Kinetic and mechanistic studies of indigocarmine oxidation by chloramine‐T and chlorine in acidic buffer media

Oxidations of indigocarmine (IC) by chloramine‐T (CAT) and aqueous chlorine (HOCl) in acidic buffer media, pH 2–6, have been kinetically studied at 30°C using spectrophotometry. The CAT reaction rate shows a first‐order dependence on [IC] 0 and an inverse fractional order on [ p ‐toluenesulfonamide]. The effect of [CAT] on the rate is strongly pH dependent with a variable order of 1–2 on [CAT] 0 in the pH range 6–2. The chlorine reaction rate follows first‐order in [IC] 0 and [HOCl] 0 each in the pH range 6–2. Addition of halide ions and variation of ionic strength of the medium have no influence on the reaction rate. There is a negative effect of dielectric constant of the solvent. The kinetics of the IC oxidation with CAT at pH 6 and of that with HOCl at pHs 2–6 are similar suggesting similarity in their rate determining steps. A two‐pathway mechanism for the CAT reaction and a one‐pathway mechanism for the HOCl reaction, consistent with the kinetic data, have been proposed. Activation parameters have been calculated using the Arrhenius and Erying plots. © 1995 John Wiley & Sons, Inc.