Phase Equilibria Among Acid Calcium Phosphates
| ISSN: |
1551-2916
|
|---|---|
| Source: |
Blackwell Publishing Journal Backfiles 1879-2005
|
| Topics: |
Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
Physics
|
| Notes: |
The stable ternary system H3PO4-Ca(OH)2-H2O has been established further by determining the equilibria among the acidic calcium phosphates. In particular, equilibria involving CaHPO4·2H2O, CaHPO4, Ca(H2PO4)2·H2O, and Ca(H2PO4)2·H2O have been established. In contrast to earlier opinions, Ca(H2PO4)2 is not a stable phase in this system at any temperature below 100°C. Ca(H2PO4)2 can be made only in boiling H3PO4 liquors at 130°C or by dehydration of Ca(H2PO4)2·H2O. Ca(H2PO4)2 converts to Ca(H2PO4)2H2O upon equilibration in calcium phosphate solutions. A liquid region seems to exist between H3PO4 and Ca(H2PO4)2·H2O. Therefore, the related invariant point between H3PO4 and Ca(H2PO4)2·H2O does not exist at 25°C. The most soluble point lies between Ca(H2PO4)2·H2O and H3PO4. The invariant point involving the solids Ca(H2PO4)2·H2O and CaHPO4 is outside its compatibility triangle, and Ca(H2PO4)2·H2O dissolves incongruently. Furthermore, the compatibility line between H2O and Ca(H2PO4)2·H2O intersects the solubility curve of hydroxyapatite, suggesting Ca(H2PO4)2H2O dissolution can form hydroxyapatite initially. The dissolution of Ca(H2PO4)2·H2O in deionized water forms CaHPO4·2H2O, having a very-thin-plate morphology below 55°C, and CaHPO4, having a rectangular-block morphology above 55°C. Over time CaHPO4·2H2O converts to CaHPO4 at temperatures above 36°C. The data obtained in this study have been combined with published data to construct a more complete H3PO4–Ca(OH)2-H2O diagram at 25°C.
|
| Type of Medium: |
Electronic Resource
|
| URL: |