A calorimetric study of the temperature effect on Calcium Phosphate precipitation

Abstract

The precipitation of calcium phosphate was studied using a heat flow twin calorimeter (C80, Setaram, France). The process was carried out using two identical membrane vessels. In the lower parts of both vessels 2 mL of a supersaturated solution (solution A) containing Ca(H2PO4)·2H2O (0.054 M) and CaCl2·H2O (0.125 M) with a molar ratio Ca/P = 1.67 were added. Then 0.05 mL of an ammonium solution (25% w/w) (solution B) and 0.05 mL of distilled water were transferred in the upper parts of sample and reference vessels, respectively. After temperature had been maintained at 303, 313, 323 and 333 K the membranes of both sample and reference vessels were broken simultaneously. The precipitation process also repeated with the same conditions for periods of 15, 60 and 120 min in a bath. The first two calorimetric curves (303, 313 K) show a single exothermic step during the process as a sharp peak in the initial stage. On the contrary at the experimental temperature of 323 K except of the sharp peak in the initial stage, a steadily exothermic tendency appears after 100 min time. In higher temperature (333 K) the sharp peak appears in the initial stage followed by a broad exothermic step between 75 and 320 min time. The XRD analyses show that the solids in the initial experimental stages are mainly consisted of dicalcium phosphate dihydrate (DCPD) for the lower temperature and a biphasic or triphasic system consisted of hydroxyapatite (HA), dicalcium phosphate anhydrous (DCPA) and octacalcium phosphate (OCP) for the rest temperatures. The XRD analyses show also that during the solution aging the initial products are transformed into the more stable thermal forms of HA and octacalcium phosphate (OCP).