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Calcium pyrophosphate

| NFPA-H = 2 | NFPA-F = 0 | NFPA-R = 0 | NFPA-S = Sodium pyrophosphate

Calcium pyrophosphate refers to any member of a series of inorganic compound with the formula . 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 (Tα/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, ρα=2.95 g/cm3), with 8 coordinate calcium, the lower temperature form β- is tetragonal (P41, a=b=6.684Å, c=24.144Å, V=915.40Å3, Z=8, ρβ=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.

References

ja:ピロリン酸カルシウム

References

  1. {{EMedicine. article. 330936. Calcium Pyrophosphate Deposition Disease
  2. (2012). "Phosphoric Acid and Phosphates".
  3. (1965). "Electronic Paramagnetic Resonance Study of the Thermal Decomposition of Dibasic Calcium Orthophosphate". Journal of the Electrochemical Society.
  4. (1945-02-01). "Nutritive Evaluation of Defluorinated Phosphates and Other Phosphorus Supplements. I. Preparation and Properties of the Samples". Journal of AOAC International.
  5. Calvo, C.. (1968-07-01). "Crystal structure of .alpha.-calcium pyrophosphate". Inorganic Chemistry.
  6. (2000). "Growth and precipitation of a monoclinic calcium pyrophosphate tetrahydrate indicating auto-inhibition at pH7". Journal of Crystal Growth.
  7. (1988). "Calcium pyrophosphate crystal deposition disease: Preparation and characterization of crystals". Journal of Crystal Growth.
  8. (1965). "Electronic Paramagnetic Resonance Study of the Thermal Decomposition of Dibasic Calcium Orthophosphate". Journal of the Electrochemical Society.
  9. (1966). "The crystal structure of β-Ca2P2O". Acta Crystallographica.
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calcium-compounds pyrophosphate-salts phosphates
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