Abstract:
The reduction behavior and properties of the 11-tungsto (aquocobalto(II)) silicate anion, K6[SIO4W11CoO6H2]•15 H20, and Keggin-structure 12-tungstophosphoric (V) anion, H3[PW12O40]•29H2O, upon addition of isopropyl alcohol (IPA), acetone and acetonitrile (ACN) in pH 4.99 buffer solution have been studied using an IBM Instruments EC/225 voltammetric analyzer with a dropping mercury electrode. Three-reduction waves for each heteropoly anion were observed. Thhalf-wave potentials did not change as the above organic substrates were added. However, the peak height did change upon adding organic substrate. The change in peak height of the first-wave was studied in detail. At low concentration, the first-wave height initially remains essentially constant. Above threshold values, the first-peak height decreases rapidly as the concentration of IPA, acetone, and ACN increases. Threshold values increase on going from IPA to acetone to ACN for the heteropoly anions studied. Three possibilities for relationships between the decrease of the first-wave current and the increase of substrate concentration are described. The association of the heteropoly (HPC) with organic substrate to form an association complex is considered to be the most likely possibility. The equilibrium constant (Kc) for the formation of association complexes decreases on going from IPA to acetone to ACN. The IPA, acetone or ACN molecules that form association complexes were deduced to associate at specific sites on the HPC.
