The ability of crown ethers to form stable complexes with cations has spurred interest in these compounds .One of the unique features of crown ethers is that they can chelate metal ions, therefore they function as phase transfer catalysts by complexing a cation then bringing the complexed-cation-anion pair into the non-polar phase [1].Studies of crown ether complexes in various solvents, show that the thermodynamic and kinetic parameters for complexation processes and even the stiochiometry of crown ether complexes are affected by the nature and composition of the solvent system [2,3]. The goal of this investigation is to study the effect of the nature and composition of the mixed solvents on selectivity and thermodynamic of complexation reactions of 4\\\\\\\'-Nitrobenzo-15-Crown-5 with Mg2+, Ca2+, Sr2+ and Ba2+ metal cations in acetonitrile (AN)-methanol (MeOH) binary systems using the conductometric technique. 4\\\\\\\'-Nitrobenzo-15C5 forms 1:1 complexes with Mg2+, Ca2+ and Sr2+ cations in solutions. The Ba2+ cation forms a 2:1 complex (ratio of ligand to cation) in solutions. The stability of the complexes is sensitive to the solvent composition and a non-linear behaviour was observed for variation of logKf of the complexes versus the composition of the binary mixed solvents. The selectivity order of 4\\\\\\\'-Nitrobenzo-15C5 for Mg2+, Ca2+ and Sr2+ metal cations is changed with the composition of AN-MeOH binary solution. The order of stability of the metal ion complexes in pure MeOH and in 50 mole% of AN in AN-MeOH binary system at 25°C is Sr2+ > Mg2+ > Ca2+ ,but in the case of 25 mole% of AN in this binary solvent, is: Mg2+ ≈ Ca2+ > Sr2+. The values of thermodynamic parameters (ΔH°c , ΔS°c) for formation of (4\\\\\\\'-Nitrobenzo-15C5,Mg)2+, (4\\\\\\\'-Nitrobenzo-15C5.Ca)2+ and (4\\\\\\\'-Nitrobenzo-15C5.Sr)2+ complexes were obtained from temperature dependence of the stability constants and the results show that the thermodynamics of complexation reactions is affected by the nature and composition of the mixed solvents. A non-linear behaviour is observed between the ΔS°c and the composition of the mixed solvents.

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