INTERACTION OF ALKALINE-EARTH METAL-IONS WITH ACETIC AND LACTIC-ACID IN AQUEOUS-SOLUTIONS STUDIED BY C-13 NMR-SPECTROSCOPY

Interaction of alkaline earth metal (magnesium, calcium, strontium and barium) ions with acetic and lactic acid in aqueous media was investi gated by C-13 NMR spectroscopy. In the acetate systems, signals whose chemical shifts were the averages of those of the free and bound aceta te ions were observed. Downfield shifts of the carboxylate carbon sign als with increasing metal ion concentration indicated that the acetate ion acted as a monodentate ligand coordinating to the metal ion using the carboxylate group. The metal ion concentration dependence of the peak positions of the methine and carboxylate carbon signals of the la ctate ion in the lactate systems suggested that the lactate ion coordi nated to a metal ion using the carboxylate and hydroxyl groups. Unique upfield shifts upon complexation in the magnesium lactate systems sug gested that the lactate ion coordinated to the magnesium ion from outs ide the primary hydration sphere. The present results were consistent with the isotope effects of the alkaline earth metals observed in cati on exchange chromatography.