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 to monitor electrochemical and mass transport properties during the cyclic voltammetry of selected isopoly and heteropoly anions. It is demonstrated that the oxidized form of these oxometalates (SiW12O404-, P2W18O626- and H2W12O406-) adsorb easily on gold electrodes, and this phenomenon is detected by the EQCM even at very low coverages. Conversely, in the same conditions, species associated with the first two redox couples of SiW12O404- and the first three couples of P2W18O626- are not adsorbed to an appreciable extent. The first redox couple of H2W12O406- is known to feature an ECE process. In this example, it appears that, in weakly acidic media (pH 3), the protonation of the reduced product associated with this first redox couple is fast enough to suppress any possible desorption and therefore only a global mass variation corresponding to a simple proton transfer during the ECE process is revealed.