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 HPO42- 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-.