KINETIC-STUDIES OF THE CATALYTIC OXYGEN-EXCHANGE OF CHROMATE IONS WITH WATER BY PERIODATE IONS

The oxygen exchange reaction between chromate ions and water is catalyzed by the addition of a small amount of periodate ions. The catalytic reaction has been studied by using O-18 tracer work (0-degrees-C, pH = 8-12, [Cr(VI)] = 0.075 mol dm-3, [I(VII)] = 10(-5) mol dm-3) and an O-17 NMR line broadening technique(30-degrees-C, pH = 4-6, [Cr(VI)] = 1 mol dm-3, [I(VII)] = 0.043 mol dm-3). The catalytic process has been interpreted in terms of the reversible condensation of chromate ions with periodate ions to form chromatoperiodate ions. The rate law is given as: R(c) = k1[HCrO4-][H4IO6-] + k2[HCrO4-][H3IO62-] + k3[CrO42-][H4IO6-] + k4[CrO42-][H3IO62-], where R(c) is defined as the increase in the rate of the oxygen exchange of chromate upon the addition of periodate ions. The catalytic rates were analyzed according to the rate law to obtain the values of the rate constants (k1 - k4); k1, k2, and k3 are on the order of magnitude of 10(3)-10(4) mol-1 dm-3 s-1, and k4 = 5 mol-1 dm3 s-1 at 0-degrees-C. For the reaction between H3IO62- and CrO42-, the energy and entropy of activation were obtained to be 26 kJ mol-1 and -143 J K-1 mol-1, respectively.