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

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Title	Solubility of Ca5(PO4)3OH in the system Ca(OH)2-H3PO4-H2O at 5-degrees-C, 15-degrees-C, 25-degrees-C and 37-degrees-C
Author	Mcdowell, H; Gregory, T. M.; Brown, W. E.
SeriesTitle	Journal of Research of the National Bureau of Standards. Section A: Physics and Chemistry
Date	1977-03
Language	English
Keywords	Dissolution; hydroxyapatite; ion pairs; solubility; solubility isotherms; solubility product; thermal coefficient of solubility; thermodynamics; tooth mineral
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-H20 at 5, 15, 25, and 37C in the pH range 3.7-6.7 by equilibration with dilute H3PO4 solutions. The solubility product K8, determined as a function of temperature by a generalized least-squares procedure from 41 experimental points, is given by the equation log K8 = - 8219.41/T - 1.6657 - 0.098215T. The values of K8 and its dispersion at 25 and 37C are 3.04 (0.25) and 2.35 (0.27) X 10-^59. There is a maximum in K8 near 16C, which may be due to the form of temperature dependence found earlier for the stability constants of the ion pairs CaH2PO+4 and CaHPOo4. 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 25C were reviewed. The solubility products obtained by three of these investigators were comparable with our value of 3.0 X 10-^59; their data were re- evaluated by the method described here. We conclude that the best value for the solubility product at 25C is 4.7 (2.0) X 10-^59.
Publisher	National Bureau of Standards
Volume	81A
Issue	2/3
Page	273
Cite	J. Res. Bur. Stand., Vol. 81A, No. 2/3, p. 273
Rights	The Journal of Research of the National Institute of Standards and Technology is a publication of the U.S. Government. The papers are in the public domain and are not subject to copyright in the United States. However, please pay special attention to the individual works to make sure there are no copyright restrictions indicated. Individual works may require securing other permissions from the original copyright holder.

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

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