A potentiometric and microcalorimetric study of the complexation of trivalent europium with lactate: The ionic strength dependency of log β' n, Δr H m,n and Δr S m,n

Highlights: • Determination of log β'_n, Δ_rH'_m,n and Δ_rS'_m,n of [Eu(Lac)_n]³⁻ⁿ (n = 1, 2, 3). • Application SIT do determine the standard state thermodynamic data. • Determination of ε_i,k and Δε_L of the Eu(III) lactate species. • Investigation of the driving force of the complexation as function of I_m. -- 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 H₂O, 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 β⁰_n) and the specific ion interaction coefficients of the ionic species (ε(i,k)). The reaction enthalpies (Δ_rH'_m,n), determined independently by isothermal calorimetry, and entropies (Δ_rS'_m,n) are also reported. The ionic strength dependence of Δ_rH'_m,n was fitted by means of the enthalpy SIT approach, giving the standard reaction enthalpies (Δ_rH⁰_m,n), entropies (Δ_rS⁰_m,n) and the partial molar enthalpy specific ion interaction coefficients (Δε_L(i,k)). The results show negative conditional reaction enthalpies for all complexes, which become more exothermic with increasing ionic strength. Simultaneously, the Δ_rS'_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_rH'_m,n and Δ_rS'_m,n at the highest value of I_m studied.