New aspects of the electrochemistry of heteropolyacids: Part IV. Acidity dependent cyclic voltammetric behaviour of phosphotungstic and silicotungstic heteropolyanions in water and N,N-dimethylformamide

Abstract

Cyclic voltammetry has been used to study the electrochemical reduction process of three heteropolyanions, two of them, α-SiW 12O 4− 40 and α-PW 12O 3− 40, having the Keggin structure and the third one, α-P 2W 18O 6− 62, a Dawson structure. The study was performed in strongly acidic aqueous solutions and in dimethylformamide with stepwise addition of perchloric acid. α-PW 12O 3− 40 is a very weak base whatever the medium and is hardly sensitive to protonation effects. For the other compounds, important morphological changes of the waves are observed in these electrolytes. In particular, their first voltammetric wave rise from a one-electron, diffusion-controlled process to an overall, apparently direct, two-electron transfer, when the acid concentration in water is raised. Due to solvent effects, much the same final results are obtained in dimethylformamide, except that it is not necessary to approach such extreme acid concentrations as in water to reach the two-electron process. It appears that exactly the same behaviour is observed for α-SiW 12O 4− 40 and α-P 2W 18O 6− 62, albeit at different proton to hetero-polyanion ratios. A clear difference is observed between the mechanisms of formation of the two-electron wave in acidic water and in dimethylformamide. In the former case, the second wave shifts toward positive potentials until it merges with the first one, while in the latter medium, a new wave more positive than the original ones appears and grows at their expense. The observations are explained on the basis of solvent effects and allow a comparison to be made between the acido-basic properties of these heteropolyanions and their one- or two-electron reduction products.