Mechanism of Oxidation of Brilliant Cresyl Blue with Acidic Chlorite and Hypochlorous Acid. A Kinetic Approach

Abstract

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, k_CAT=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.