{"title":"Solubility of Ca5(PO4)3OH in the System Ca(OH)2-H3PO4-H2O at 5, 15, 25, and 37 °C","authors":"H. McDowell, T. M. Gregory, W. E. Brown","doi":"10.6028/jres.081A.017","DOIUrl":null,"url":null,"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.","PeriodicalId":94340,"journal":{"name":"Journal of research of the National Bureau of Standards. Section A, Physics and chemistry","volume":null,"pages":null},"PeriodicalIF":0.0000,"publicationDate":"1977-04-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":"65","resultStr":null,"platform":"Semanticscholar","paperid":null,"PeriodicalName":"Journal of research of the National Bureau of Standards. Section A, Physics and chemistry","FirstCategoryId":"1085","ListUrlMain":"https://doi.org/10.6028/jres.081A.017","RegionNum":0,"RegionCategory":null,"ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":null,"EPubDate":"","PubModel":"","JCR":"","JCRName":"","Score":null,"Total":0}
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.