Pyrophosphoric acid

Pyrophosphoric acid
Names
	IUPAC names Diphosphoric acid; μ-oxido-bis(dihydroxidooxidophosphorus)
	Other names Diphosphoric acid
Identifiers
CAS Number	2466-09-3 ;
3D model (JSmol)	Interactive image;
ChEBI	CHEBI:29888 ;
ChEMBL	ChEMBL1160571 ;
ChemSpider	996 ;
ECHA InfoCard	100.017.795
IUPHAR/BPS	3151;
PubChem CID	1023;
UNII	4E862E7GRQ ;
CompTox Dashboard (EPA)	DTXSID4075035 ;
	InChI InChI=1S/H4O7P2/c1-8(2,3)7-9(4,5)6/h(H2,1,2,3)(H2,4,5,6) Key: XPPKVPWEQAFLFU-UHFFFAOYSA-N ; InChI=1/H4O7P2/c1-8(2,3)7-9(4,5)6/h(H2,1,2,3)(H2,4,5,6) Key: XPPKVPWEQAFLFU-UHFFFAOYAX;
	SMILES O=P(O)(O)OP(=O)(O)O;
Properties
Chemical formula	H4P2O7
Molar mass	177.97 g/mol
Melting point	71.5 °C (160.7 °F; 344.6 K)
Solubility in water	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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	Infobox references

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.

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.[1]

H₄P₂O₇ + H₂O ⇌ 2H₃PO₄

Pyrophosphoric acid is a medium strong inorganic acid.

Safety

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

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.[3]

References

Corbridge, D. (1995). "Chapter 3: Phosphates". Studies in inorganic Chemistry vol. 20. Elsevier Science B.V. pp. 169–305. doi:10.1016/B978-0-444-89307-9.50008-8. ISBN 0-444-89307-5.
Material Safety Data Sheet: Pyrophosphoric acid MSDS www.sciencelab.com
Beck, Lewis Caleb (1834). A Manual of Chemistry: Containing a Condensed View of the Present State of the Science, with Copious References to More Extensive Treatises, Original Papers, Etc. E.W & C Skinner. p. 160. Retrieved January 30, 2015.

External links

Pyrophosphoric acid at PubChem

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[StudiesinInorganicvol20-1] Corbridge, D. (1995). "Chapter 3: Phosphates". Studies in inorganic Chemistry vol. 20. Elsevier Science B.V. pp. 169–305. doi:10.1016/B978-0-444-89307-9.50008-8. ISBN 0-444-89307-5.

[2] Material Safety Data Sheet: Pyrophosphoric acid MSDS www.sciencelab.com

[3] Beck, Lewis Caleb (1834). A Manual of Chemistry: Containing a Condensed View of the Present State of the Science, with Copious References to More Extensive Treatises, Original Papers, Etc. E.W & C Skinner. p. 160. Retrieved January 30, 2015.