A group of chemical compounds called crown ethers has aroused a great interest in recent years because of their characteristics which are of importance in preparative chemistry and in biology. New fields of application in research and potential uses in industry have been found for these macrocyclic compounds. A variety of physico-chemical techniques such as calorimetry[1], polarography[2], potentiometry[3], NMR spectrometry[4] and conductometry[5] have been used to study the complex formation between crown ethers and various metal ions in solutions. In the present paper, we report the results which are obtained in conductometric study of complexation of La3+, UO22+, Ag+ and NH4+ cations by DCH18C6 in acetonitrile-tetrahydrofuran binary systems at different temperatures. The results show that with the exception of complexation of NH4+ cation with DCH18C6 in pure acetonitrile, the stoichiometry of all the complexes is 1:1(M:L). The stability constants of the complexes were determined using a GENPLOT computer program. The non-linear behaviour which was observed for changes of logKf of the complexes versus the composition of the mixed solvent was discussed in terms of solvent-solvent interactions in their binary solutions which result in changing the chemical and physical properties of the constituent solvents when they mix whit one another and, therefore, changing the solvation capacities of the metal cations, crown ether molecules and even the resulting complexes with changing the mixed solvent composition. The results show that the selectivity of DCH18C6 for the studied cations changes with the composition of the AN/THF binary systems. The sequence of stabilities of complexses in an acetonitrile- tetrahydrofuran binary solution (mol%AN=75.0) at 25 ˚C is:(DCH18C6,La)3+> (DCH18C6,UO2)2+> (DCH18C6,Ag)+> (DCH18C6, NH4)+,but in the case of other binary systems of AN/THF (mol%AN=25.0and50.0) is: (DCH18C6,La)3+ > (DCH18C6,NH4)+ > (DCH18C6,UO2)2+ > (DCH18C6,Ag)+. The values of enthalpy changes (∆H ºC) for complexation reactions were obtained from the slope of the van.t Hoff plots and the changes in standard entropy (∆S ºC) were calculated from the relationship: ∆GºC,298.15 = ∆H ºC – 298.15 ∆S ºC . The obtained results show that the thermodynamics of complexation reactions is affected by nature and compositions of the mixed solvents and in all cases, positive values of ∆S ºC characterize the formation of the complexes. Anon-monotonic behaviour was observed for changes of ∆S ºC and composition of AN-THF binary solvent.

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