Kinetics of disproportionation and pKa of bromous acid

Abstract

The kinetics of the disproportionation reaction of bromine(III), 2Br(III) → Br(I) + Br(V), was studied in phosphate buffer, in the pH range 5.9-8.0, by monitoring optical absorbance at 294 nm using stopped-flow. The dependences on aH+ and [Br(III)] were of order 1 and 2, respectivley, and no dependence on [Br-] was found. The reaction was also studied in acetate buffer, in the pH range 3.9-5.6. Within experimental error, no evidence was found for a direct reaction between two BrO2- ions. This study yields rate constants 39.1 ± 2.6 M-1 s-1 for the reaction HBrO2 + BrO2- → HOBr + BrO3- and 800 ± 100 M-1 s-1 for the reaction 2HBrO2 → HOBr + BrO3- + H+ and an equilibrium quotient for HBrO2 ⇄ H+ + BrO2- of 3.7 ± 0.9 × 10-4 M (pKa = 3.43) at ionic strength 0.06 M and 25.0 ± 0.1°C. The bromous acid dissociation constant is essentially identical to the value previously obtained from a kinetics study of the bromine(III)-I- reaction (J. Phys. Chem. 1992, 96, 6861). © 1994 American Chemical Society.