KINETICS AND MECHANISM OF THE COPPER(II)-CATALYZED OXIDATION OF THE IODIDE-ION BY CHROMATE(VI) ION IN AN AQUEOUS-SOLUTION

Although the chromate(VI) ions, CrO42- and HCrO4-, which are described as Cr-VI herein, could hardly oxidize the iodide ion to iodine above pH 3.0, it was catalytically oxidized by adding traces of copper(II) i on. When a copper(II) solution of 10(-7)-10(-5) mol dm(-3) was mixed w ith an iodide solution, after a fast formation of iodine, the iodine c oncentration remained constant. After such an iodine formation had occ urred, if a chromate solution was added into the reacting mixture, the iodine began to form again in accordance with a rate law of 2/3(d[I-2 ]/dt) = -d[Cr-VI]/dt = k[CuI][Cu-VI], in which [CuI] is the steady-sta te concentration in the chain cycle CuI+/CuI. The rate of the chain re action was independent of the time to add chromate, and the formation of iodine I-2 (or triiodide ion I-3(-)) stopped at the time when all o f the chromate was consumed by the chain reaction. The stoichiometry w as 3I(-) + Cr-VI --> 1.5I(2) + Cr-III. The iodine formation was extrem ely inhibited by the presence of either radical scavengers or complex- forming reagents as well as by increasing pH. Although addition of alc ohols to the reaction mixture extremely accelerated the iodine formati on before adding chromate, it hardly affected on this process after th e addition of chromate. The reaction mechanisms are presented in order to account for the observed results.