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Pentacalcium hydroxide tris(orthophosphate) is an inorganic phosphate. Thus biological degradation is not relevant for the substance. Pentacalcium hydroxide tris(orthophosphate) exists also naturally, known as hydroxyapatite (HAP) with the formula Ca5(PO4)3(OH), usually written Ca10(PO4)6(OH)2. Hydroxyapatite is a stable and the least soluble calcium phosphate. It is similar to the human hard tissues in morphology and composition, i.e. mineral component of teeth and bone. Chemically synthesized pentacalcium hydroxide tris(orthophosphate) and natural hydroxyapatite share the similar properties. When HAP is in contact with water, the following reaction occurs:




5 Ca2+(aq)+ 3 PO43-(aq)+ OH-(aq)


3 PO43-(aq)

+ H+




+ H+



With a solubility product Ksp is 2.91 × 10−58  (Synthetic hydroxyapatite, Ca:P ratio of 1.664) over the pH range 4.56 to 9.67 (Bell, 1978), hydroxyapatite is very poorly soluble in water. With decreasing pH the hydrogen ions remove hydroxyl ions to form water, the phosphate equilibrium shifts and HPO42-and H2PO4-will be formed. The substance will dissolve. With increasing the pH the solution will be supersaturated and mineral will precipitate. Thus, when released to the environment a distribution between water and soil is most likely whereby the water solubility is inversely related to the pH value.

Calcium and phosphorus are generally abundant natural elements that are ubiquitous in the aqueous and terrestrial environment. Calcium, the fourth most abundant element, is an essential nutrient for higher plants, algae and animals. Calcium occurs only in compound form in the environment. It is expected to adsorb to clay and organic matter in soil and thus to be relatively immobile in natural soils. However, the mobility strongly depends on the cation-exchange capacity of the soil. The calcium concentration increases with the CEC of soils. The availability of free calcium increases with soil pH. The free calcium may interact with other ions, for example phosphorus. Like calcium, phosphate is also ubiquitous in natural waters and an essential micronutrient for many organisms. Triphosphate species can be hydrolysed forming orthophosphate (PO43-) in sewerage systems, sewage treatment plants and in the environment. These same orthophosphates are also formed by natural hydrolysis of human urine and faeces, animal wastes, food and organic wastes, mineral fertilisers, bacterial recycling of organic materials in ecosystems, etc. Phosphates are bio-assimilated by the bacterial populations and the aquatic plants and algae found in these different compartments and are an essential nutrient (food element) for plants, and stimulate the growth of water plants (macrophytes) and/or algae (phytoplankton) if they represent the growth-limiting factor. 

Distribution of pentacalcium hydroxide tris(orthophosphate) to air is not likely. The substance is an inorganic solid and can thus be considered non-volatile. Due to the low vapour pressure is a significant release to the atmosphere not anticipated.

Bioaccumulation and secondary poisoning are not relevant for pentacalcium hydroxide tris(orthophosphate). Phosphate and calcium are essential micronutrients for many organisms and the uptake and concentration of the ions in organisms regulated by a number of mechanisms.


Bell, L. C., Mika, H., and Kruger, B.J. (1978) Synthetic hydroxyapatite solubility product and stoichiometry of dissolution, Arch. Oral. Biol., 23, 329–336.