Crown ethers which first synthesized in 1967 by Pederson[1], are macrocyclic polyethers which are noted for their remarkable selectivity towards specific cations [2] and formation of stable and selective complexes with them [3]. Cryptands are three dimensional analogues of crown ethers [4]. Most of them are available as the commercial name ‘kryptofix’. In the present work, the complexation reactions between Cd2+ and Y3+metal cations with the macrocyclic ligand, kryptofix222 (4,7,13,16,21,24-hexaoxa-1,10-diazabicyclo[8.8.8] hexacosane),were studied in acetonitrile-ethanol (AN-EtOH), acetonitrileethylacetate (AN-EtOAc) binary mixed solvent solutions, and in pure acetonitrile (AN), pure ethanol (EtOH) and pure ethylacetate at different temperatures using conductometric method. The conductance data show that in most solvent systems, kryptofix222 forms a 1:1 complex [M:L] with Cd2+ and Y3+cations, but in some of the solvent systems,in addition of formation a 1:1 [M:L] complex, 1:2 [ ] and 1:3[ ] complexes are probably formed in solutions. The stability order of (krytofix222.Cd) 2+ and (kryptofix222.Y) 3+ complexes in pure studied solvents at 25 was found to be: AN> EtOH. The stability constants of the 1:1 complexes were determined using a computer program, Genplot [5] . The results obtained in this study, show that the (kryptofix222.Cd)2+ complex is more stable than (kryptofix222.Y) 3+ in most solvent systems at various temperatures and the stability constants of the 1:1 complexes are affected by the nature and also the composition of the binary solvent solutions. A non-linear behavior was observed for the changes of logKf of the complexes versus the composition of the binary mixed solvents. The values of the standard thermodynamic parameters (ΔH°c , ΔS°c) were obtained from temperature dependence of the equilibrium constants of (kryptofix222.Cd) 2+ and (kryptofix222.Y) 3+ complexes using the van‘t Hoff plots. The obtained results show that the complexation processes between Cd2+ and Y3+ cations with C222 in all cases are entropy stabilized but from the enthalpy view point, depending on the nature and composition of the mixed solvents they are exothermic and endothermic or athermic.

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