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+kb[B–][H+]2[IO3–][I–]2/(1 +k′[I–]). At 25° and I= 1·00M in a perchlorate medium, k0= 3·0 × 108M–4 sec.–1 and Ea= 5·0 kcal. mole–1. The values of kb(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; HPO42–, 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.