Equilibrium study on mixed ligand complexes of UO 2+2 and Th 4+ with EDTA as primary ligand and various nucleosides and their bases as secondary ligands

Abstract

Formation constants of mixed ligand complexes of UO 2+ 2 and Th 4+ with EDTA various nucleosides, purines and pyrimidines such as adenosine, guanosine, cytidine, uridine, adenine, 2,6-diamino-purine, 8-azaadenine, cytosine, thymine and uracil in a 1:1:1 ratio were determined in aqueous solution at 35°C by potentiometric equilibrium measurements. The acid dissociation constants of the above mentioned secondary ligands were determined at 35, 45 and 55°C together with the thermodynamic parameters involved in their ionization reactions. Formation constants of 1:1 binary complexes of UO 2+ 2 and Th 4+ with the above secondary ligands are also reported at 35°C. All the measurements were made in aqueous solution and μ = 0.1 M (KNO 3). Both enthalpy and entropy factors involved in the dissociation reactions of secondary ligands were found to be favourable for the first dissociation reactions. There was a good correlation between the basicity of secondary ligands and stability of their 1:1 binary complexes determined. In general the binary complexes of Th 4+ showed greater stability in comparison to the corresponding UO 2+ 2 complexes. Excepting the case of adenosine and cytidine, formation of protonated ternary complex in the early buffer region followed by dissociation to normal complex was visualized. In all the cases the binary complexes were more stable than their corresponding ternary complexes. Among the secondary ligands, thymine, uracil and uridine showed no measurable interaction with both the metal ions in binary and ternary complex systems. Cytosine and cytidine also showed very weak interactions with the UO 2EDTA system.