Mechanistic aspects for the oxidation of sunset yellow dye by chloramine-T in presence of perchloric acid and in sodium hydroxide medium catalyzed by Os(VIII): A spectrophotometric kinetic approach

The kinetics of oxidation of sunset yellow (SY) by sodium-N-chloro-p-toluenesulfonamide or chloramine-T ( CAT) was studied spectrophotometrically in HClO4 and NaOH media with Os(VIII) as a catalyst in the latter medium at 298 K and 303 K, respectively. In acid medium, the experimental rate law is -d[CAT]/dt = k[CAT](0)[SY](0)[HClO4] (0.46). Alkali accelerates the rate of reaction and the rate law takes the form -d[CAT]/dt = k[CAT](0)[SY](0)[NaOH] (0.23)[OsO4] (0.84). The solvent isotope effect was studied using D2O. Benzenesulfonic acid and 1,2-naphthoquinone-6-sulfonic acid were characterized as the oxidation products of SY. Under identical set of experimental conditions in alkaline medium, Os(VIII) catalyzed reaction is about seven-fold faster than the uncatalyzed reaction. Activation parameters for the overall reaction and also with respect to catalyst have been evaluated. The observed results have been explained by plausible mechanisms and the related rate laws have been deduced. (C) 2008 Elsevier B.V. All rights reserved.