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|Section2= }} Phosphotungstic acid (PTA), tungstophosphoric acid (TPA), is a heteropoly acid with the chemical formula 31240. 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,〔''Contribution to the chemistry of phosphomolybdic acids, phosphotungstic acids and allied substances'' H Wu The Journal of Biological Chemistry 43, 1, (1920), 189〕 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.〔''On phosphotungstic staining, I'' G Quintarelli, R Zito, J.A Cifonelli The Journal of Histochemistry and Cytochemistry 19, 11, (1971, 641〕 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. ==Structure== Gouzerh〔''From Scheele and Berzelius to Müller: polyoxometalates (POMs) revisited and the "missing link" between the bottom up and top down approaches'' P. Gouzerh, M. Che; L’Actualité Chimique, 2006, 298, 9〕 summarises the historical views on the structure of phosphotungstic acid leading up to Keggin's determination of the structure as: *H7() proposed by Miolati and further developed by Rosenheim *H3() (Pauling) The structure was determined by J.F Keggin first published in 1933〔''Structure of the Molecule of 12-Phosphotungstic Acid'' J. F. Keggin, Nature 1933, 131, 908.〕 and then in 1934〔''The Structure and Formula of 12-Phosphotungstic Acid'' J.F. Keggin. Proc. Roy. Soc., A, 144, 851, 75-100 (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.〔''Dodecatungstophosphoric acid hexahydrate, (H5O2+)3(PW12O403 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Phosphotungstic acid」の詳細全文を読む スポンサード リンク
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