Crown ethers which first synthesized in 1967 by Pedersen, are macrocyclic polyethers which are able to form stable and selective complexes with various metal cations. Complexes formed by crown ethers, can be utilized as solvent extraction agents, construction of ion selective electrodes, selective separation, selective ion-transport and liquid chromatography [1-3]. The complex formation between UO2 2+ metal cation with 1,13-bis(8-quinolyl)-1, 4,7,10,13- pentaoxatridecane (kryptofix5) as microcyclic ligand was studied in ethylacetate -methanol (EtOAc-MeOH), ethylacetate-acetonitrile (EtOAc-AN), ethylacetate-ethanol (EtOAc-EtOH) and ethylacetate-methylacetate (EtOAc-MeOAc) binary solvents system at different temperatures (15oC, 25 oC, 35 oC, 45 oC) using conductometric methods [4,5]. The conductance data show that the stoichiometry of the complexes in most binary mixed solvents is 1:1 [L:M]. The stability constants of the complex was obtained from fitting of molar conductivity curves using a computer program, Genplot [6]. The nonlinear behavior which was observed from variations of log Kf of the complex versus the composition of the binary mixed solvents was discussed in terms of changing the chemical and physical properties of constituent solvents when they mix with each other and , therefore, changing the solvation capacities of the metal cations , crown ether molecules and even the resulting complex with changing the mixed solvent composition. The standard thermodynamic parameters for formation of complex was determined from Van,t Hoff plots. The results show that the values and also the sign of these parameters are influenced by the nature and composition of the mixed solvents.

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