The formation of sparingly soluble species of Ca2+ with carboxylic ligands: speciation and thermoanalysis

This work reports the results of studies into the solubility and thermal decomposition of various sparingly soluble complexes of calcium with two isomeric benzene tricarboxylates, and with benzene penta- and hexa-carboxylates. The dependence of their solubility on ionic strength and, for the calcium benzene hexacarboxylate, the dependence of solubility on Na+ concentration in solution were studied. Comparison between structure and solubility of the calcium insoluble compounds here and previously studied, is also reported. In order to perform solubility calculations, we determined also the formation constants of trimellitic and trimesic anions with H+, Na+ and Ca2+ potentiometrically (H+-ISE), since these are not reported in literature. All solid compounds were characterised by TG-FTIR measurements, and comparable behaviours in thermal decomposition as a function of ligand structure were found.