Dicalcium phosphate

Dicalcium phosphate is the calcium phosphate with the formula CaHPO₄ and its dihydrate. The "di" prefix in the common name arises because the formation of the HPO₄^2– anion involves the removal of two protons from phosphoric acid, H₃PO₄. It is also known as dibasic calcium phosphate or calcium monohydrogen phosphate. Dicalcium phosphate is used as a food additive, and it is found in some toothpastes as a polishing agent and biomaterial.

Preparation

Dibasic calcium phosphate is produced by neutralizing calcium hydroxide with phosphoric acid, precipitating the dihydrate as a solid. At 60 °C the anhydrous form is precipitated:
To prevent degradation that would form hydroxyapatite, sodium pyrophosphate or trimagnesium phosphate octahydrate are added when, for example, dibasic calcium phosphate dihydrate is to be used as a polishing agent in toothpaste.
In a continuous process CaCl₂ can be treated with (NH₄)₂HPO₄ to form the dihydrate:
A slurry of the dihydrate is then heated to around 65–70 °C to form anhydrous CaHPO₄ as a crystalline precipitate, typically as flat diamondoid crystals, which are suitable for further processing.
Dibasic calcium phosphate dihydrate is formed in "brushite" calcium phosphate cements, which have medical applications. An example of the overall setting reaction in the formation of "β-TCP/MCPM" calcium phosphate cements is:

Structure

Three forms of dicalcium phosphate are known:

dihydrate, CaHPO₄•2H₂O, the mineral brushite
monohydrate, CaHPO₄•H₂O
anhydrous CaHPO₄, the mineral monetite. Below pH 4.8, the dihydrate and anhydrous forms of dicalcium phosphate are the most stable calcium phosphates.

The structure of the anhydrous and dihydrated forms has been determined by X-ray crystallography, and the structure of the monohydrate was determined by electron crystallography. The dihydrate as well as the monohydrate, adopt layered structures.

Uses and occurrence

Dibasic calcium phosphate is mainly used as a dietary supplement in prepared breakfast cereals, dog treats, enriched flour, and noodle products. It is also used as a tableting agent in some pharmaceutical preparations, including some products meant to eliminate body odor. Dibasic calcium phosphate is also found in some dietary calcium supplements. It is used in poultry feed. It is also used in some toothpastes as a tartar control agent.
Heating dicalcium phosphate gives dicalcium diphosphate, a useful polishing agent:
In the dihydrate form, it is found in some kidney stones and dental calculi.