The oxidation reaction of amaranth (trisodium 2-hydroxy-1-(4-sulfonato-1-naphthylazo)naphthalene-3,6-disulfonate or AM(-)) by chlorine dioxide (ClO(2)) in aqueous conditions was investigated in detail. The major reaction products immediately after decolorization of AM(-) were 1,2-naphthoquinone disulfonate sodium salt and 1,4-napthalenedione. The reaction had first-order dependence on both AM(-) and ClO(2). The rate-limiting step involved the reaction between AM(-) and OH(-) ions. The role of hydroxide ion as a catalyst was established. The second-order rate constant increased with pH, from (19.8 ± 0.9) M(-1) s(-1) at pH 7.0, (97.1 ± 2.3) M(-1) s(-1) at pH 8.0 to (132.5 ± 2.8) M(-1) s(-1) at pH 9.0. In the pH range of 6.0-7.5, the catalytic constant for OH(-) ion was 4.0 × 10(9) M(-2) s(-1). The energy and entropy of activation values for the reaction were 50.0 kJ mol(-1) and -658.7 J K(-1) mol(-1), respectively. A probable reaction mechanism was elucidated and was validated by simulations.

The reaction mechanism of the oxidation of Amaranth dye (2-hydroxy-1-(4-sulfonato-1-naphthylazo) naphthalene-3,6-disulfonate) with hypochlorite under varied pH conditions was elucidated by a kinetic approach. Under excess concentration of oxidant, the reaction followed pseudo-first-order kinetics with respect to Amaranth, and the oxidation was found to occur through two competitive reactions, initiated by hypochlorite and hypochlorous acid. The reaction order with respect to both OCl(-) ion and HOCl was unity. While the latter reaction was fast, the significance of the oxidation paths depended on the relative concentration of the two oxidizing species, which was dictated by the reaction pH. The role of the H(+) ion in the reaction was established. For the hypochlorite ion and hypochlorous acid facilitated reactions, the second-order rate coefficients were 1.9 and 23.2 M(-1) s(-1), respectively. The energy parameters were E(a) = 33.7 kJ mol(-1), ΔH(‡) = 31.2 kJ mol(-1) and ΔS(‡) = -190.6 J K(-1) mol(-1) for the OCl(-) ion-driven oxidation, and E(a) = 26.9 kJ mol(-1), ΔH(‡) = 24.3 kJ mol(-1) and ΔS(‡) = -222.8 J K(-1) mol(-1) for the reaction with HOCl-initiated oxidation. The major oxidation products for both the pathways were 3,4-dihydroxy naphthalene-2,7-disulfonic sodium salt (P(1)), dichloro-1,4-naphthoquione (P(2)) and naphtha(2,3)oxirene-2, 3-dione (P(3)). On the basis of the primary salt effect and other kinetic data, the rate law for the overall reaction and probable reaction mechanism was elucidated. The proposed mechanism was validated by simulations using Simkine-2.

As an oxidant,

The kinetics of the oxidation of a triphenylmethane dye, Brilliant Blue-R (BB -Na + ), in aqueous solution by hypochlorite as a func-

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