A study of the decomposition behaviour of 12-tungstophosphate heteropolyacid in solution

In this paper, the decomposition of H3PW12O40 in aqueous solution or in mixed solutions of water-ethanol or water-acetone is investigated by potentiometric titration and P-31 NMR. Identification of the products from H3PW12O40 decomposition over a pH range of 1-12 was achieved using preparation high performance liquid chromatography (Pre-HPLC) combined with IR, UV-vis spectroscopy, and inductively coupled plasma atomic emission spectroscopy (ICP). It is found that H3PW12O40 in aqueous solution decomposes in a stepwise fashion with increasing pH, with the following solution compositions: [PW12O40](3-) (at pH similar to 1) approaches [PW12O40](3-) + [P2W21O71](6-) + [PW11O39](7-) (at pH 2.2) approaches [PW12O40](3-) + [P2W21O71](6-) + [PW11O39](7-) + [P2W18O62](6-) + [P2W19O67](10-) (at pH 3.5) approaches [P2W21O71](6-) + [PW11O39](7-) + [P2W18O62](6-) (at pH 5.4) approaches [PW9O34](9-) (at pH 7.3) approaches PO43- + WO42- (pH > 8.3). In the first stages at pH < 8, H3PW12O40 decomposes partially with removal of W=O units. In the second stage at pH > 8, tungstophosphoric completely decomposes to PO43-. In contrast, the decomposition of H3PW12O40 is reduced, or the stability of the [PW12O40](3-) anion is enhanced, in ethanol-water or acetone solution at pH < 8.