A study of 12-tungstosilicic and 12-molybdophosphoric acids in solution

The behaviour of two heteropolyacids (HPAs) with quite different stability in aqueous solutions was systematically investigated by UV, IR and NMR spectroscopy and potentiometric titration. It was shown that the Keggin structure of 12-tungstosilicic acid (H(4)SiW(12)O(40), WSiA) anion was Sustained over a wide range of pH from 1.0 to 7.0, while the same anion type of 12-molybdophosphoric acid (H(3)PMo(12)O(40), MoPA) was present only at pH 1.0. This means that under physiological conditions WSiA is dominantly present in the form of a Keggin-anion, whereas the structure of MoPA is completely decomposed to molybdate and phosphate. The obtained results are of special importance for bio-medical and catalytic applications of these compounds and for a better understanding of the mechanism of their action.