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Published in Soil Sci Soc Am J 40:39-42 (1976)
© 1976 Soil Science Society of America
677 S. Segoe Rd., Madison, WI 53711 USA
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Solubility and Solubility Product of Dicalcium Phosphate Dihydrate in Aqueous Solutions and Soil Solutions1

A. C. Bennett and Fred Adams2

ABSTRACT

The solubility product (Ca2+)(HPO42-) of CaHPO4 · 2H2O was determined in several aqueous solutions containing ions common to soil solutions. Extrapolation of analytical Ca and P concentrations to zero ionic strength yielded pKSD values ranging from 6.54 to 6.59 and averaging 6.57 for 10 aqueous salt solutions. A method of successive approximation that computed ionic activity was used to determine the pKSD for three concentrations of 15 different aqueous salt solutions saturated with CaHPO4 · 2H2O. Calculated pKsp values ranged from 6.44 to 6.63; the average was 6.55. Eight soil samples that varied widely in pH and soil-test P were treated with CaHPO4 · 2H2O at rates of 600 to 3,000 ppm P. Soil solutions were displaced, analyzed, and (Ca2+)(HPO42-) was calculated. Eight of the 20 samples of soil were considered to be saturated with CaHPO4 · 2H2O because their (Ca2+)(HPO42-) values were approximately the same as that for similar aqueous salt solutions. It was concluded that the presence of excess CaHPO4 · 2H2O in soil could be inferred by ion-activity products of displaced soil solutions.

NOTES

1 Contribution from the Dep. of Agronomy and Soils, Auburn Univ. Agric. Exp. Stn. Auburn, Al 36830.

2 Assistant Professor and Professor of Soils, respectively.

Received for publication March 19, 1975. Accepted for publication September 30, 1975.






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Copyright © 1976 by the Soil Science Society of America.