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 3 ) with 0.0266 mol•dm -3 Ca(NO3)2 and 0.0200 mol•dm -3 (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 1000m of mean long-axis length due to the heating of 0.0500 mol•dm -3 Ca(NO3)2 and 0.0300 mol•dm -3 (NH4)2HPO4 solution (Ca/P ratio=1.67) under reflux and approximately 1200m due to the heating of 0.0500 mol•dm -3 Ca(NO3)2 and 0.0250 mol•dm -3 (NH4)2HPO4 solution (Ca/P ratio=2.00)), both using 0.20 mol•dm -3 urea.
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