Revisiting the Kinetics and Mechanism of Bromate-Bromide Reaction

Abstract

The bromate-bromide reaction was investigated in an acidity range not studied yet. The reaction was followed at the Br2/Br3-isosbestic point (λ = 446 nm). It was observed a first-order behavior for bromate and bromide ions and a second-order behavior for H+ ion that results in the rate law v = k[BrO3-][Br-][H+]2. This rate law suggests a mechanism involving two successive protonation of bromate followed by the interaction of the intermediate species H2BrO3+ with bromide. These results disagree with the obtained by other authors who observed a second-order behavior for the bromide and first-order for H+, and have proposed intermediate species like H2Br2O3 and HBr2O3-. The second-order for [H+] observed in the range 0.005 ≤ [H+] ≤ 2.77 mol L-1 sets down that the pKa of bromic acid, HBrO3, must be lower than -0.5 (T = 25°C), different from all other values for this pKa proposed in the literature.