Reactions of aqueous iodine

Abstract

The rate of disappearance of iodine or triiodide was measured by mixing either OH⁻, Hg(II), H₃AsO₃, or H₂O₂ with the iodine solutions. It was expected that these species would shift the hydrolysis equilibrium, and that the rate of hydrolysis would be an upper limit for the rates of the other reactions. Kinetic measurements on the Durrum-Gibson Stopped-Flow Spectrophotometer showed instead that all these reactions were much faster than the limit set by the temperature-jump data of Eigen and Kustin on the reaction between I₂ (aq) and H₂O. Reactions of aqueous iodine with hydroxide and iodide ions and with a mercuric salt were studied with a special mixing chamber that gives a dead time of only 1.2 (10⁻³) sec. Even so, only the reaction between I₃⁻ and OH⁻ at high iodide concentrations gave measurable rates. This study of reactions between triiodide and arsenious acid indicates that the reaction is complex and may proceed by several mechanisms. At high [H⁺] the data obtained were linked to earlier results. The forward rate measured was used to test the relationship k₁/k₂ = k, and the calculated equilibrium constant agreed satisfactorily with earlier reported values. There is a transition in mechanisms as the [H⁺] is reduced..