Kinetics and Mechanism of the Oxidation of Coomassie Brilliant Blue-R Dye by Hypochlorite and Role of Acid Therein

Abstract

The kinetics of the oxidation of a triphenylmethane dye, Brilliant Blue-R (BB^–Na⁺), in aqueous solution by 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±0.2M^–1 s^–1 and 22.2±1.2M^–1 s^–1, respectively. The reaction energy parameters were determined as E_a=35.5 kJ mol^–1,ΔH^t=33.1 kJ mol^–1 and ΔS^t=–191.9 JK^–1 mol^–1 for the OCl^– driven oxidation; and Ea= 26.8 kJ mol^–1, ΔH^t=29.3 kJ mol^–1 and ΔS^t=–204.6 JK^–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’-hydroxybicyclohexylidene-2,5,2’-triene-4,4’-dione.

KEYWORDS Brilliant Blue-R, oxidative degradation, hypochlorite, hypochlorous acid, kinetics, simulations.

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