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Phosphotungstic acid
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<p>Phosphotungstic acid (PTA) or tungstophosphoric acid (TPA), is a <a href="/facts/Heteropoly_acid/rQyeQAYM">heteropoly acid</a> with the chemical formula <a href="/facts/Proton/clCjepXP">H</a>3<a href="/facts/Phosphorus/27xCeIqs">P</a><a href="/facts/Tungsten/WYwbNDYX">W</a>12<a href="/facts/Oxide/DXtoVe1C">O</a>40]. It forms hydrates H3[PW12O40]·<i>n</i>H2O. It is normally isolated as the <i>n</i> = 24 hydrate but can be <a href="/facts/Desiccation/X6IUsM3R">desiccated</a> to the hexahydrate (<i>n</i> = 6). EPTA is the name of <a href="/facts/Ethanol/3lCLfBSP">ethanolic</a> 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 names and acronyms (see 'other names' section of infobox). </p><p>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, (P2W24O80H6)·29H2O, which correctly identifies the atomic ratios of P, W and O. This formula was still quoted in papers as late as 1970. </p><p>Phosphotungstic acid is used in <a href="/facts/Histology/yrI90zlU">histology</a> as a component for <a href="/facts/Staining/jqGqWEE1">staining</a> of cell specimens, often together with <a href="/facts/Haematoxylin/BAXp4hTI">haematoxylin</a> as <a href="/facts/PTAH/tnfY0VXq">PTAH</a>. It binds to <a href="/facts/Fibrin/BFar47y5">fibrin</a>, <a href="/facts/Collagen/KsyAb72K">collagen</a>, and fibres of <a href="/facts/Connective_tissue/mal3fc7k">connective tissues</a>, and replaces the anions of dyes from these materials, selectively decoloring them. </p><p>Phosphotungstic acid is electron dense, opaque for <a href="/facts/Electron/L0KBo5cW">electrons</a>. It is a common <a href="/facts/Negative_stain/neI3a22v">negative stain</a> for <a href="/facts/Virus/mf88Bcbl">viruses</a>, <a href="/facts/Nerve/q7GsAfYV">nerves</a>, <a href="/facts/Polysaccharide/lqXq2cJU">polysaccharides</a>, and other biological tissue materials for imaging by a <a href="/facts/Transmission_electron_microscope/Rpr8jpCn">transmission electron microscope</a>. </p>