Kinetics and Mechanisms of Bromine Chloride Reactions with Bromite and Chlorite Ions

Chloride ion catalyzes the reactions of HOBr with bromite and chlorite ions in phosphate buffer (p[H⁺] 5 to 7). Bromine chloride is generated in situ in small equilibrium concentrations by the addition of excess Cl^- to HOBr. In the BrCl/ClO₂^- reaction, where ClO₂^- is in excess, a first-order rate of formation of ClO₂ is observed that depends on the HOBr concentration. The rate dependencies on ClO₂^-, Cl^-, H⁺, and buffer concentrations are determined. In the BrCl/BrO₂^- reaction where BrCl is in pre-equilibrium with the excess species, HOBr, the loss of absorbance due to BrO₂^- is followed. The dependencies on Cl^-, HOBr, H⁺, and HPO₄^2- concentrations are determined for the BrCl/BrO₂^- reaction. In the proposed mechanisms, the BrCl/ClO₂^- and BrCl/BrO₂^- reactions proceed by Br⁺ transfer to form steady-state levels of BrOClO and BrOBrO, respectively. The rate constant for the BrCl/ClO₂^- reaction ( ) is 5.2 × 10⁶ M^-1 s^-1 and for the BrCl/BrO₂^- reaction ( ) is 1.9 × 10⁵ M^-1 s^-1. In the BrCl/ClO₂^- case, BrOClO reacts with ClO₂^- to form two ClO₂ radicals and Br^-. However, the hydrolysis of BrOBrO in the BrCl/BrO₂^- reaction leads to the formation of BrO₃^- and Br^-.