A Potentiometric and Microcalorimetric Study of the Complexation of Trivalent Europium with Lactate: The Ionic Strength Dependency of log β’n, ΔrHm,n and ΔrSm,n

Abstract The complexation of Eu(III) by lactate and the ligand protonation were studied at 25 °C as a function of the ionic strength ( I m  = 0.10 - 5.61 mol kg −1 H 2 O, NaCl). The experimental formation constants of the three [Eu(Lac) n ] 3-n (n = 1, 2, 3) mononuclear complexes (log β ’ n ) and the protonation constant of lactate (log β ’ HLac ) were determined by potentiometric titrations. The experimental values were extrapolated to zero ionic strength by means of the SIT approach, yielding the thermodynamic constants (log β 0 n ) and the specific ion interaction coefficients of the ionic species ( e (i,k)). The reaction enthalpies (Δ r H ’ m,n ), determined independently by isothermal calorimetry, and entropies (Δ r S ’ m,n ) are also reported. The ionic strength dependence of Δ r H ’ m,n was fitted by means of the enthalpy SIT approach, giving the standard reaction enthalpies (Δ r H 0 m,n ), entropies (Δ r S 0 m,n ) and the partial molar enthalpy specific ion interaction coefficients (Δe L (i,k)). The results show negative conditional reaction enthalpies for all complexes, which become more exothermic with increasing ionic strength. Simultaneously, the Δ r S ’ m,n decrease steadily with I m , resulting in a gradual change of the driving force of the reactions: at low ionic strength the reaction is driven almost exclusively by entropy. As I m increases the contribution of the enthalpy term becomes relevant and the reactions are driven almost equally by Δ r H ’ m,n and Δ r S ’ m,n at the highest value of I m studied.