No abstract

The solubilities of synthetic basic calcium phosphate precipitates in carbon dioxide free aqueous systems were studied over a wide range of conditions. T h e pH of the suspensions and the calcium and phosphate concentrations in the solutions were determined both after precipitation atid after dissolution of the solid phases. Solubility criteria applied to these measurements indicated that hydroxyapatite has a definite solubility product.

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.

A method is presented for calculating the Al3+ ion activity from measurements of the concentration of Al, pH, and the ionic strength of the soil extract. The colorimetric method for determining the concentration of Al in soil extracts makes use of the aluminon reagent with provisions for removing organic matter and silica and for preventing the interference of Fe by the addition of thioglycollic acid.The validity of the method was established by verifying the existence of a theoretical equilibrium relationship between the Al3+ and the H+ activities in soil extracts. The value of (pH — ⅓ pAl) for CaCl2 extracts of three acid soils was found to remain remarkably constant as the concentration of CaCl2 in the extracting solution was increased from 0.001 to 0.10M. This constancy of (pH — ⅓ pAl) for soil extracts can be predicted from the Donnan and Gouy theories and does not necessarily indicate the presence in the soil of some crystalline form of hydrous aluminum oxide having a definite solubility product.