Solubility of CaHPO4 · 2H2O and Formation of Ion Pairs in the System Ca(OH)2- H3PO4 - H2O at 37.5 °C.

Abstract

The solubility isotherm for CaHPO4 · 2H2O (DCPD) in the three-component system Ca(OH)2 - H3PO4 - H2O was determined in the pH range 3.5 to 6.8 by leaching a thermostated column of DCPD with dilute phosphoric acid solutions. In confirmatory experiments, equilibrium was approached both from super- and under-saturation by shaking DCPD with appropriate solutions. The calculated ionic activity product (Ca++) × ( HPO 4 = ), appeared to be a parabolic function of pH with a minimum near pH 5.0. The pH dependence of the ionic product could be accounted for by considering the ion pairs [CaHPO4]° and [CaH2PO4]+ as semi-empirical parameters. Under the condition of saturation with respect to DCPD, the activity of the pair [CaHPO4]° must be a constant. The activity of the species [CaH2PO4]+ was shown to vary directly with hydrogen ion activity. The activities of the two ion pairs were adjusted to give a set of pH-independent ionic activity products with a mean of 2.19±0.011 × 10-7. The stability constants for [CaHPO4]° and [CaH2PO4]+ are 5.88±0.031 × 102 and 7.49 ±0.039, respectively. Experiments were conducted to study the hydrolysis of DCPD to more basic calcium phosphates and the kinetics of these transformations is discussed. The significance of the ion pairs in human serum is considered.