Kinetics and mechanisms of redox reactions in aqueous solutions. Part II. The reaction between chromium(VI) and iodide

Kinetics of the reaction between dichromate and iodide in acid solution have been measured under a variety of conditions. By taking account of the equilibria present in the system we find that the initial reaction is accurately of the first order in acid chromate ion and of second order in both iodide and hydrogen ion. At a relatively early stage however the rate falls below that indicated by the initial kinetics. The redox potentials for the CrVI/CrIII and l0/l–1 couples show that there can be no overall detectable position of equilibrium; and the fall seems to be due to a reversible step in the mechanism involving l2 and CrIV. A satisfactory complete rate equation is thereby obtained.