The complexation reactions of dibenzo-18-crown-6 (DB18C6) with Ce3+, Y3+, UO2+2 and Sr2+ cations were studied in acetonitrile–dioxane (AN–dioxane) binary solvent solutions at different temperatures by the conductometric method. The stability constants of the resulting 1:1 complexes were determined from computer fitting of the conductance–mole ratio data. The results show that dibenzo-18-crown-6 does not exhibit selectivity for the cation whose ionic size is closest to the cavity size of this macrocyclic ligand in AN–dioxane binary solvent solutions. A nonlinear relationship was observed between the stability constants (log10Kf) of these complexes with the composition of the AN–dioxane binary solvent. Values of thermodynamic parameters (_H◦c ,_S◦c ) for complexation reactions were obtained from the temperature dependence of the stability constants. The results show that the values along with the sign of these parameters are influenced by the nature and composition of the mixed solvent.

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