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Kinetics of Heterogeneously Initiated Precipitation of Calcium Phosphates¹

ABSTRACT

The precipitation of calcium phosphate from stable supersaturated solutions was studied under conditions of pH/Ca-stat and pH-stat. The pH and Ca²⁺ activity of the titrand were kept constant by means of specific ion electrodes coupled to automatic titrators. The precipitation was initiated by the addition of well-characterized Domino soil, which had no identifiable calcium phosphate solids. The precipitate was characterized as CaHPO₄·2H₂O (DCPD) by x-ray diffraction, scanning electron microscopy, energy dispersive x-ray, chemical analysis, and equilibrium solution data. The rate of precipitation was compared using H⁺, Ca²⁺, and H₂PO^-₄ data. The rate of H⁺ release was shown to be the most reliable indicator of the rate of precipitation under the chosen experimental conditions. The proton release data were used to calculate the empirical order of reaction (n) and the rate constant (log K). Under conditions of pH/Ca-stat, n was found to be 1.73 ± 0.17 and log K was found to be 2.60 ± 0.60. Under conditions of pH-stat, n was found to be 1.27 ± 0.12 and log K was found to be 0.69 ± 0.44. No relationship was found between the amount of seed material used and the kinetic parameters under the chosen experimental conditions.

¹ Contribution from the Dep. of Soil and Environmental Sciences, Univ. of California, Riverside, CA 92521. Presented Before Div. S-2, Soil Sci. Soc. Am., Las Vegas, NV, 27 Nov. 1984.

² Graduate Research Assistant and Assistant Professor, respectively, Univ. of California, Riverside. Present address of the second author is Geochemistry Section, Earth Sci. Dep., Battelle Pacific N.W. Labs, Richland, WA 99352.

Received for publication May 15, 1986.