Interaction of some alkali metal citrates with hydroxyapatite - Ion-exchange adsorption and role of charge balance

Proper balancing of the concentration of citrate ions in soft drinks is important in order to regulate the cariogenic potential of these ions. The uptake, or ion-exchange adsorption, of sodium or potassium citrates from their aqueous solutions onto synthetic hydroxyapatite at 22 degrees C was investigated in the present study as a means to elucidate the role of citrate ions in the cariogenic process. For each salt, the amount of uptake, or removal of citrate ions from its aqueous solutions by the apatite, bears a simple ratio to the amount of calcium or phosphate ions released into the solution by the substrate. The uptake isotherms may be represented by their Langmuir plots. The interaction is purely ion-exchange adsorption; there is no indication of the presence of any foreign phase. Plausible equations for ion-exchange equilibria show that the surface is electrically charged upon the adsorption of salts, but remains neutral upon the adsorption of the citric acid, and this may have a bearing on the effective area occupied by the absorbate species. (C) 1998 Published by Elsevier Science B.V. All rights reserved.