Phosphotungstic acid

Phosphotungstic acid (PTA), tungstophosphoric acid (TPA), is a heteropoly acid with the chemical formula H3PW12O40. It is normally present as a hydrate. EPTA is the name of ethanolic phosphotungstic acid, its alcohol solution used in biology. It has the appearance of small, colorless-grayish or slightly yellow-green crystals, with melting point 89 °C (24 H2O hydrate). It is odorless and soluble in water (200 g/100 ml). It is not especially toxic, but is a mild acidic irritant. The compound is known by a variety of different names and acronyms (see 'other names' section of infobox). Phosphotungstic acid (PTA), tungstophosphoric acid (TPA), is a heteropoly acid with the chemical formula H3PW12O40. It is normally present as a hydrate. EPTA is the name of ethanolic phosphotungstic acid, its alcohol solution used in biology. It has the appearance of small, colorless-grayish or slightly yellow-green crystals, with melting point 89 °C (24 H2O hydrate). It is odorless and soluble in water (200 g/100 ml). It is not especially toxic, but is a mild acidic irritant. The compound is known by a variety of different names and acronyms (see 'other names' section of infobox). In these names the '12' or 'dodeca' reflects the fact that the anion contains 12 tungsten atoms. Some early workers who did not know the structure, such as Hsien Wu, called it phospho-24-tungstic acid, formulating it as 3H2O·P2O5 24WO3·59H2O, (P2W24O80H6)·29H2O, which correctly identifies the atomic ratios of P, W and O. This formula was still quoted in papers as late as 1970. Phosphotungstic acid is used in histology as a component for staining of cell specimens, often together with haematoxylin as PTAH. It binds to fibrin, collagen, and fibres of connective tissues, and replaces the anions of dyes from these materials, selectively decoloring them. Phosphotungstic acid is electron dense, opaque for electrons. It is a common negative stain for viruses, nerves, polysaccharides, and other biological tissue materials for imaging by a transmission electron microscope. Gouzerh summarises the historical views on the structure of phosphotungstic acid leading up to Keggin's determination of the structure as: The structure was determined by J.F Keggin first published in 1933 and then in 1934 and is generally known as the Keggin structure. The anion has full tetrahedral symmetry and comprises a cage of twelve tungsten atoms linked by oxygen atoms with the phosphorus atom at its centre. The picture on the right shows the octahedral coordination of oxygen atoms around the tungsten atoms, and that the surface of the anion has both bridging and terminal oxygen atoms. Further investigation showed that the compound was a hexahydrate not a pentahydrate as Keggin had proposed. 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: 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.