Thermodynamic study of Th(IV) complexes with dicarboxylates by potentiometry and calorimetry

Abstract

The thermodynamic quantities (ΔG_c, ΔH_c and ΔS_c) for formation of Th(IV) complexes with dicarboxylic ligands, namely, malonate, succinate, glutarate and adipate were determined using potentiometry and calorimetry. The protonation constants of the dicarboxylate ligands were determined by potentiometric titration of the ligand solution, while the corresponding enthalpy values were taken from the literature. In the case of Th(IV)-malonate, multiple species (ML_j, j=1–3) were revealed from the potentiometric data, while in the case of higher homologues, the data for only 1:1 complex could be obtained owing to precipitation at higher ligand concentration. The effect of chain length on the thermodynamic parameters of Th(IV)-dicarboxylate complexation was studied. All the complexation reactions were found to be highly entropy driven, which is the characteristic of the hard acid (metal ion) and hard base (ligand) interactions. The TΔS_c values for 1:1 complexes were found to be nearly constant while the ΔH_c values increased from malonate to glutarate and then leveled off in adipate. The thermodynamic data of Th(IV) have been compared with corresponding data for U(VI) and rare earths.