The complexation reaction of N-phenylaza-15- crown-5 (PhA15C5) with UO2 2? cation was studied in acetonitrile– methanol (AN–MeOH), acetonitrile–butanol (AN– BuOH), acetonitrile–dimethylformamide (AN–DMF) and methanol–propylencarbonate (MeOH–PC) binary solutions, at different temperatures by conductometry method. The conductance data show that the stoichiometry of the complex formed between PhA15C5 with UO2 2? cation in most cases is 1:1 [M:L], but in some solvent systems a 1:2 [M:L2] complex is formed in solutions. The results revealed that, the stability constant of (PhA15C5UO2)2? complex in the binary mixed solvents varies in the order: AN–BuOH[AN– MeOH[AN–DMF. In the case of the pure organic solvents, the sequence of the stability of the complex changes as: AN[PC[BuOH[DMF. A non-linear relationship was observed for changes of logKf of (PhA15C5UO2)2? complex versus the composition of the binary mixed solvents. The corresponding standard thermodynamic parameters (DHc,DSc) were obtained from temperature dependence of the stability constant. The results show that the values and also the sign of these parameters are influenced by the nature and composition of the mixed solvents.

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