Phase Equilibria Among Acid Calcium Phosphates

Abstract

The stable ternary system H 3 PO 4 ‐Ca(OH) 2 ‐H 2 O has been established further by determining the equilibria among the acidic calcium phosphates. In particular, equilibria involving CaHPO 4 ·2H 2 O, CaHPO 4 , Ca(H 2 PO 4 ) 2 ·H 2 O, and Ca(H 2 PO 4 ) 2 ·H 2 O have been established. In contrast to earlier opinions, Ca(H 2 PO 4 ) 2 is not a stable phase in this system at any temperature below 100°C. Ca(H 2 PO 4 ) 2 can be made only in boiling H 3 PO 4 liquors at 130°C or by dehydration of Ca(H 2 PO 4 ) 2 ·H 2 O. Ca(H 2 PO 4 ) 2 converts to Ca(H 2 PO 4 ) 2 H 2 O upon equilibration in calcium phosphate solutions. A liquid region seems to exist between H 3 PO 4 and Ca(H 2 PO 4 ) 2 ·H 2 O. Therefore, the related invariant point between H 3 PO 4 and Ca(H 2 PO 4 ) 2· H 2 O does not exist at 25°C. The most soluble point lies between Ca(H 2 PO 4 ) 2 ·H 2 O and H 3 PO 4 . The invariant point involving the solids Ca(H 2 PO 4 ) 2 ·H 2 O and CaHPO 4 is outside its compatibility triangle, and Ca(H 2 PO 4 ) 2 ·H 2 O dissolves incongruently. Furthermore, the compatibility line between H 2 O and Ca(H 2 PO 4 ) 2· H 2 O intersects the solubility curve of hydroxyapatite, suggesting Ca(H 2 PO 4 ) 2 H 2 O dissolution can form hydroxyapatite initially. The dissolution of Ca(H 2 PO 4 ) 2· H 2 O in deionized water forms CaHPO 4 ·2H 2 O, having a very‐thin‐plate morphology below 55°C, and CaHPO 4 , having a rectangular‐block morphology above 55°C. Over time CaHPO 4 ·2H 2 O converts to CaHPO 4 at temperatures above 36°C. The data obtained in this study have been combined with published data to construct a more complete H 3 PO 4 –Ca(OH) 2 ‐H 2 O diagram at 25°C.