Kinetics and mechanism of the alkali metal ions promoted electron transfer between 12‐tungstocobaltate(III) and citric acid

Abstract

AbstractThe kinetics of the interactions of the Keggin type heteropolyanion [CoW12O40]5−, with all the molecular and anionic forms of citric acid have been investigated spectrophotometrically in the pH range 0.60–5.45 at 60°C. With the exception of the molecular form of the acid (H3L), all the other species (H2L−, HL2−, and L3−) undergo oxidation through an alkali metal ion catalyzed path in addition to the uncatalyzed one. The catalytic power increases with increasing size of the alkali metal ion. The contributions of each path have been evaluated from the kinetic data with the help of the measured dissociation constant values of citric acid. A plausible mechanism involving the formation of a bridged outer‐sphere complex of the reacting species with the alkali cation, has been suggested. The observed limiting dependence of kobs on [citrate] at high concentrations owing to an equilibrium between the reactants preceeding the electron transfer step supports the proposed mechanism. The pH‐rate profile gives an excellent fit with the evaluated kinetic parameters.