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Hydroxyapatite is a stable calcium phosphate wtih a low water solubility. It is similar to the human hard tissues in morphology and composition, i.e. the mineral component of teeth and bone. Chemically synthesized pentacalcium hydroxide tris(orthophosphate) and natural hydroxyapatite share similar properties. On contact with water, the following reaction occurs:




5 Ca2+(aq)+ 3 PO43-(aq)+ OH-(aq)


3 PO43-(aq)

+ H+




+ H+



The solubility product of Ksp is 2.91 × 10−58 (Synthetic hydroxyapatite, Ca:P ratio of 1.664) over a pH range of 4.56 - 9.67 (Bell, 1978), hydroxyapatite is sparingly soluble in water. A decrease in pH will lead to the formation of water by hydrogen ions and hydroxyl ions. The phosphate equilibrium will shift leading to the formation of HPO42- and H2PO4-. The substance will dissolve. At higher pH values the solution will be become supersaturated and mineral will precipitate. Thus the water solubility is inversely related to the pH value.

A determination of the hydrolysis of the calcium cation and the phosphate anion according to OECD guideline 111 was not conducted since both ions have no potential mechanism for further hydrolysis or degradation.

Phosphoric acid is a weak acid that does not fully dissociate in water. Salts containing the anion H2PO4¯ are weakly acidic. When dihydrogen phosphate salt is dissolved in solution, equilibria will establish among the following four species H3PO4 (phosphoric acid itself), H2PO4-1 (dihydrogen phosphate anion), HPO4-2 (hydrogen phosphate anion), and PO4-3 (phosphate anion). The tendency of H2PO4¯ ion to dissociate is greater than its tendency to hydrolyse to HPO4-2. The salts of HPO4-2 are weakly basic, and the tendency of this ion to hydrolyse is greater than its tendency to dissociate. Various phosphate ions maintain a dissociation equilibrium state and are present as dominant phosphorus species in water under the normal environmental conditions. 

H3PO4 + H2O < --- > H2PO4¯ + H3O+                    pK1 = 2.12

H2PO4¯ + H2O < --- > HPO4-2 + H3O+                   pK2 = 7.21

HPO4-2 + H2O < --- > PO4-3 + H3O+                      pK3 = 12.44

Monohydrogen and dihydrogen phosphates act as buffer over a pH range of 6 – 8 and have an important biological function in the aquatic environment.

As ions monohydrogen and dihydrogen phosphates are well soluble in water. However, they can be incorporated into either biological solids (e.g. microorganisms) or chemical precipitates and removed from water, such as during formation of insoluble aluminum hydroxide at a pH value between 6 and 8.

Key value for chemical safety assessment

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