Phase Relationships in the Ternary System CaO─P2O5─H2O at 25°C

The portion of the stable phase diagram for the ternary system CaO‐P2O5‐H2O involving calcium phosphate phases more basic than Ca(H2PO4)2 has been constructed. Construction involves plotting concentrations as their fifth roots to allow phases having very different solubilities to be represented on a...

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Bibliographic Details
Published in:Journal of the American Ceramic Society 1992-01, Vol.75 (1), p.17-22
Main Author: Brown, Paul Wencil
Format: Article
Language:English
Subjects:
calcium phosphate
Chemistry
equilibrium dissolution
Exact sciences and technology
General and physical chemistry
hydroxyapatite
Liquid-solid equilibria
phase diagrams
Phase equilibria
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Summary:The portion of the stable phase diagram for the ternary system CaO‐P2O5‐H2O involving calcium phosphate phases more basic than Ca(H2PO4)2 has been constructed. Construction involves plotting concentrations as their fifth roots to allow phases having very different solubilities to be represented on a single diagram. Hydroxyapatite is shown to be congruently soluble. The stability region for hydroxyapatite includes compositions having Ca/P ratios extending between 1.5 and 1.67. Calcium‐deficient hydroxyapatite forms a stable invariant point with CaHPO4 and a metastable invariant point with CaHPO4. 2H2O. Inspection of the diagram indicates that the compounds CaHPO4 and CaPHO4.2H2O dissolve incongruently. Incongruent dissolution of either CaHPO4 or CaHPO4.2H2O results in the formation of hydroxyapatite during the dissolution process. Such incongruency between these compounds and hydroxyapatite influences their microstructural relationships.
ISSN:0002-7820
1551-2916
DOI:10.1111/j.1151-2916.1992.tb05435.x