Calcium pyrophosphate

Calcium pyrophosphate

File:Calcium pyrophosphate.png
Names
IUPAC name Calcium diphosphate
Other names Diphosphoric acid, calcium salt (1:2) Dicalcium diphosphate Dicalcium pyrophosphate
Identifiers
CAS Number	7790-76-3 checkY
3D model (JSmol)	Interactive image
ChEBI	CHEBI:32598 checkY
ChemSpider	23034 checkY
ECHA InfoCard
E number
MeSH	Calcium+pyrophosphate
PubChem CID	24632
UNII	X69NU20D19 checkY
CompTox Dashboard (EPA)	{{#property:P3117}}
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 checkY Key: JUNWLZAGQLJVLR-UHFFFAOYSA-J checkY 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). checkY verify (what is checkY☒N ?) Infobox references

Calcium pyrophosphate refers to any member of a series of inorganic compound with the formula Ca₂P₂O₇(H₂O)_n. They are white solids that are insoluble in water. They contain the pyrophosphate anion, although sometimes they are referred to as phosphates. The inventory includes an anhydrous form, a dihydrate (Ca₂P₂O₇·2H₂O), and a tetrahydrate (Ca₂P₂O₇·4H₂O). Deposition of dihydrate crystals in cartilage is 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 because of its insolubility and nonreactivity toward fluoride.

Preparation

Crystals of the tetrahydrate can be prepared by treating a solution of sodium pyrophosphate with calcium nitrate with careful control of pH and temperature:

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:

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

The anhydrous forms can be prepared by heating dicalcium phosphate:

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.

Structure of anhydrous and hydrated forms

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 three polymorphs: α-, β-, and metastable γ (T_α/β=1140ºС). 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, two that are 7 coordinate, one 8 and one 9. In both the pyrophosphates are essentially eclipsed.