Crown ethers and cryptands have attracted significant attetion from various field of science [1]. Crown ethers are macrocyclic polyethers which form stable and selective complexes with suitable cations [2]. The binding ability and selectivity of crown ethers to metal cations depend on several factors: such as the size of the crown ether cavity, metal ion radius, type of donor atoms, conformation of the crown ether, the nature of the solvent and etc [3]. Cryptands are three dimensional analogues of crown ethers but are more selective and complex the guest ions more strongly. The resulting complexes are lipophilic. The 116 three dimensional interior cavity of a cryptand provides a binding site - or nook - for \"guest\" ions. Cryptands form complexes with many \"hard cations\" including NH4+, lanthanoids, alkali metals, and alkaline earth metals. In contrast to crown ethers, cryptands bind the guest ions using both nitrogen and oxygen donors. Many cryptands are commercially available under the tradename \"Kryptofix\" [4] . In this paper, we report the results of thermodynamic study of complexation reactions between kryptofix 21 with Y3+ and Ce3+ cations in methanol-acetonitrile (MeOH/AN) and methanol-methyl acetate (MeOH/MeOAc) binary mixtures and their pure solvents at different temperatures using conductometric method. Kryptofix 21 forms a 1:1 [M:L] complex with Y3+ and Ce3+ cations in most solvent systems, but in the case of Y3+ cation, in addition of formation of a 1:1[M:L] complex, 2:1[M2L] and also 1:2[ML2] complexes are formed in pure MeOAc solution at all studied temperatures, which shows that the stoichiometry of the complexes may change by the nature of the solvent. The order of stability of the metal-ion complexes in (MeOH/AN) binary solution, at 25°C was found to be: (kryptofix21.Y)3+ > (kryptofix21.Ce)3+, but in the case of (MeOH/MeOAc) binary solution at the same temperature, it changes to: (kryptofix21.Ce)3+ > (kryptofix21.Y)3+. The values of stability constants of the 1:1[M:L] complexes were determined from conductometric data using a GENPLOT computer program. The thermodynamic parameters (ΔH°c and ΔS°c) for formation of (kryptofix21.Y)3+ and (kryptofix21.Ce)3+ complexes were obtained from temperature dependence of the stability constant using the van,t Hoff plots. The results show that the values of standard enthalpy (ΔH°c) and standard entropy (ΔS°c) change with the nature of the non-aqueous solvents.

Keywords