EFFECT OF PH ON THE SYSTEM I-/I-3(-)/H2O2 - APPLICATION TO IODINE HYDROLYSIS

The reaction mechanism involved in the iodide anion oxidation initiated by hydrogen peroxide is relatively well known although several elementary steps are not well defined. Particularly, the two reverse steps of iodine hydrolysis and the reaction of hydrogen peroxide with hypoiodous acid had to be determined. New experimental kinetic data have been obtained concerning I-/H2O2 ([I-](0) = 10(-2) to 5 x 10(-1)M, [H2O2](0) = 0.9 x 10(-5) to 7.4 x 10(-4) M) at different pH (4.7, 7,8 and 9), and I-/I-3(-)/H2O2 ([I-](0) = 10(-1)M, [H2O2](0) = 1.1 to 7.4 x 10(-4)M, [I-3(-)](0) = 3 x 10(-4)M) at pH 8. According to these data, we have established(1) the important reactivity of the basic forms (HO2- and/or IO-), even at pH values less than the corresponding pKa's (10.6-11.2), (2) that the two reverse reactions of iodine hydrolysis equilibrium are competitive with the other usual reations of the mechanism (this competitivity had not before been taken into consideration). By computation of all the experimental curves we have determined the following rate constants: k(I-2+OH-) = (7 +/- 0.5) x 10(7) M(-1) s(-1), k(I-+IOH) = (7.5. +/- 0.5) x 10(5) M(-1) s(-1) (K(I-2 hydrolysis) = 93), and k(IOH+HO2-) = (1 +/- 0.2) x 10(8) M(-1) s(-1).