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) x 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 x 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.