Chemistry:Pyrophosphoric acid

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Pyrophosphoric acid
Chemical structure of pyrophosphoric acid
3D model of pyrophosphoric acid
IUPAC names
Diphosphoric acid
Other names
Diphosphoric acid
3D model (JSmol)
Molar mass 177.97 g/mol
Melting point 71.5 °C (160.7 °F; 344.6 K)
Extremely soluble
Solubility Very soluble in alcohol, ether
Conjugate base Pyrophosphate
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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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 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.


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.


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.[1]

H4P2O7 + H2O ⇌ 2H3PO4

Pyrophosphoric acid is a medium strong inorganic acid.


While pyrophosporic acid is corrosive, it is not known to be otherwise toxic.[2]


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.[3]

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