Phosphotungstic acid

Phosphotungstic acid (PTA) or tungstophosphoric acid (TPA), is a heteropoly acid with the chemical formula . It forms hydrates . It is normally isolated as the n = 24 hydrate but can be desiccated to the hexahydrate (n = 6). 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 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 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 called it phospho-24-tungstic acid, formulating it as 3H2O·P2O5 24WO3·59H2O, , 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: proposed by Miolati and further developed by Rosenheim (Pauling) 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.