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

Abstract

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 cm³) 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 80°C 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 μm 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 1200 μm 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.