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

In this paper, the decomposition of H 3 PW 12 O 40 in aqueous solution or in mixed solutions of water–ethanol or water–acetone is investigated by potentiometric titration and 31 P NMR. Identification of the products from H 3 PW 12 O 40 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 H 3 PW 12 O 40 in aqueous solution decomposes in a stepwise fashion with increasing pH, with the following solution compositions: [PW 12 O 40 ] 3– (at pH ~ 1) [PW 12 O 40 ] 3– + [P 2 W 21 O 71 ] 6– + [PW 11 O 39 ] 7– (at pH 2.2) [PW 12 O 40 ] 3– + [P 2 W 21 O 71 ] 6– + [PW 11 O 39 ] 7– + [P 2 W 18 O 62 ] 6– + [P 2 W 19 O 67 ] 10– (at pH 3.5) [P 2 W 21 O 71 ] 6– + [PW 11 O 39 ] 7– + [P 2 W 18 O 62 ] 6– (at pH 5.4) [PW 9 O 34 ] 9– (at pH 7.3) PO 4 3– + WO 4 2– (pH > 8.3). In the first stages at pH < 8, H 3 PW 12 O 40 decomposes partially with removal of W=O units. In the second stage at pH > 8, tungstophosphoric completely decomposes to PO 4 3– . In contrast, the decomposition of H 3 PW 12 O 40 is reduced, or the stability of the [PW 12 O 40 ] 3– anion is enhanced, in ethanol–water or acetone solution at pH < 8. Key words: 12-tungstophosphate heteropolyacid, decomposition behaviour, potentiometric titration, 31 P NMR, preparation high performance liquid chromatography.