Solubility of Ca5(PO4)3OH in the System Ca(OH)2-H3PO4-H2O at 5, 15, 25, and 37 °C

H. McDowell, T. M. Gregory, W. E. Brown
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引用次数: 65

Abstract

Solubility isotherms of hydroxyapatite, Ca5(PO4)3OH (OHAp), prepared by titrating a boiling aqueous suspension of Ca(OH)2 with 0.5 M H3PO4, were determined in the ternary system Ca(OH)2–H3PO4–H2O at 5, 15, 25, and 37 °C in the pH range 3.7–6.7 by equilibration with dilute H3PO4 solutions. The solubility product Ks, determined as a function of temperature by a generalized least-squares procedure from 41 experimental points, is given by the equation logKs=−8219.41/T−1.6657−0.098215T.The values of Ks and its dispersion at 25 and 37 °C are 3.04 (0.25) and 2.35 (0.27) × 10−59. There is a maximum in Ks near 16 °C, which may be due to the form of temperature dependence found earlier for the stability constants of the ion pairs CaH2PO4+ and CaHPO40. The relative positions of the isotherms show that OHAp has a negative thermal coefficient of solubility. Thermodynamic functions for the dissolution of the salt are reported. The solubility data previously reported by others for OHAp at 25 °C were reviewed. The solubility products obtained by three of these investigators were comparable with our value of 3.0 × 10−59; their data were reevaluated by the method described here. We conclude that the best value for the solubility product at 25 °C is 4.7 (2.0) × 10−59.
Ca5(PO4)3OH在体系Ca(OH)2-H3PO4-H2O中5、15、25和37℃的溶解度
用0.5 M H3PO4滴定Ca(OH)2的沸水悬浮液,测定了Ca(OH)2 - H3PO4 - h2o三元体系中Ca(OH)2 - H3PO4 - h2o在5、15、25、37℃、pH 3.7 ~ 6.7范围内的溶解度等温线。用广义最小二乘法从41个实验点确定溶解度乘积Ks作为温度的函数,由方程logKs=−8219.41/T−1.6657−0.098215T给出。在25°C和37°C时,Ks及其色散值分别为3.04(0.25)和2.35 (0.27)× 10−59。k在16°C附近有一个最大值,这可能是由于前面发现的CaH2PO4+和CaHPO40离子对的稳定性常数的温度依赖形式。等温线的相对位置表明OHAp的溶解度热系数为负。报道了盐溶解的热力学函数。回顾了前人报道的OHAp在25℃下的溶解度数据。其中三位研究者得到的溶解度产物与我们的值3.0 × 10−59相当;他们的数据用这里描述的方法重新评估。我们得出结论,25°C时溶解度乘积的最佳值为4.7 (2.0)× 10−59。
本文章由计算机程序翻译,如有差异,请以英文原文为准。
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