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Stability of Dicalcium Phosphate Dihydrate in Aqueous Solutions and Solubility of Octocalcium Phosphate¹

ABSTRACT

The hydrolysis of dicalcium phosphate dihydrate, CaHPO₄·2H₂O, in dilute phosphoric acid solutions and in water was investigated at 25° C. in the absence of CO₂. Octocalcium phosphate, Ca₄H(PO₄)₃·3H₂O, was found to precipitate when the pH of the solution was higher than 6.38. The extent of the hydrolysis was larger the higher the initial pH of the solution. The solution composition followed the metastable solubility isotherm for dicalcium phosphate dihydrate until a singular point between octocalcium phosphate and dicalcium phosphate dihydrate was reached. At this point the solution had a pH of 6.38 and Ca and P concentrations of 1.36 x 10^-3M and 2.21 x 10^-3M, respectively. The solubility product of octocalcium phosphate was calculated to be (Ca)⁴(H)(PO₄)³ = 1.25 x 10^-47.

Relative rates of solution and precipitation of both dicalcium and octocalcium phosphates are discussed. No formation of anhydrous dicalcium phosphate or hydroxyapative was detected in equilibrations for as long as 90 days, even though both of these compounds are more stable than dicalcium phosphate dihydrate under the experimental conditions.

¹ Contribution from the Soils and Fertilizer Research Branch, Division of Agricultural Relations, and the Fundamental Research Branch, Division of Chemical Development, TVA, Wilson Dam, Ala.

² Soil Chemist, Research Chemist, and Analytical Chemist, respectively. The authors wish to express their appreciation to J. R. Lehr and A. W. Frazier, Fundamental Research Branch, for petrographic analyses, and to J. P. Smith, Fundamental Research Branch, for X-ray examinations.

Received for publication August 24, 1959. Accepted for publication December 21, 1959.