Samples of arsenic hydroxyapatite and its solid solutions with calcium hydroxyapatite are synthesized in aqueous media. The samples are characterized by X-ray, infrared spectra, the assignments of phosphate, arsenate, and hydroxyl bands are recorded. The possibility of anion migration in the apatite lattice is proposed. Presence of water, adsorbed in nature, is indicated in the TGA and DTA analysis of the samples. The solubility product, Ksp, of the samples of arsenic hydroxyapatite Ca10(AsO4)6(OH)2, and the solid solutions, Ca10(PO4)6−x(AsO4)x(OH)2, (x=1 to 5) are determined considering the formation of ion pairs [CaHAsO4]0, [CaHPO4]0, [Ca(H2AsO4)]+, and [Ca(H2PO4)]+ in solution. The instability of the samples in solution is indicated from the experimental ΔG0soln. values.
028ChemInform Abstract A pure and homogeneous sample of arsenate hydroxyapatite (I) is synthesized by precipitation from an aqueous solution of Ca(NO3)2 and (NH4)2HAsO4 at 100 rc C and pH 12. (I) is found to be isomorphous with hydroxyapatite belonging to the space roup P63/m. Between 308 and 323 K in the pH range 4.0-8.0 it shows a decreasing trend in solubility with increasing pH and temperature. This behaviourcan be explained on the basis of its chemical potential and the free energy of solution. The standard thermodynamic arameters ∆H rc , ∆S rc , and ∆C rc p are calculated and the solubility product is determined.
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