Calcium pyrophosphate (Ca₂P₂O₇) is a chemical compound, an insoluble calcium salt containing the pyrophosphate anion. There are a number of forms reported: an anhydrous form, a dihydrate, Ca₂P₂O₇·2H₂O and a tetrahydrate, Ca₂P₂O₇·4H₂O. Deposition of dihydrate crystals in cartilage are responsible for the severe joint pain in cases of calcium pyrophosphate deposition disease (pseudo gout) whose symptoms are similar to those of gout.^[1] Ca₂P₂O₇ is commonly used as a mild abrasive agent in toothpastes,^{[citation needed]} because of its insolubility and nonreactivity toward fluoride.^[2]

Quick Facts Names, Identifiers ...

Calcium pyrophosphate

Names
IUPAC name Calcium diphosphate
Other names Diphosphoric acid, calcium salt (1:2) Dicalcium diphosphate Dicalcium pyrophosphate
Identifiers
CAS Number	7790-76-3 Y
3D model (JSmol)	Interactive image
ChEBI	CHEBI:32598 Y
ChemSpider	23034 Y
ECHA InfoCard	100.029.292
E number	E450(vi) (thickeners, ...)
MeSH	Calcium+pyrophosphate
PubChem CID	24632
UNII	X69NU20D19 Y
CompTox Dashboard (EPA)	DTXSID50872512
InChI InChI=1S/2Ca.H4O7P2/c;;1-8(2,3)7-9(4,5)6/h;;(H2,1,2,3)(H2,4,5,6)/q2*+2;/p-4 Y Key: JUNWLZAGQLJVLR-UHFFFAOYSA-J Y InChI=1/2Ca.H4O7P2/c;;1-8(2,3)7-9(4,5)6/h;;(H2,1,2,3)(H2,4,5,6)/q2*+2;/p-4 Key: JUNWLZAGQLJVLR-XBHQNQODAN
SMILES [Ca+2].[Ca+2].[O-]P([O-])(=O)OP([O-])([O-])=O
Properties
Chemical formula	Ca2O7P2
Molar mass	254.053 g/mol
Appearance	White powder
Density	3.09 g/cm3
Melting point	1,353 °C (2,467 °F; 1,626 K)
Solubility in water	insoluble
Solubility	soluble in HCl, nitric acids
Refractive index (nD)	1.585
Hazards
NFPA 704 (fire diamond)	2 0 0
Flash point	Non-flammable
Related compounds
Other anions	Calcium phosphate
Other cations	Magnesium pyrophosphate Sodium pyrophosphate
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa). Y verify (what is YN ?) Infobox references

Close

Crystals of the tetrahydrate can be prepared by reacting sodium pyrophosphate, Na₄P₂O₇ with calcium nitrate, Ca(NO₃)₂, at carefully controlled pH and temperature:^[3]

Na₄P₂O₇(aq)+2 Ca(NO₃)₂(aq)→ Ca₂P₂O₇·4 H₂O + 4 NaNO₃

The dihydrate, sometimes termed CPPD, can be formed by the reaction of pyrophosphoric acid with calcium chloride:^{[citation needed]}

CaCl₂ + H₄P₂O₇(aq) → Ca₂P₂O₇·2 H₂O + HCl.

The anhydrous forms can be prepared by heating dicalcium phosphate:^{[citation needed]}

2 CaHPO₄ → Ca₂P₂O₇ + H₂O

At 240-500 °C an amorphous phase is formed, heating to 750 °C forms β-Ca₂P₂O₇, heating to 1140 - 1350 °C forms the α-Ca₂P₂O₇.

The stable tetrahydrate was originally reported to be rhombohedral but is now believed to be monoclinic. Additionally there is an unstable monoclinic form.^[3]

The dihydrate is triclinic, with hydrogen bonding between the two water molecules and hydrogen bonds to the O atoms on the anion.^{[citation needed]} An hexagonal dihydrate has also been reported.^[4]

The anhydrous form has 3 polymorphs, α-, β-, and metastable γ^[5] (T_α/β=1140ºС^[6]). The high temperature form α- is monoclinic (P2₁/n, a=12.66(1)Å, b=8.542(8)Å, c=5.315(5)Å, Z=4, ρ_α=2.95 g/cm³), with 8 coordinate calcium, the lower temperature form β- is tetragonal (P4₁, a=b=6.684Å, c=24.144Å, V=915.40ų, Z=8, ρ_β=3.128 g/cm³), with calcium in four different coordination environments, 2 that are 7 coordinate, one eight and one 9. In both the pyrophosphates are essentially eclipsed.^[7][8][9]