Department of Chemistry, Faculty of Science Ferdowsi University of Mashhad, Mashhad, I.R. IRAN
The complexation reactions between Mg2+, Ca2+, Sr2+ and Ba2+ metal cations with 15-crown-5 (15C5) were studied in acetonitrile (AN)-methanol (MeOH) binary mixtures at different temperatures using conductometric method. 15C54 forms 1:1 complexes with Mg2+, Ca2+ and Sr2+ cations in solutions. The Ma2+ cation forms a 2:1 complex (ratio of lighand to cation) in solutions. A linear relation was observed for variation of logKf of 15C5-Ca2+ and 15C5-Sr2+ complexes vs the composition of An-MeOH binary systems, but a non-linear behavior was observed in the case of 15C5-Mg2+ complex in these binary mixtures. Selectivity of 15C5 for Mg2+, Ca2+ and Sr2+ cations is sensitive to solvent composition. In some cases and in certain compositions of the mixed solvent systems, the selectivity order is changed. The values of thermodynamic parameters (DH0c , DS0c ) for formation of 15C5-Mg2+ , 15C5-Ca2+ and 15C5-Sr2+ complexes were obtained from temperature dependence of stability constants and the results show that the thermodynamics of complexation reactions is affected by the nature and composition of the mixed solvents.