Tribasic calcium phosphate
|3D model (Jmol)||Interactive image|
|Appearance||White amorphous powder|
|Melting point||Liquifies under high pressure at 1670 K (1391 °C)|
|0.002 g/100 g|
Std enthalpy of
|-4126 kcal/mol (α-form)|
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
|what is ?)(|
Tricalcium phosphate (sometimes abbreviated TCP) is a calcium salt of phosphoric acid with the chemical formula Ca3(PO4)2. It is also known as tribasic calcium phosphate and bone phosphate of lime (BPL). Calcium phosphate is one of the main combustion products of bone (see bone ash). Calcium phosphate is also commonly derived from inorganic sources such as mineral rock.
It has three crystalline polymorphs α, α' and β. The α and α' states are formed at high temperatures. As rock, it is found in Whitlockite.
Calcium phosphate refers to minerals containing calcium ions (Ca2+) together with orthophosphates (PO43−), metaphosphates or pyrophosphates (P2O74−) and occasionally hydrogen or hydroxide ions. Especially, the common mineral apatite has formula Ca5(PO4)3X, where X is F, Cl, OH, or a mixture; it is hydroxyapatite if the extra ion is mainly hydroxide. Much of the "tricalcium phosphate" on the market is actually powdered hydroxyapatite.
Preparation of pure Ca3(PO4)2
It is generally believed that tricalcium phosphate cannot be precipitated directly from aqueous solution. Typically a double decomposition reaction involving a soluble phosphate and calcium salts (e.g. (NH4)2HPO4 + Ca(NO3)2) is performed under carefully controlled pH conditions. The precipitate will either be "amorphous tricalcium phosphate", ATCP, or calcium deficient hydroxyapatite, CDHA, Ca9(HPO4)(PO4)5(OH), (note CDHA is sometimes termed apatitic calcium triphosphate). Crystalline tricalcium phosphate can be obtained by calcining the precipitate. β-Ca3(PO4)2 is generally formed, higher temperatures are required to produce α-Ca3(PO4)2.
An alternative to a wet procedure is to heat a dry mixture of a calcium phosphate and calcium carbonate which has an overall Ca/P ratio of 3:2, for example:
- CaCO3 + Ca2P2O7 → Ca3(PO4)2 + CO2
Crystal structure of β-, α- and α'- Ca3(PO4)2 polymorphs
Tricalcium phosphate has three recognised polymorphs, the rhombohedral β- form, and two high temperature forms, monoclinic α- and hexagonal α'-. β-tricalcium phosphate has a crystallographic density of 3.066 g cm−3 while the high temperature forms are less dense, α-tricalcium phosphate has a density of 2.866 g cm−3 and α'-tricalcium phosphate has a density of 2.702 g cm−3 They all have complex structures and have been described as containing "columns" of cations and anions. The β-form has two types of columns, each containing calcium and phosphate ions. The high temperature forms each have two types of columns, one containing only calcium ions and the other both calcium and phosphate.
There are differences in chemical and biological properties between the beta and alpha forms, the alpha form is more soluble and biodegradeable. Both forms are available commercially and are present in formulations used in medical and dental applications.
Tricalcium phosphate occurs naturally in several forms, including:
- as a rock in Morocco, Israel, Philippines, Egypt, and Kola (Russia) and in smaller quantities in some other countries. The natural form is not completely pure, and there are some other components like sand and lime which can change the composition. In terms of P2O5, most calcium phosphate rocks have a content of 30% to 40% P2O5 in weight.
- in the skeletons and teeth of vertebrate animals
- in milk.
Biphasic tricalcium phosphate, BCP
Biphasic tricalcium phosphate, BCP, was originally reported as tricalcium phosphate but X-Ray diffraction techniques showed that the material was an intimate mixture of two phases, hydroxyapatite, HA, and β-tricalcium phosphate. It is a ceramic. Preparation involves the sintering causing the irreversible decomposition of calcium deficient apatites alternatively termed non-stoichiometric apatites or basic calcium phosphate, an example is:
- Ca10-δ(PO4)6-δ(HPO4)δ(OH)2-δ → (1-δ)Ca10(PO4)6(OH)2 + 3δCa3(PO4)2
β-TCP can contain impurities, for example calcium pyrophosphate, CaP2O7 and apatite. β-TCP is bioresorbable. The biodegradation of BCP involves faster dissolution of the β-TCP phase followed by elimination of HA crystals. β-TCP does not dissolve in body fluids at physiological pH levels, dissolution requires cell activity producing acidic pH.
Calcium phosphate is an important raw material for the production of phosphoric acid and fertilizers, for example in the Odda process. Phosphate ore quality and quantity is often specified as percent BPL (bone phosphate of lime), where 1% BPL is equivalent to 0.458% P2O5.
As a mineral salt found in rocks and bones, it is used in cheese products.
It is also used as a nutritional supplement and occurs naturally in cow milk, although the most common and economical forms for supplementation are calcium carbonate (which should be taken with food) and calcium citrate (which can be taken without food). There is some debate about the different bioavailabilities of the different calcium salts.
It is commonly used in porcelain and dental powders, and medically as an antacid or calcium supplement.
It can be used as a tissue replacement for repairing bony defects when autogenous bone graft is not feasible or possible. It may be used alone or in combination with a biodegradable, resorbable polymer such as polyglycolic acid. It may also be combined with autologous materials for a bone graft.
Porous beta-Tricalcium phosphate scaffolds are employed as drug carrier systems for local drug delivery in bone.
Another practical application of the compound is its use in gene transfection. The calcium ions can make a cell competent to allow exogenous genes to enter the cell by diffusion. A heat shock afterwards then invokes the cell to repair itself. This is a quick and easy method for transfection, albeit a rather inefficient one.
Calcium triphosphate is used to remove fluoride from water in water filtration systems.
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