Kinetics of the iodate–iodide reaction: catalysis by carboxylate and phosphate ions

Abstract

The iodate–iodide reaction was investigated by a direct amperometric method of iodine determination, and catalysis by carboxylate and phosphate ions (B^–) was found to occur. The reaction rate may be expressed by v=–d[IO₃ ^–]/dt=k₀[H⁺]²[IO₃ ^–][I^–]²+k_b[B^–][H⁺]²[IO₃ ^–][I^–]²/(1 +k′[I^–]). At 25° and I= 1·00M in a perchlorate medium, k₀= 3·0 × 10⁸ M ^–4 sec.^–1 and E_a= 5·0 kcal. mole^–1. The values of k_b(M ^–5 sec.^–1) and k′(M ^–1) for the bases studied are as follows: ClCH₂CO₂ ^–, 1·7 × 10⁹, 0; H₂PO₄ ^–, 1·7 × 10⁹, 0; ClCH₂CH₂CO₂ ^–, 3·2 × 10⁹, 75; CH₃CO₂ ^–, 6·1 × 10⁹, 54; HPO₄ ^2–, 9·4 × 10⁹, 0. Previous kinetic studies have been evaluated critically, and the structure of the iodate ion in aqueous solution is discussed. The mechanism probably involves direct attack by B^– and I^– on the central iodine(V) atom, giving an increase in co-ordination number in the rate-determining step, rather than formation of IO₂ ⁺ as previously believed.