Kinetics of orthoperiodate dimerisation studied by temperature-jump spectrophotometry

Abstract

Relaxation spectra have been obtained for the formation and decomposition of H₂I₂O₁₀^4– at 25.0 ± 0.2 °C and I= 0.25 ± 0.01 mol dm^–3(Na₂[SO₄]) in the pH range 9.1–9.6 by the temperature-jump method. The observed relaxation times (in the range 50–200 µs, depending on concentration and pH) could be fitted to a scheme involving dimerisation reactions of H₃IO₆^2– with itself (k_f), and with H₂IO₆^3–(k_f′). The values (dm² mol^–1 s^–1) determined by least-squares analysis of the pH dependence are: for 2H₃IO₆^2–, k_f=(1.54 ± 0.04)× 10⁵; and for H₃IO₆^2–+ H₂IO₆^3–, k_f′⩽(6.94 ± 0.50)× 10⁶. The value of k_f′ is close to the diffusion-control limit. The results are interpreted in terms of a nucleophilic mechanism in which there is attack on the iodine by –O– followed by elimination.