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The solubilities of synthetic calcium hydroxyapatites [Ca(OH)A], Ca5(P04)30H, are determined in the ternary system Ca(OH)2+H3P04+H20 at 25°C in the pH range 5-7 by equilibration with dilute &PO4 solutions. The thermodynamic solubility product Ksp of precipitated Ca(OM)A, prepared by titrating a boiling aqueous suspension of Ca(OH), with M3P04, is (3.2s k 0.29) x 11101~ dm-27 . Ca(OH)A.prepared by solid state reaction of CaC03 and CaHP04 and precipitated Ca(0H)A heated at high temperatures in a water vapour atmosphere showed no constant Ksp. After being washed, these samples equilibrate according to the usual solubility product principles with a Ksp value of (3.d4+0&) xThe influence of heat treatment and washing on the solubility behaviour is discussed. Deviations from solubility behaviour which accords with the usual solubility product principles are caused by reactions occurring at the surface of the solid. The latter are induced by a high free energy of the solid and/or the presence of a fraction with high solubility associated with the Ca(0H)A sample. and (2. 6, -, k0.21) x mo19 dm-27, respectively. MATERIALSCa(0H)A samples were prepared by precipitation or by solid state reaction.

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Der Apatit wurde entsprechend Gleichung A in stöchiometrischer Zusammensetzung und kristallisierter Form durch eine Festkörperreaktion dargestellt.

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