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Solubility of Dicalcium Phosphate Dihydrate in Aqueous Systems¹

ABSTRACT

The metastable solubility isotherm for dicalcium phosphate dihydrate, CaHPO₄·2H₂O, (DCPD), in the pH range 3.5 to 7.5 was established by (a) leaching solutions of dilute phosphoric acid through columns of DCPD, and (b) by shaking suspensions of DCPD in dilute phosphoric acid solutions.

In other experiments, equilibrium was approached both from undersaturated and supersaturated conditions. In both cases the final solution compositions were represented by points on the isotherm obtained by the methods mentioned above, and dissolution or precipitation occurred with a Ca/P ratio equal to one.

The solubility product constant of DCPD was found to be 2.77 x 10^-7 for all concentrations tested when appropriate corrections were made in the calculation of ionic activities.

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.