To understand the toxic effects of vanadium on the human skeletal system, the solubility products of phosphate and vanadate apatites of calcium and eight of their solid solutions, spread over the entire compositional range, were investigated at 37°C in 0.165 M sodium chloride solution. They were found to increase with increase in the vanadium content, the dissolution being found to be stoichiometric. A theoretical interpretation based on changes in lattice and hydration energies resulting from isomorphous ionic substitution is advanced.
By appropriate modifications of existing precipitation methods, a sample of calcium vanadate apatite, Cal0(VO4)e(OH)2 was prepared at 11 0 ° C. It was characterized through X-ray, electron microscopic, infrared and thermoanalytical analyses in addition to chemical analysis. Equilibrating the sample in buffered aqueous media at 37 ° C, the solubility was measured using microanalytical techniques of Ca 2+ and VO 3-present in the saturated solutions after separating the colloidal component of the solute. Duration of equilibration for the attainment of saturation of the solution of the sample was obtained from a study on its dissolution kinetics. The pK~p of calcium vanadate apatite was found to be 1 07.09. The investigation showed that pK~p was independent of the presence of the common ions.
The toxicity of lead and vanadium and their salts [1][2][3][4][5] is well known. The mechanism [6] of their incorporation in the humal skeletal system is based on the isomorphous substitution of Ca 2+ by Pb 2+ (ionic radii 0.099 and 0.120nm respectively [7]) and of PO 3-by VO]-(covalent radii 0.110 and 0.122 nm respectively [7]) in calcium phosphate apatite, Cal0(PO4)6(OH)2 , the principal inorganic constituent [8,9] of bones and teeth, resulting in the formation of lead vanadate apatite (LVA), Pb~0(VO4)6(OH)2. The preparation, characterization and determination of the solubility of LVA are reported in this letter; the method adopted for the preparation of LVA is the first of its kind.The preparation of LVA brought about by a modification of Hayek's method [10,11] is based on the following equation, with ethylenediamine being used instead of ammoniaStock solutions of Pb 2+ and VO 3-obtained by dissolving lead acetate and vanadium pentoxide in water and sodium hydroxide, respectively, were preserved in polyethylene containers and their amounts of lead and vanadium were estimated [12,13]. Details of the method adopted for the preparation and purificiation of the sample were the same as those described elsewhere for an allied system [14,19]. A part of the sample washed with acetone and air dried was heated to ~ 300°C for about 2 h and cooled in a desiccated atmosphere for use in chemical [15], X-ray and infrared analyses, as well as for density determinations [16,17], whereas electron-microscopic and thermogravimetric investigations were performed on the air-dried sample. The sample was characterized by X-ray, infrared, electron-microscopic and thermogravimetric analyses and the experimental details are the same as those described elsewhere [14]. The solubility studies carried out at 37°C in buffered aqueous media, raised to 0.165 M with respect to sodium chloride in the pH range 2 to 3, were based on complexometric [12] and spectrophotometric [18] determinations, respectively, of lead and vanadium. The buffer comprised potassium acid phthalate and hydrochloric acid. Investigations on the dissolution kinetics proved the optimum period of equilibration for the formation of a saturated solution of the sample to be 3 h. In order to be quite sure that the condition of saturation had been achieved, the period was extended to 12 h; the remaining experimental details for solubility studies are the same as those described before [19] for allied systems.The effect of common ions on the PKsp of the sample was also investigated at a convenient pH value.The formation of tertiary phosphates/vanadates of heavy metals through a conventional precipitation from aqueous media was complicated because of co-precipitation of their hydroxides, primary and secondary species [20]. Among the methods available for elimination of such a co-precipitation [21, 22], that of Hayek [22], involving complexing of the heavy metal ions with an appropriate ligand, was found to be suitable because of the ability of such ions to form well-defined soluble...
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