as an autocatalyst and as an inhibitor. The added bromide increased the initial rate of depletion of SA + , but delayed the transition to rapid reaction. The overall stiochiometric reaction was found to be 6SA + + 4 BrO 3 − = 6SP + 3N 2 O + 3H 2 O + 6H + + 4Br − , where SP is 3‐amino‐7‐oxo‐2,8‐dimethyl‐5‐phenylphenazine. The fast kinetics of the reaction between aqueous bromine and safranine‐ O ( k = (2.2 ± 0.1) × 10 3 M −1 s −1 ) are also reported in this paper A 17‐step mechanism, consistent with the overall reaction dynamics and supported by simulations, is proposed and the role of various bromo and oxybromo species is also discussed. © 2002 Wiley Periodicals, Inc. Int J Chem Kinet 34: 542–549, 2002