Solubility Criteria for the Existence of Calcium and Aluminum Phosphates in Soils

Abstract

The solubilities of calcium phosphates may be represented on a single solubility diagram in which functions of the chemical potentials of Ca(OH)2 and Ca(H2PO4)2 are used as co‐ordinates. In the presence of solid phase CaCO3, the chemical potential of lime (the value of pH ‐ ½pCa) depends upon the partial pressure of CO2 in the atmosphere. Solubility determinations showed that neither dicalcium phosphate nor hydroxyapatite was present in the soil or clay systems studied. A similar solubility diagram may be constructed for the aluminum phosphates using functions of the chemical potentials of Al(OH)3 and Al(H2PO4)3 as co‐ordinates. In the absence of reliable solubility data, the existence of the aluminum phosphates in the soils studied could not be established.

About the Genetic Mechanisms of Apatites: A Survey on the Methodological Approaches

“…On the other hand, some reports claimed the existence of a single solubility product, as Clark did in his papers. In fact, in 1955, Clark and Peech [94] presented a solubility diagram for HAp in soils. At the same time, Clark [77] proposed a further paper, demonstrating that, within the variability due to analytical errors, HAp possesses a unique solubility product over a wide range of experimental conditions.…”

Solubility Criteria for the Existence of Hydroxyapatite

“…In contrast t o this opinion, the thermodynamic properties of crystalline hydroxyapatite have been reported by the Chemical Engineering Division of the TVA (8). Since in a previous paper solubility criteria were proposed as a means of establishing the existence of calcium phosphate compounds in soils (2), it was considered desirable t o resolve the contrasting views on the solubility of hydroxyapatite in order t o evaluate the validity of this approach. The object of the work reported here was t o determine whether hydroxyapatite possesses a definite solubility product over a wide range of conditions.…”

Evidence of Dicalcium Phosphate Precipitation in a Calcareous Soil

“…Hagin and Hadas (1962) found that the solubility of phosphate was higher than that of octocalcium phosphate and increased beyond that of CaHPO, with increasing additions of Ca(H,PO,),, and attributed this to lack of phosphate equilibrium. Clark and Peech (1955) found a phosphate solubility less than that of hydroxyapatite after adding KH,PO, and storin the mixtures moist for 5 months. Murrmann and Peech (1968) foun a phosphate solubility corresponding to that of and lime potentia P (pH-&pCa) (Fig.…”