Pyrophosphoric acid
Pyrophosphoric acid, also known as diphosphoric acid, is the inorganic compound with the formula HPO or, more descriptively, [(HO)P(O)]O. Colorless and odorless, it is soluble in water, diethyl ether, and ethyl alcohol. The anhydrous acid crystallizes in two polymorphs, which melt at 54.3 °C and 71.5 °C. The compound is not particularly useful, except that it is a component of polyphosphoric acid and the conjugate acid of the pyrophosphate anion. Anions, salts, and esters of pyrophosphoric acid are called pyrophosphates. Preparation. It is best prepared by ion exchange from sodium pyrophosphate or by treating lead pyrophosphate  with hydrogen sulfide. It is not prepared by dehydration of phosphoric acid. Instead, pyrophosphoric acid is produced as only one of the products. Reactions. When molten, pyrophosphoric acid rapidly forms an equilibrium mixture of phosphoric acid, pyrophosphoric acid and polyphosphoric acids. The percentage by weight of pyrophosphoric acid is around 40% and it is difficult to recrystallise from the melt. In aqueous solution pyrophosphoric acid, like all polyphosphoric acids, hydrolyses and eventually an equilibrium is established between phosphoric acid, pyrophosphoric acid, and polyphosphoric acids. Pyrophosphoric acid is a medium strong inorganic acid. Safety. While pyrophosphoric acid is corrosive, it is  not known to be otherwise toxic. History. The name pyrophosphoric acid was given by a "Mr. Clarke of Glasgow" in 1827 who is credited with its discovery following the heating to red heat of a sodium phosphate salt. It was found that phosphoric acid when heated to red heat formed pyrophosphoric acid that was readily converted to phosphoric acid by hot water.