Europium(III) Complexation with Salicylic Acid at Elevated Temperatures

The complex formation of europium(III) with salicylic acid was investigated with time-resolved laser-induced fluorescence spectroscopy (TRLFS) in aqueous solution at elevated temperatures. Three complexes with metal-to-ligand stoichiometries of 1:1, 1:2 and 1:3 could be identified. Stability constants have been calculated for these complexes at various temperatures between 25 and 60 °C. Using the van´t Hoff equation, thermodynamic data (reaction enthalpy ΔrH, reaction entropy ΔrS and reaction energy ΔrG) could be determined. The formation of the 1:1 complex with ΔrH = -2.1 ± 4.9 kJ • mol-1 is nearly temperature independent, whereas the formation of the 1:2 and 1:3 complexes with reaction enthalpies of 12.7 ± 4.8 kJ • mol-1 and 23.3 ± 3.7 kJ • mol-1, respectively, is clearly endothermic.