Dioxouranium(VI)–carboxylate complexes: A calorimetric and potentiometric investigation of interaction with oxalate at infinite dilution and in NaCl aqueous solution at I = 1.0 mol L−1 and T = 25 °C

In this paper we investigated the interactions between dioxouranium(VI) and oxalate using (H+-glass electrode) potentiometry and titration calorimetry. Potentiometric measurements were carried out in NaCl aqueous solutions and at T = 25 °C in a wide range of experimental conditions (concentrations, ligand/metal molar ratio, pH, titrants) at low ionic strength values (I ≤ 0.090 mol L−1, without supporting electrolyte) and at I = 1.0 mol L−1; different procedures were employed for the acquisition of experimental data and careful analysis of these data performed. In all cases the speciation model that best fits experimental data takes into account the formation of the binary mononuclear species UO2(ox)0, UO2(ox)22−, UO2(ox)34− widely reported in literature, the ternary hydroxyl mononuclear species UO2(ox)OH−, UO2(ox)(OH)22−, UO2(ox)2OH3−, UO2(ox)3OH5−, the protonated ternary mononuclear species UO2(ox)3H3− and the binuclear species (UO2)2(ox)56−. metric measurements were carried out following similar procedures and in the same experimental conditions as employed for the potentiometric measurements at I = 1.0 mol L−1 in NaCl. The stability of UO22+–oxalate2− complexes is fairly high and their main contribution to stability is entropic in nature. Some linear empirical relationships were found which make it possible to calculate (i) the contribution of a single bond: Δ G sb ° = − 16.6 ± 0.2   kJ   mo l − 1 and T Δ S sb ° = 16.2 ± 0.5   kJ   mo l − 1 ; (ii) chelate stabilisation per ring: Δ G r ° = − 1.30 ± 0.04   kJ   mo l − 1 and T Δ S r ° = 1.27 ± 0.23   kJ   mo l − 1 and (iii) the mean stability of negatively charged Na+-ion pair complexes: logTK = (0.46 ± 0.02)·|z| (z = charge of complex species), ΔG° = −(2.60 ± 0.1)·|z| kJ mol−1 and TΔS° = 2.5 ± 0.5 kJ mol−1. Both potentiometric and calorimetric results provide evidence of the penta-coordination of the species UO2(ox)34−. SIT parameters were calculated from the data at I = 0 and I = 1.02 mol kg−1. Comparisons are made with literature data. An insoluble dioxouranium(VI) ternary complex was synthesised (at I = 1.0 mol L−1 in NaCl) and characterised by thermoanalysis and elemental analysis.