The main characteristic of a crown ether is the complexation of the ether oxygens with various ionic species. The factors which influence the stability of the metal cation -crown ether complexes include: the relative size of the cation and the cavity of the crown ether, the number of oxygen atoms in the ring, the coplanarity of the oxygen atoms ,the symmetrical placement of the oxygen atoms, the basicity of the oxygen atoms, steric hindrance in the polyether ring ,the nature of the cation and electrical charge on the cation [2,3] .The ability of the solvent molecules to compete with the donor atoms of the ligand towards the coordination sites of the cation and also the solvation of the ligand and the resulting complex are another factors that can thermodynamically influence the complexation process between such macrocyclic ligands and the metal cations [4,5].The complexation reactions between Co 2+, Cu2+, Ni2+ metal cations with the macrocyclic ligand , kryptofix -21,in pure acetonitrile , ethanol and also in acetonitrile- ethanol (AN-EtOH) binary mixtures have been studied at different temperatures using conductometric method. the conductance data show that the stoichiometry of the complexes in most cases is 1:1 [ML]. But in the case of Cu2+ cation a 1:2 [ML2] complex is formed in pure EtOH , which shows that stoichiometry of the complexes may be changed by the nature of the medium. The stability constants of the 1:1[M: L] complexes were determined using a computer program, GENPLOT. The stability order of complexes between kryptofix -21 and Co2+, Ni2+, Cu2+ cations at 250C in the binary solvent solution of AN- EtOH (mol % AN 0) is : ( kryptofix-21. Ni ) 2+ > (kryptofix- 21. Cu) 2+ > ( kryptofix-21. Co ) 2+, but in case of another AN- EtOH binary solvent solution (mol % AN 25) it was found to be : (kryptofix-21. Cu ) 2+> (kryptofix-21. Co) 2+> ( kryptofix-21. Ni) 2+ and in case of AN- EtOH binary mixed systems (mol % AN 50 and 75) the stability order changes as : ( kryptofix - 21. Co) 2+> ( kryptofix-21. Cu) 2+> ( kryptofix-21. Ni) 2+ The order of the stability of the complexes in pure AN is (kryptofix - 21. Co) 2+> (kryptofix-21. Ni) 2+.> (kryptofix-21. Cu) 2+ .The values of stability constants of complexes ,which were obtained from conductometric data , show that the stability of complexes is affected by the nature and composition of the binary mixed solvents and a non-linear behavior was observed for variation logKf of the complexes versus the composition of the solvent system .The values of thermodynamic parameters (ΔHc 0, ΔSc 0 ) for formation of complexes show that in most solution systems , the complexes are enthalpy destabilize but entropy stabilized.

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