Abstract

NADUPALLI, Srinivasu; DASIREDDY, Venkata D.B.C.; KOORBANALLY, Neil A.  and  JONNALAGADDA, Sreekantha B.. Kinetics and mechanism of the oxidation of Coomassie Brilliant Blue-R dye by hypochlorite and role of acid therein. S.Afr.j.chem. (Online) [online]. 2015, vol.68, pp.85-92. ISSN 1996-840X.  https://doi.org/10.17159/0379-4350/2015/V68A13.

The kinetics of the oxidation of a triphenylmethane dye, Brilliant Blue-R (BB^- Na⁺), in aqueous solutionby hypochlorite as a function of pH was investigated. While the degradation of dye obeyed pseudo-first-order kinetics, the oxidation of the dye occurred through two competitive reactions facilitated by [OCl^-] and [HOCl]. Both reactions exhibited first-order dependence on [OCl^-] and [HOCl], respectively, but the hypochlorous acid initiated reaction was about ten times faster. The relative importance of the two paths rested on the pH-dependent concentrations of hypochlorite and hypochlorous acid. The overall second-order rate constants for the OCl^- and HOCl initiated reactions are 1.2 ± 0.2 M^-1 s^-1 and 22.2 ±1.2 M^-1 s^-1, respectively. The reaction energy parameters were determined as E_a = 35.5 kJ mol^-1, ∆H^‡ = 33.1 kJ mol^-1 and ∆S^‡ = -191.9 J K^-1 mol^-1 for the OCl^- driven oxidation; and E_a = 26.8 kJ mol^-1, ∆H^‡ = 29.3 kJ mol^-1 and ∆S^‡ = -204.6 J K^-1 mol^-1 for the HOCl facilitated reaction. The governing rate law and probable reaction mechanism were elucidated and validated by simulation. The three main oxidation products were 4-(4-ethoxyphenylamino)benzoic acid, 3-[(ethyl-hydroxyamino)methyl]benzene sulfonic acid and 6'-chloro-5'-hydroxy-bicyclohexylidene-2,5,2'-triene-4,4'-dione.

Keywords : Brilliant Blue-R; oxidative degradation; hypochlorite; hypochlorous acid; kinetics; simulations.

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