ABSTRACT
The elucidation of the mechanisms of precipitation in solutions containing calcium and phosphate ions provides a challenge to the chemist not only because of the need to calculate the activities of ionic species in the solution phase but also because of the numerous calcium and phosphate phases that may be involved in the crystallization reactions. At least 5 sparingly soluble crystalline phases have been characterized, including dicalcium phosphate dihydrate (DCPD, CaHPO4·2H2O), dicalcium phosphate (DCPA, CaHPO4), tricalcium phosphates (α and β TCP, Ca3(PO4)2), octacalcium phosphate (OCP, Ca4H(PO4)3·2.5H2O) and calcium hydroxyapatite (HAP, Ca5(OH)(PO4)3). 1 Typical solubility isotherms as a function of pH are given in Fig. 1. Although the thermodynamically most stable phase is HAP, it is now generally accepted that the other phases, including defect apatites may participate in the overall precipitation process. 2–4 In the spontaneous precipitation of calcium phosphate in highly supersaturated solutions, the formation of apatite is usually preceded by the precipitation of one or more precursor phases including amorphous calcium phosphate (ACP) in which no long range order can be detected by X-ray diffraction studies. 5 The composition of this phase appears to depend upon the precipitation conditions and its formation may be followed by the nucleation of OCP which serves as a template for HAP growth. 6 In biological mineralization and demineralization, DCPD, OCP and HAP have been shown to be actively involved. 7 It can be seen in Fig. 1 that as the acidity of the solution is increased, OCP or DCPD may be the appropriate precursor phase in accordance with Ostwald’s Rule of Stages which states that the least stable salt with the highest solubility will always form first in a sequential precipitation reaction. Solubility isotherms of calcium phosphate phases. Logarithmic products of total molar calcium concentration plotted against pH of the saturated solutions. (Equimolar calcium and phosphate; ionic strength 0.10 molL<sup>−1</sup>) https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9780203751367/af122379-0bf8-4e0b-aa88-63eca6dfbd14/content/fig4_1.tif"/>
