Precipitation and solubility of calcium phosphates and oxalates in the system Ca(OH) 2-H 3PO 4-H 2C 2O 4-NaCl-H 2O

Abstract

The influence of the initial reactant concentrations ( c(Ca) = 1 × 10 -4 to 1 × 10 -1 mol dm -3, c(PO 4) = 8 × 10 -4 to 5 × 10 -2 mol dm -3, c(C 2O 4) = 3×10 -4 to 2×10 -2 mol dm -3) and temperature (298 and 310 K) on the composition and morphology of the precipitates formed in the system Ca(OH) 2-H 3PO 4-H 2C 2O 4-NaCl(0.3 mol dm -3)-H 2O at initial pH 6.5 has been investigated. Precipitation diagrams are presented showing the concentration regions within which calcium oxalates (calcium oxalate monohydrate, COM, and dihydrate, COD), or mixtures of calcium oxalates and calcium phosphates (octacalcium phosphate, OCP, and calcium hydrogen phosphate dihydrate, DCPD), have been formed. The solubilities of the solid phases in this system have also been determined. It is shown that phosphate ions inhibit calcium oxalate crystal growth and retard equilibration. Calcium oxalate does not influence the precipitation and solubility of calcium phosphates. The appearance of the different calcium phosphate phases (OCP or DCPD) depends on the calcium and phosphate concentrations in solution, the pH and the temperature. The temperature effect can be explained by considering that the solubility of OCP decreases with increasing temperature.