Английская Википедия:Calcium pyrophosphate

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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,Шаблон:Cn because of its insolubility and nonreactivity toward fluoride.

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:

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:Шаблон:Cn

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

The anhydrous forms can be prepared by heating dicalcium phosphate:Шаблон:Cn

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.Шаблон:Cn An hexagonal dihydrate has also been reported.

The anhydrous form has 3 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, 2 that are 7 coordinate, one eight and one 9. In both the pyrophosphates are essentially eclipsed.