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 and at ionic strengths of 0.1 and 1 mol kg−1. The constants were extrapolated to infinite dilution applying SIT. 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 ΔrH0 = 4.3 ± 3.6 kJ mol−1 is nearly temperature independent, whereas the formation of the 1:2 and 1:3 complexes with standard reaction enthalpies of 17.8 ± 3.8 and 33.4 ± 5.8 kJ mol−1, respectively, is clearly endothermic.

Graphical abstractThe complex formation of europium with salicylate (Sal) was investigated at pH 5 and temperatures between 25 and 60 °C with time-resolved laser-induced fluorescence spectroscopy; three different complexes, Eu(Sal)2+, Eu(Sal)2+ and Eu(Sal)3, could be identified, and thermodynamic data (stability constants, reaction enthalpies, reaction entropies, reaction energies) were determined.Figure optionsDownload full-size imageDownload as PowerPoint slideHighlights► In aqueous solution the Eu(III) salicylate system forms 1:1, 1:2 and 1:3 complexes. ► The formation of the 1:1 complex is temperature independent (isoenthalpic). ► The formation of the 1:2 and 1:3 complexes is endothermic. ► With rising temperature the formation of the 1:3 complex increases strongly.