ELECTROCHEMICAL QUARTZ-CRYSTAL MICROBALANCE - EVIDENCE FOR THE ADSORPTION OF HETEROPOLY AND ISOPOLY ANIONS ON GOLD ELECTRODES

The electrochemical quartz crystal microbalance (EQCM) has been used t o monitor electrochemical and mass transport properties during the cyc lic voltammetry of selected isopoly and heteropoly anions. It is demon strated that the oxidized form of these oxometalates (SiW12O404-, P2W1 8O626- and H2W12O406-) adsorb easily on gold electrodes, and this phen omenon is detected by the EQCM even at very low coverages. Conversely, in the same conditions, species associated with the first two redox c ouples of SiW12O404- and the first three couples of P2W18O626- are not adsorbed to an appreciable extent. The first redox couple of H2W12O40 6- is known to feature an ECE process. In this example, it appears tha t, in weakly acidic media (pH 3), the protonation of the reduced produ ct associated with this first redox couple is fast enough to suppress any possible desorption and therefore only a global mass variation cor responding to a simple proton transfer during the ECE process is revea led.