Calcium pyrophosphate

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] because of its insolubility and nonreactivity toward fluoride.[3]

Calcium pyrophosphate
Names
IUPAC name
Calcium diphosphate
Other names
Diphosphoric acid, calcium salt (1:2); Dicalcium diphosphate; Dicalcium pyrophosphate
Identifiers
3D model (JSmol)
ChEBI
ChemSpider
ECHA InfoCard 100.029.292
E number E450(vi) (thickeners, ...)
MeSH Calcium+pyrophosphate
UNII
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 (fire diamond)
Flammability code 0: Will not burn. E.g. waterHealth code 2: Intense or continued but not chronic exposure could cause temporary incapacitation or possible residual injury. E.g. chloroformReactivity code 0: Normally stable, even under fire exposure conditions, and is not reactive with water. E.g. liquid nitrogenSpecial hazards (white): no code
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

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:[4]

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:[2]

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

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

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.[4]

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.[5]

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.[6][7]

References
  1. Calcium Pyrophosphate Deposition Disease at eMedicine
  2. Ropp, R.C. (2013). "Group 15 (N, P, As, Sb and Bi) Alkaline Earth Compounds". Encyclopedia of the Alkaline Earth Compounds. pp. 199–350. doi:10.1016/B978-0-444-59550-8.00004-1. ISBN 978-0-444-59550-8.
  3. Klaus Schrödter; Gerhard Bettermann; Thomas Staffel; Friedrich Wahl; Thomas Klein; Thomas Hofmann (2012). "Phosphoric Acid and Phosphates". Ullmann's Encyclopedia of Industrial Chemistry. Weinheim: Wiley-VCH. doi:10.1002/14356007.a19_465.pub3.
  4. Christoffersen, Margaret R.; Balic-Zunic, Tonci; Pehrson, Søren; Christoffersen, Jørgen (2000). "Growth and precipitation of a monoclinic calcium pyrophosphate tetrahydrate indicating auto-inhibition at pH7". Journal of Crystal Growth. 212 (3–4): 500–506. Bibcode:2000JCrGr.212..500C. doi:10.1016/S0022-0248(00)00231-1.
  5. Mandel, Gretchen S.; Renne, Kathleen M.; Kolbach, Ann M.; Kaplan, Wayne D.; Miller, Jay D.; Mandel, Neil S. (1988). "Calcium pyrophosphate crystal deposition disease: Preparation and characterization of crystals". Journal of Crystal Growth. 87 (4): 453–462. Bibcode:1988JCrGr..87..453M. doi:10.1016/0022-0248(88)90093-0.
  6. 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.
  7. Webb, N. C. (1966). "The crystal structure of β-Ca2P2O". Acta Crystallographica. 21 (6): 942–948. doi:10.1107/S0365110X66004225.
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