Crown ethers have been demonstrated to be highly selective complexing agents for many metal ions and can potentially be applied in thier separation and determination[1]. Thermodynamic studies of macrocyclic complexation reactions with metal ions not only result in important information on the thermodynamics of complexation reaction, but also lead to a better understanding of the high selectivity of these ligands towards different metal cations. It is well known that the stability and selectivity of complexations strongly depend on the solvating ability of the solvent[2,3]. The complexation reaction between a macrocyclic polyether, 4\',4\'\'(5\'\')-Di-tert-butyldibenzo-18-crown-6 (DtBDB18C6) and lantanium(III) ion was studied in methylacetat (MeOAc)-acetonitrile (AN) and tetrahydrofuran (THF) -acetonitrile (AN) binary solvent systems at different temperatures using a conductometric method. It was of interest to us, therefore, to investigate the dependence of the stability constant and thermodynamic parameters of the DtBDB18C6-La3+ complex on the composition of several binary solvent mixtures in order to see how the thermodynamics of complexation are affected by the solvent composition. In addition it was found that the stability of the 1:1 complex formed between La3+ ion. A Non –linear behavior was observed for the variation of logK f of the complexes versus the composition of binary mixed solvents, which was discussed in terms of heteroselective salvation and solvent –solvent interactions in binary solutions . It was found that the stability order of the complexes changes with changing the composition of the mixed solvents. Standard enthalpies and standard entropies of the complex formation were obtained from the temperature dependence of the stability constant. In all cases negative ΔH° c and ΔS° c values characterize the formation of DtBDB18C6-La3+ complex. The results obtained show that the stability of the complex is governed by the solvent medium and the thermodynamic parameters ΔH° c, ΔS° c and ΔG° c are sensitive to the composition of the mixed solvents. The values of stability constants of the1:1[M:L] complex was determined from conductometric data using a GENPLOT computer program[4]. The results revealed that, the stability constant of(DtBDB18C6.La)3+ complex in pure organic solvents varies in the order: AN>MeOAc>THF and in the case of binary mixed solutions at 25°C it follows the order: AN– MeOAc >AN–THF.

Keywords