Effects of Various Ions on the Synthesis of Strontium Phosphates Obtained by Homogeneous Precipitation Method

Chiharu KATO*, Kazumi FUJITA and Keizo MATSUDA

Department of Chemistry, School of Science, Tokai University; 1117 Kitakaname, Hiratsuka-shi 259-1292 Japan

Two processes are known in the formation of strontium apatite (Sr-Ap) by homogeneous precipitation method using hydrolysis of urea. The alpha strontium hydrogenphosphate (α-SrHPO4) is obtained as the initial product in all the reaction conditions. Sr-Ap is transformed from α-SrHPO4 via strontium phosphate (Sr3(PO4)2) or beta strontium hydrogenphosphate (β-SrHPO4). The effects of reaction conditions on the formation of various strontium phosphates and on the composition of Sr-Ap are discussed. Sr-Ap was prepared at 98(}1)°C by the homogeneous precipitation method from the mixed solution (Sr2+/P mole ratio=1.00--5.00) of Sr2+ ion, NaH2PO4 and urea. Each of α-SrHPO4, β-SrHPO4 or Sr3(PO4)2 was aged in various reaction solutions.
At pH 6.2, α-SrHPO4 transformed easily into Sr3(PO4)2 by aging in the dilute solution of Sr2+ ions and into β-SrHPO4 by aging in the concentrated solution of Sr2+ ions. But, regardless of Sr2+ ions concentration, α-SrHPO4 transformed into β-SrHPO4 at pH 7.2. As the concentration of Sr2+ ions or Cl ions was high, Sr3(PO4)2 or β-SrHPO4 transformed into Sr-Ap in a short time.
The incorporation of Cl ions or CO32− ions into the crystal of Sr-Ap obtained by hydrolysis of urea is unavoidable. It is known that the incorporation of CO32− ions could be reduced by heating urea solutions of low concentration for a long time. It is an effective method to prepare the high concentration of Sr2+ ions in the initial solution with Sr2+/P mole ratio=1.67. On the other hand, the heightening Cl ions concentration in the initial solution increased the amount of incorporation of Cl ions into the obtained crystal of Sr-Ap.
The single phase of α-SrHPO4, β-SrHPO4, Sr3(PO4)2 or Sr-Ap could be obtained readily by the control of the reaction conditions. And, it was clarified that the incorporation of Cl ions or CO32− ions into the obtained crystal of Sr-Ap could be controlled.

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