Precipitation of calcium phosphates from electrolyte solutions. VII. The influence of di- and tricarboxylic acids

Abstract

The influence of a series of di- and tricarboxylic acid anions (CAA) on the metastability of amorphous calcium phosphate (ACP) (I) and on the morphology of calcium hydrogen phosphate dihydrate (DCPD) (II) at 298 K and initial pHi 7.4 was investigated. In system I, precipitation was initiated from solutions of equimolar (3 · 10−3 mol dm−3 concentrations of calcium chloride and sodium phosphate. Malic, succinic, maleic and fumaric acid anions had no significant effect on the metastability period while the effectiveness of the other CAA increased in the succession: transaconitic < tartaric < tricarballylic < citric < dihydroxytartaric acid anion. In system II, precipitation was initiated at initial reactant concentrations of Ca = 5 · 10−2 mol dm−3 and PO4 = 1 · 10−2 mol dm−3. The resulting solid phase was an intercrystalline mixture of calcium deficient apatites (DA) and DCPD. Habit modification of DCPD was induced by the following CAA: tartaric < tricarballylic < dihydroxytartaric < transaconitic < citric < malic acid anion. All these anions inhibited crystal growth in the direction perpendicular to the (001) plane, promoting the growth of needles instead of platelets. Succinic, fumaric and maleic acid anions did not affect the shape of DCPD crystals.