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

Abstract

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.