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Reference.org
Pyrophosphoric acid
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<p>Pyrophosphoric acid, also known as diphosphoric acid, is the inorganic compound with the formula H4P2O7 or, more descriptively, [(HO)2P(O)]2O. Colorless and odorless, it is soluble in <a href="/facts/Water_(molecule)/FFsl65l1">water</a>, <a href="/facts/Diethyl_ether/Ep20vEdj">diethyl ether</a>, and <a href="/facts/Ethyl_alcohol/3lCLfBSP">ethyl alcohol</a>. The anhydrous acid crystallizes in two <a href="/facts/Polymorphism_(materials_science)/bu2tqsB9">polymorphs</a>, which melt at 54.3 and 71.5 °C. The compound is a component of polyphosphoric acid, an important source of phosphoric acid. <a href="/facts/Anion/wrbwhF3z">Anions</a>, <a href="/facts/Salt_(chemistry)/lsSKwGpn">salts</a>, and <a href="/facts/Ester/FoUAIIw4">esters</a> of pyrophosphoric acid are called <a href="/facts/Pyrophosphate/L9FPEkhF">pyrophosphates</a>. </p>