Preparation of millimeter-scale fibrous octacalcium phosphate particles by homogeneous precipitation method

Preparation conditions of millimeter-scale fibrous octacalcium phosphate (Ca8H2(PO4)6·5H2O: OCP) particles by homogeneous precipitation method were examined, using Ca(NO3)2, (NH4)2HPO4, urea ((NH2)2CO; precipitant) and conc. HNO3 (precipitation inhibitor). The starting solution (200 cm) with 0.0266 mol∙dm Ca(NO3)2 and 0.0200 mol∙dm (NH4)2HPO4, i.e., the stoichiometric Ca/P ratio of OCP (=1.33), was heated under reflux at 80oC for 24 h in order to encourage the precipitation, due to the hydrolysis of urea to dissociate ammonia; however, the reaction products were not only OCP but also CaHPO4. The increase in Ca/P ratio and diluted concentrations of starting calcium phosphate solution made the formation of longer fibrous OCP particles possible, i.e., approximately 1000m of mean long-axis length due to the heating of 0.0500 mol∙dm Ca(NO3)2 and 0.0300 mol∙dm (NH4)2HPO4 solution (Ca/P ratio=1.67) under reflux and approximately 1200m due to the heating of 0.0500 mol∙dm Ca(NO3)2 and 0.0250 mol∙dm (NH4)2HPO4 solution (Ca/P ratio=2.00)), both using 0.20 mol∙dm urea. (Received Mar 26, 2017; Accepted Apr 20, 2017)