No abstract

When solid hydroxyapatite and solid calcium fluoride were equilibrated at 25C with dilute solutions of HCl, analysis of the solutions yielded an average value of 118.8 for the negative logarithm of the ion‐activity product for fluorapatite (10 pCa + 2 pF + 6 pPO4). This value compares favorably with values of 118.4, 119.2, and 120.9 reported by other investigators for the negative logarithm of the activity solubility‐product constant for fluorapatite. A plot of the negative logarithm of the activity‐product of (H)3(PO4) against the negative logarithm of the activity‐product of (Ca)(OH)2 in solution yielded a regression line with a slope of −1.52, which is close to the theoretical value of −1.50 for fluorapatite in a solution saturated with calcium fluoride and is greater than the theoretical value of −1.67 for hydroxyapatite. The results indicate that at least the surface of the hydroxyapatite had been converted to fluorapatite during the equilibration with solid calcium fluoride.

Dilute HCl solutions equilibrated with Florida phosphate rock and Tennessee brown phosphate rock in the presence of solid fluorite (CaF2) developed a condition of supersaturation with respect to fluorapatite at 25C. In suspensions boiled before equilibration at 25C, however, the solutions were in equilibrium with fluorapatite, as evidenced by ion‐activity products for fluorapatite in the solutions. The pH and concentrations of calcium and phosphorus in solution decreased due to boiling a suspension of Florida phosphate rock, as expected if fluorapatite had been precipitated. A condition of supersaturation with respect to fluorapatite redeveloped upon long‐continued equilibration of boiled suspensions of Florida and Tennessee phosphate rocks at 25C. Dilute HCl solutions equilibrated with phosphate rock or with phosphate rock plus solid fluorite became supersaturated with respect to fluorite at low pH values (about pH 3) where considerable calcium and fluoride were released from the phosphate rock, and the supersaturation persisted until the end of the equilibration period of 47 days at 25C. At pH values above about 4, the solutions became saturated with fluorite if solid fluorite was present but remained unsaturated where only phosphate rock was present as the solid phase.

No abstract

The enhancement of mineralization of soil organic P that previous investigators had reported as resulting from incubation of soils with added inorganic P was not found in laboratory experiments. Previously reported recoveries of more than 100% of inorganic P added to certain alkaline soils before incubation, which also might be a consequence of enhanced mineralization of soil organic P, could not be reproduced except where the P was added in an acidified solution. Presumably an acidified solution was used by mistake in the previous work.