Mechanism of Oxidation of Brilliant Cresyl Blue with Acidic Chlorite and Hypochlorous Acid. A Kinetic Approach
The kinetics and mechanism of oxidation of brilliant cresyl blue (7-amino-3-diethylamino-8-methyl-phenoxazine chloride) (BB+) by chlorite in the presence of acid is reported. Under [H+]0>[ClO2–]0>[BB+]0 conditions, the oxidation reaction followed pseudo first-order kinetics with respect to BB+. During the reaction, chlorite ion disproportionates, resulting in accumulation of chlorine dioxide. The overall reaction is third-order with first-order dependence on both H+ and ClO2– ions. The rate coefficient for the overall reaction is (0.158 ± 0.003) l2 mol–2 s–1. The stoichiometry of the reaction is 2BB+ + 7ClO2 – + 2H+ 2P + → 2CH3COOH + 4ClO2 + 3Cl– where P=7-amino-3-ethylamino-8-methyl-phenoxazine-10-N-oxide, and it varied with the initial concentrations of chlorite and acid. Near neutral pH, the hypochlorite-initiated oxidation of BB+ proceeded through two second-order pathways, one driven by OCl– ion and the other by HOCl. The latter reaction is much faster, with k=(1.26±0.04)×103 l mol–1 s–1. At low pH, the reaction was much faster and had first-order dependence on the concentrations of BB+, H+ and HOCl. Ru(III) catalysed the BB+-chlorite reaction with efficiency and the kinetics of the catalysed reaction are reported. Ru(III) had a catalytic constant, kCAT=1.2 × 106 l3 mol–3 s–1. The activation parameters for both uncatalysed and catalysed reactions were also reported. The kinetic profiles of the title reaction were computed using the proposed 11-step mechanism and Simkine software. The simulated curves agreed well with the experimental curves.
Keywords: Kinetics, reaction mechanisms, dyes, water chemistry, computer chemistry.
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