Ternary complexes : equilibrium study of mixed complexes of uranyl ion with dipicolinic acid as a primary ligand and dicarboxylic acids as secondary ligands

A pH titration method has been used to determine the equilibrium constants for the mixed-ligand systems of uranyl ion with dipicolinic acid as primary ligand and malonic, succinic, itaconic, glutaric, adipic, thiomalic, thiodiglycolic and thiodipropionic acids as secondary ligands at 30deg and ionic strength μ=0.1 M (NaClO4). Dipicolinic, malonic and glutaric acids form 1:1 and 1:2 chelates with uranyl ion while remaining acids form only 1:1 chelates in the pH range 2.5-5.0. It is observed that the coordination of UO22+ with thiodicarboxylic takes place through oxygens of carboxylate groups, whereas dipicolinic acid bonds through N-O donor atoms filling the equitorial position of coordination sphere of the metal ion. The preferential formation of the ternary complexes over binary ones have been discussed on the basis of ring size of chelating acid, basicity of the secondary ligands and the distribution of simple and mixed species present as a function of pH. (author)