PRECIPITATION OF CALCIUM PHOSPHATES FROM ELECTROLYTE-SOLUTIONS .5. INFLUENCE OF CITRATE IONS

The influence of citrate ions on the precipitation of crystalline apatitic precipitates with low Ca/P molar ratios [octacalcium phosphate (OCP) and calcium-deficient apatites (DA) (system A)] and of the intercrystalline mixtures of calcium hydrogen phosphate dihydrate (DCPD) and DA (system B) was investigated. Samples were prepared by direct mixing of calcium chloride solutions (A, 6·10-3 mol dm-3; B, 1·10-1 mol dm-3) and sodium phosphate solutions (A, 6·10-3 mol dm-3; B, 2·10-2 mol dm-3) containing citrate (0-2·10-3 mol dm-3) and preadjusted to pH 7.4. In the presence of citrate ions: (a) crystal growth of OCP and DA was slowed down; (b) habit modification of DCPD crystals occurred; and (c) equilibration in intercrystalline mixtures of DCPD and DA's was slowed down. All phenomena were caused by surface adsorption of negatively charged ions, most probably CaC6H5O7-, which is the prevalent calcium citrate species under the given experimental conditions. Habit modification of DCPD was induced by preferential adsorption at the (001) crystal plane. © 1979 Springer-Verlag.