Calcium pyrophosphate

Calcium pyrophosphate
Names
IUPAC name
Calcium diphosphate
Other names
Diphosphoric acid, calcium salt (1:2)
Identifiers
7790-76-3 YesY
3D model (Jmol) Interactive image
ChEBI CHEBI:32598 YesY
ChemSpider 23034 YesY
ECHA InfoCard 100.029.292
MeSH Calcium+pyrophosphate
PubChem 24632
UNII X69NU20D19 YesY
Properties
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)
insoluble
Solubility soluble in HCl, nitric acids
1.585
Hazards
NFPA 704
Flammability code 0: Will not burn. E.g., water Health code 2: Intense or continued but not chronic exposure could cause temporary incapacitation or possible residual injury. E.g., chloroform Reactivity code 0: Normally stable, even under fire exposure conditions, and is not reactive with water. E.g., liquid nitrogen Special hazards (white): no codeNFPA 704 four-colored diamond
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).
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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.[1] Ca2P2O7 is commonly used as a mild abrasive agent in toothpastes.[2]

Preparation

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) +2Ca(NO3)2(aq) → Ca2P2O7·4H2O + 2Na2NO3

The dihydrate, sometimes termed CPPD, can be formed by the reaction of pyrophosphoric acid with calcium chloride:[2]

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

The anhydrous forms can be prepared by heating dicalcium phosphate:[2]

2CaHPO4 → 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.[3]

The dihydrate is triclinic, with hydrogen bonding between the two water molecules and hydrogen bonds to the O atoms on the anion.[2] An hexagonal dihydrate has also been reported.[4]

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.[5][6]

References

  1. ↑ "Calcium Pyrophosphate Deposition Disease". Medscape.
  2. 1 2 3 4 Ropp, R.C. (2013). "Chapter 4 - Group 15 (N, P, As, Sb and Bi) Alkaline Earth Compounds". Encyclopedia of the Alkaline Earth Compounds. 1. Elsevier. doi:10.1016/B978-0-444-59550-8.00004-1. ISBN 978-0-444-59550-8. – via ScienceDirect (Subscription may be required or content may be available in libraries.)
  3. 1 2 Christoffersen, Margaret R.; Balic-Zunic, Tonci; Pehrson, Søren; Christoffersen, Jørgen (May 2000). "Growth and precipitation of a monoclinic calcium pyrophosphate tetrahydrate indicating auto-inhibition at pH7". Journal of Crystal Growth. 212 (3–4): 500–506. doi:10.1016/S0022-0248(00)00231-1. ISSN 0022-0248.  – via ScienceDirect (Subscription may be required or content may be available in libraries.)
  4. ↑ Mandel, Gretchen S.; Renne, Kathleen M.; Kolbach, Ann M. Kaplan; Wayne D., Miller; Jay D., Mandel; Neil S. (March 1983). "Calcium pyrophosphate crystal deposition disease: Preparation and characterization of crystals". Journal of Crystal Growth. 87 (4): 453–462. doi:10.1016/0022-0248(88)90093-0. ISSN 0022-0248.  – via ScienceDirect (Subscription may be required or content may be available in libraries.)
  5. ↑ Parodi, J. A.; Hickok, R. L.; Segelken, W. G.; Cooper, J. R. (1965). "Electronic Paramagnetic Resonance Study of the Thermal Decomposition of Dibasic Calcium Orthophosphate". Journal of The Electrochemical Society. 112 (7): 688. doi:10.1149/1.2423665. ISSN 0013-4651.
  6. ↑ Webb, N. C. (1966). "The crystal structure of β-Ca2P2O". Acta Crystallographica. 21 (6): 942–948. doi:10.1107/S0365110X66004225. ISSN 0365-110X.
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