KINETICS OF DISPROPORTIONATION AND PK(A) OF BROMOUS ACID

The kinetics of the disproportionation reaction of bromine(III), 2Br(I II) --> Br(I) + Br(V), was studied in phosphate buffer, in the pH rang e 5.9-8.0, by monitoring optical absorbance at 294 nm using stopped-fl ow. The dependences on alpha(H+) and [Br(III)] were of order 1 and 2, respectivley, and no dependence on [Br-] was found. The reaction was a lso studied in acetate buffer, in the pH range 3.9-5.6. Within experim ental error, no evidence was found for a direct reaction between two B r-2(O)- 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 2HBrO(2) --> HOBr + BrO3- + H+ and an equilib rium quotient for HBrO2 reversible arrow H+ + BrO2- of 3.7 +/- 0.9 x 1 0(-4) M (pK(a) = 3.43) at ionic strength 0.06 M and 25.0 + 0.1 degrees C. The bromous acid dissociation constant is essentially identical to the value previously obtained from a kinetics study of the bromine(II I)-I- reaction(J. Phys, Chem, 1992, 96, 6861).