The complexation reactions between Mg2+, Ca2+ and Sr2+ cations with themacrocyclic ligand, benzo-15-crown-5 (B15C5), in pure acetonitrile, water, methanol and tetrahydrofuran and also in acetonitrile–water (AN–H2O) and in methanol–tetrahydrofuran (MeOH–THF) binary mixtures have been studied at different temperatures using conductometric method. The conductance data show that the stoichiometry of the complexes in most cases is 1:1 [ML]. But in the case of Ca2? cation a 1:2 [ML2] complex is formed in pure THF, which shows that the stoichiometry of the complexes may be changed by the nature of the medium. The values of stability constants of complexes, which were obtained from conductometric data, show that the stability of complexes is affected by the nature and composition of the binary mixed solvents and a non-linear behavior was observed for variation of logKf of the complexes versus the composition of the solvent systems. The results show that the selectivity order of B15C5 for the metal cations in two AN–H2O binary solutions (mol% AN = 25.71 and 50.94) at 25 _C is: Mg2+>Sr2+>Ca2+. The values of thermodynamic parameters (DHc0, DSc0) for formation of complexes were obtained from temperature dependence of stability constants of complexes using the van’t Hoff plots. The results show that the values and also the sign of these parameters are influenced by the nature and also the composition of the binary mixed solvents.

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