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

The reaction mechanism involved in the iodide anion oxidation initiate d by hydrogen peroxide is relatively well known although several eleme ntary steps are not well defined. Particularly, the two reverse steps of iodine hydrolysis and the reaction of hydrogen peroxide with hypoio dous acid had to be determined. New experimental kinetic data have bee n 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 h ave established(1) the important reactivity of the basic forms (HO2- a nd/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 equilib rium are competitive with the other usual reations of the mechanism (t his competitivity had not before been taken into consideration). By co mputation of all the experimental curves we have determined the follow ing 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), a nd k(IOH+HO2-) = (1 +/- 0.2) x 10(8) M(-1) s(-1).