Kinetics and mechanism of oxidation of amido black by sodium N-halo-p-toluenesulfonamides in acidic medium: Spectrophotometric approach

Abstract

The kinetics of oxidative decolorization of amido black (AB, Naphthol blue black) by chloramine-T (CAT) and bromamine-T (BAT) in acidic medium has been investigated spectrophotometrically (λ_max = 618 nm) at 298 K. Kinetic runs were performed under pseudo first-order conditions of [Oxidant]_o >> [AB]_o. Under identical experimental conditions, reactions with both the oxidants follow identical kinetics with a first-order dependence on each [Oxidant]_o and [AB] and a fractional-order dependence on [HClO₄]. Stoichiometry of the reaction was found to be 1:4 and the oxidation products were identified. The reaction was studied at different temperatures and various activation parameters have been computed. Effects of p-toluenesulfonamide, halide ions, ionic strength and dielectric constant of the medium have been investigated. Reaction mixture fails to induce polymerization of acrylonitrile. The rate of oxidation of AB is about three-fold faster with BAT as compared to CAT. This may be attributed to the difference in electrophilicity of Cl⁺ and Br⁺ ions and also the Vander Waals radii of chlorine and bromine. Plausible mechanism and related rate law have been deliberated for the observed kinetics.