Cyclic polyethers(crown ethers) are selective complexing agents for the cations[1-3]. They are utilized to extract cations from aqueous solutions into hydrophobic, nonpolar organic solvents and to carry these cations selectively through solvent-polymeric liquid membranes[4,5]. The complexation processes between mercury(II) metal cation with macrocyclic ligand, 1,13-bis(8-quinolyl)-1,4,7,10,13-pentaoxatridecane(kryptofix5), were studied in acetonitrilemethanol( AN-MeOH), acetonitrile-tetrahydrofuran(AN-THF), acetonitrile-methylacetate(AN-MeOAc) and acetonitrile-nitrobenzen(AN-NB) binary solvents, at different temperatures using conductometric method. The conductance data show that the stoichiometry of the complexes formed between this macrocyclic ligand and Hg2+ cation in most cases is1:1(ML). The values of stability constants of the1:1[M:L] complex was determined from conductometric data using a GENPLOT computer program[6]. The results revealed that, the stability constant of(kryptofix5.Hg)2+ complex in pure organic solvents varies in the order: NB>MeOAc>AN>MeOH>THF and in the case of binary mixed solutions at 25°C it follows the order: AN–NB>AN–MeOAc>AN– 83 MeOH>AN–THF. A non –linear relationship was observed for changes of log Kf of(kryptofix5.Hg)2+complex versus the composition of the binary mixed solvents. The values of thermodynamic parameter(ΔHc °and ΔSc °) for formation of the complex were obtained from temperature dependence of the stability constants, using the van’t Hoff plots. The results show that the values of standard enthalpy(ΔHc °) and standard entropy(ΔSc °) change with the nature of the non-aqueous solvents. The corresponding standard thermodynamic parameters(ΔH° c and ΔS° c) were obtained from temperature dependence of the stability constant. 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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