Kinetics and Mechanism of the Oxidation of Coomassie Brilliant Blue-R Dye by Hypochlorite and Role of Acid Therein
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 Ea=35.5 kJ mol–1,ΔHt=33.1 kJ mol–1 and ΔSt=–191.9 JK–1 mol–1 for the OCl– driven oxidation; and Ea= 26.8 kJ mol–1, ΔHt=29.3 kJ mol–1 and ΔSt=–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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