Preparation and Chemical Properties
Phosphotungstic acid can be prepared by the reaction of sodium tungstate, Na2WO4.2H2O, with phosphoric acid, H3PO4, acidified with hydrochloric acid, HCl.
Phosphotungstic acid solutions decompose as the pH is increased. A step-wise decomposition has been determined and the approximate compositions at various pH values are as follows:
pH principal components 1.0 3− 2.2 3−, 6−, 7− 3.5 3−, 6−, 7−, 6−, 10− 5.4 6−, 7−, 6− 7.3 9− 8.3 PO43−, WO42−
The species 7− is a lacunary, or defective Keggin ion. The 6− has a Dawson structure. At pH less than 8, the presence of ethanol or acetone stabilises the anion, 3−, reducing decomposition.
Tungstophosphoric acid is thermally stable up to 400 °C, and is more stable than the analogous silicotungstic acid, H4SiW12O40.
Large quantities of polar molecules such as pyridine are absorbed into the bulk phase and not simply on the surface. Solid state NMR studies of ethanol absorbed in the bulk phase show that both protonated dimers, ((C2H5OH)2H+) and monomers, (C2H5OH2+) are present.
Phosphotungstic acid is less sensitive to reduction than phosphomolybdic acid. Reduction with uric acid or iron(II) sulfate produces a brown coloured compound. the related silicotungstic acid when reduced forms a similar brown compound where one of the four W3 units in the Keggin structure becomes a metal-metal bonded cluster of three edge shared W(IV) octahedra.
Phosphotungstic acid is the strongest of heteropolyacids. Its conjugate base is the PW12O403− anion. Its acidity in acetic acid has been investigated and shows that the three protons dissociate independently rather than sequentially, and the acid sites are of the same strength. One estimate of the acidity is that the solid has an acidity stronger than H0 =−13.16, which would qualify the compound as a superacid. This acidic strength means that even at low pH the acid is fully dissociated.
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