Pyrophosphoric acid, also known as diphosphoric acid, is the inorganic compound with the formula H₄P₂O₇ or, more descriptively, HO)₂P(O)sub>2O. Colorless and odorless, it is soluble in

water Water (chemical formula ) is an inorganic, transparent, tasteless, odorless, and nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living organisms (in which it acts as ...

diethyl ether Diethyl ether, or simply ether, is an organic compound in the ether class with the formula , sometimes abbreviated as (see Pseudoelement symbols). It is a colourless, highly volatile, sweet-smelling ("ethereal odour"), extremely flammable li ...

, and ethyl alcohol. The anhydrous acid crystallizes in two polymorphs, which melt at 54.3 and 71.5 °C. The compound is a component of polyphosphoric acid, an important source of phosphoric acid.

Anion An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by conve ...

salt Salt is a mineral composed primarily of sodium chloride (NaCl), a chemical compound belonging to the larger class of salts; salt in the form of a natural crystalline mineral is known as rock salt or halite. Salt is present in vast quant ...

s, and

ester In chemistry, an ester is a compound derived from an oxoacid (organic or inorganic) in which at least one hydroxyl group () is replaced by an alkoxy group (), as in the substitution reaction of a carboxylic acid and an alcohol. Glycerides ...

s of pyrophosphoric acid are called pyrophosphates.

Preparation

It can be prepared by reaction of phosphoric acid with phosphoryl chloride: : → It can also be prepared by ion exchange from sodium pyrophosphate or by treating lead pyrophosphate with

hydrogen sulfide Hydrogen sulfide is a chemical compound with the formula . It is a colorless chalcogen-hydride gas, and is poisonous, corrosive, and flammable, with trace amounts in ambient atmosphere having a characteristic foul odor of rotten eggs. The under ...

Reactions

Pyrophosphoric acid is a tetraprotic acid, with four distinct pKa's: :, pKa = 0.85 :, pKa = 1.96 :, pKa = 6.60 :, pKa = 9.41 The pKa's occur in two distinct ranges because deprotonations occur on separate phosphate groups. For comparison with the pKa's for phosphoric acid are 2.14, 7.20, and 12.37. At physiological pH's, pyrophosphate exists as a mixture of doubly and singly protonated forms. When molten, pyrophosphoric acid rapidly converts to 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. Even in cold water, pyrophosphoric acid hydrolyses to phosphoric acid. All polyphosphoric acids behave similarly. :

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.

References

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