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 Gout ( ) is a form of inflammatory arthritis characterized by recurrent attacks of a red, tender, hot and swollen joint, caused by deposition of monosodium urate monohydrate crystals. Pain typically comes on rapidly, reaching maximal intensi ...

. Ca₂P₂O₇ is commonly used as a mild abrasive agent in toothpastes, because of its insolubility and nonreactivity toward fluoride. __TOC__

Preparation

Crystals of the tetrahydrate can be prepared by reacting

sodium pyrophosphate Tetrasodium pyrophosphate, also called sodium pyrophosphate, tetrasodium phosphate or TSPP, is an inorganic compound with the formula Na4P2O7. As a salt, it is a white, water-soluble solid. It is composed of pyrophosphate anion and sodium ions. Tox ...

, Na₄P₂O₇ with

calcium nitrate Calcium nitrate, also called ''Norgessalpeter'' (Norwegian salpeter), is an inorganic compound with the formula Ca(NO3)2(H2O)x. The anhydrous compound, which is rarely encountered, absorbs moisture from the air to give the tetrahydrate. Both anhy ...

, 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 Calcium chloride is an inorganic compound, a salt with the chemical formula . It is a white crystalline solid at room temperature, and it is highly soluble in water. It can be created by neutralising hydrochloric acid with calcium hydroxide. Ca ...

: :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 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=915.40Å³, 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.

References

{{DEFAULTSORT:Calcium Pyrophosphate Calcium compounds Pyrophosphates Phosphates ja:ピロリン酸カルシウム