KINETICS AND MECHANISM OF THE REDUCTION OF DODECATUNGSTOCOBALTATE(III) BY ALPHA-HYDROXYACIDS IN AQUEOUS ACID-SOLUTION - EVIDENCE FOR ALKALI-METAL ION CATALYSIS

The kinetics of the redox reaction of dodecatungstocbaltate(III) [abbreviated as Co(III)W] with three alpha-hydroxycarboxylic acids: e.g., mandelic, glycolic and lactic acid, were investigated in the 0-5.4 pH range where the complex remains stable and does not decompose. All reactions are first-order both in oxidant and reductant over the experimental pH range. A specific metal ion effect has been observed. Oxidations of the molecular form of the three acids and the anionic form of mandelic acid are catalysed by alkali metal ions and follow a general rate law: -d[Co(III)W]/dt = 2(k(s) + k(c)[M+]) [R] [Co(III)W] where k(s) and k(c) account for the spontaneous and catalysed paths respectively, M+ is either an alkali metal ion (Li+, Na+ or K+) and [R], the reductant concentration. The oxidation of lactate and glycolate ions occurs only in the presence of alkali cations. The cation catalytic order: K+ > Na+ > Li+ observed in this study is in accord with the polarizability of the cations. An outer-sphere alkali cation-bridged mechanism is suggested for the electron transfer step.