Effect of Temperature and pH on Calcium Phosphate Precipitation

Calcium orthophosphates (CaP) synthesis involves several chemical equilibria that define the phases present in the final product. From the biomaterials standpoint, it is important to gain knowledge on how synthesis parameters affect phase formation and particle size. This study evaluates the interaction between temperature (24 or 45 °C) and pH conditions (4.5, 6.5, or drifting) on CaP precipitation in terms of yield, phase formation, density, morphology, and size distribution. Calcium and phosphate solutions (Ca/P = 1.0) are mixed and kept under stirring for 3 h. The precipitate is freeze‐dried and characterized. Under drifting pH and pH 4.5, dicalcium phosphate dihydrate (DCPD, CaHPO4 × 2H2O) is the predominant phase at both temperatures; however, some samples also present peaks ascribed to dicalcium phosphate anhydrous (DCPA, CaHPO4). At pH 6.5, diffractograms reveal a mixture of low‐crystallinity DCPD and DCPA (24 °C) or low crystallinity hydroxyapatite [HAP, Ca10(OH)2(PO4)6] (45 °C). In spite of the different morphologies (plates or aggregates), particle size remains within a relatively narrow range (D50 = 12–28 µm). DCPD precipitation is favored under more acidic or drifting pH, while HAP is formed under nearly neutral pH 6.5.