Calcium pyrophosphate

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 (Jmol)	Interactive image
ChEBI	CHEBI:32598 Y
ChemSpider	23034 Y
ECHA InfoCard	100.029.292
MeSH	Calcium+pyrophosphate
PubChem CID	24632
UNII	X69NU20D19 Y
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	0 2 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  (what is YN ?)
Infobox references

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. Ca₂P₂O₇ is commonly used as a mild abrasive agent in toothpastes.

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

The dihydrate, sometimes termed CPPD, can be formed by the reaction of pyrophosphoric acid with calcium chloride:

The anhydrous forms can be prepared by heating dicalcium phosphate:

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.

The dihydrate is triclinic, with hydrogen bonding between the two water molecules and hydrogen bonds to the O atoms on the anion. An hexagonal dihydrate has also been reported.

The anhydrous form has 3 polymorphs, α-, β-, and a metastable γ. The high temperature form α- is monoclinic, with 8 coordinate calcium, the lower temperature form β- is tetragonal, with calcium in four different coordination environments, 2 that are 7 coordinate, one eight and one 9. In both the pyrophosphates are essentially eclipsed.