Changes in Composition and Structure of Tricalcium Phosphate under Different pH Conditions

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  • リン酸三カルシウムの各種ｐＨ水溶液中での組成・構造変化
  • リンサン サンカルシウム ノ カクシュ pH スイヨウエキチュウ デ ノ ソセ

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Aqueous suspensions of α-Ca3(PO4)2 powder were treated at 80°C and at various initial and fixed values of pH by using the “pH spontaneous drift” and “pH-stat” methods. The products were characterized by X-ray diffraction, chemical analysis, infrared absorption spectroscopy, thermogravimetry and electron microscopy.<br>Products obtained above pH 4.6 consisted of a single phase of apatite having various Ca/P ratios. As the pH was controlled higher, the Ca/P molar ratio tended to be higher values ranging from 1.50 to 1.67, while the rate of apatite formation was decreased by the controlling. The shape of the apatite crystals formed by the “pH-spontaneous drift” method was thin leaves elongated along the {1010}. In the “pH-stat” experiments, the apatite crystals were either thin leaves or cylindrical rods depending upon the fixed pH. In the range of pH 4.6 to 4.3 mixtures of apatite and octacalcium phosphate crystallized as thin blades. In the range of pH 4.2 to 3.0 platelet CaHPO4 crystals were a major constituent of products. The apatites prepared by using this reaction system had a high crystallinity compared with that of apatites obtained by usual precipitation reactions, The dehydration process of apatites was similar to that of octacalcium phosphate, and the process was divided into, at least, three regions: below 140°C, 140°-600°C, and 750°-850°C. The amount of dehydration in the each region increased as the Ca/P ratio decreased. HPO42- groups in the apatite structure converted to P2O74- groups near 250°C. This temperature was considerably lower than that (-400°C) for CaHPO4. Consequently, it was suggested that these apatites were close to octacalcium phosphate with respect to the nature of water and HPO42- contained in both the structures.

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