Kinetics of the bromate–iodide reaction: catalysis by acetate and other carboxylate ions

Abstract

A direct amperometric method for continuous determination of iodine concentrations of the order of 10^–5 M was used to investigate the kinetics of the bromate–iodide reaction. Marked catalysis by buffer anions was discovered. The rate law was found to be v=-d[BrO₃ ^–]/dt=k₀[H⁺]²[BrO₃ ^–][I^–]+k_b[B^–][H⁺]²[BrO₃ ^–][I^–]. At 25° and ionic strength 1·00M in a perchlorate medium, k₀= 49 M ^–3 sec.^–1, and for B^–= acetate, β-chloropropionate, and chloroacetate, k_b= 5·4 × 10⁴, 1·7 × 10⁴, and 2·5 × 10³ M ^–4 sec.^–1 respectively. For acetate solutions more concentrated than 0·05M the second term predominated. The mechanism is discussed, and it is shown that the intermediate BrO₂ ⁺ is unlikely to exist. Reaction schemes are proposed involving additions and replacements on the bromate ion.