Conductometric titrations have been performed in some binary solvent solutions of acetonitrile (AN), 1,2–dichloroethane (DCE), ethylacetate (EtOAc) and methylacetate (MeOAc) with methanol (MeOH), at 288, 298, 308, and 318 K to give the complex stability constant and the thermodynamic parameters for the complexation of lanthanum(III) cation with 4,7,13,16,21,24–hexaoxa–1,10–diazabicy clo[8.8.8]–hexacosane (cryptand 222). The stability constant of the resulting 1:1 complex at each temperature was determined from computer fitting of the conductancemole ratio data. The results revealed that, the stoichiometry and the stability order of (cryptand 222 ⋅ La)3+ complex changes with the nature and also the composition of the solvent system. A nonlinear relationship was observedbetween the stability constant (logKf) of (cryptand 222 ⋅ La)3+ complex versus the composition of the binary mixed solvents. Thermodynamically, the complexation of lanthanum(III) cation with the cryptand 222, is mainly entropy governed and the values of these parameters are influenced by the nature and composition of the binary mixed solvent solutions.

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