PHASE-EQUILIBRIA AMONG ACID CALCIUM PHOSPHATES

The stable ternary system H3PO4-Ca(OH)(2)-H2O has been established fur ther by determining the equilibria among the acidic calcium phosphates . In particular, equilibria involving CaHPO4 . 2H(2)O, CaHPO4, Ca(H2PO 4). H2O, and Ca(H2PO4)(2) . H2O have been established. In contrast to earlier opinions, Ca(H2PO4), is not a stable phase in this system at a ny temperature below 100 degrees C. Ca(H2PO4)(2) can be made only in b oiling H3PO4 liquors at 130 degrees C or by dehydration of Ca(H2PO4)(2 ) . H2O. Ca(H2PO4)(2) converts to Ca(H2PO4)(2) . H2O upon equilibratio n in calcium phosphate solutions. A liquid region seems to exist betwe en H3PO4 and Ca(H2PO4)(2) . H2O. Therefore, the related invariant poin t between H3PO4, and Ca(H2PO4). H2O does not exist at 25 degrees C. Th e most soluble point lies between Ca(H2PO4)(2) . H2O and H3PO4. The in variant point involving the solids Ca(H2PO4)(2) . H2O and CaHPO4 is ou tside its compatibility triangle, and Ca(H2PO4)(2) . H2O dissolves inc ongruently. Furthermore, the compatibility line between H2O and Ca(H2P O4)(2) . H2O intersects the solubility curve of hydroxyapatite, sugges ting Ca(H2PO4)(2) . H2O dissolution can form hydroxyapatite initially. The dissolution of Ca(H2PO4)(2) . H2O in deionized water forms CaHPO4 . 2H(2)O, having a very thin-plate morphology below 55 degrees C, and CaHPO4, having a rectangular-block morphology above 55 degrees C. Ove r time CaHPO4 . 2H(2)O converts to CaHPO4 at temperatures above 36 deg rees C. The data obtained in this study have been combined with publis hed data to construct a more complete H3PO4- Ca(OH)(2)-H2O diagram at 25 degrees C.