The complexation reaction between UO22+ cation with diaza-15-crown-5 (DA15C5) was studied in acetonitrile-methanol (AN-MeOH),acetonitrile-ethanol (AN-EtOH), water-methanol (H2O-MeOH), waterethanol(H2O-EtOH), dimethyl formamide-ethanol (DMF-EtOH), dimethylformamide-ethyl acetate (DMF-EtOAc), propylene carbonate-methanol(PC-MeOH), propylene carbonate-ethanol (PC-EtOH) and dimethyl ormamide-propylene carbonate (DMF-PC) binary mixtures at 25 ºCusing the conductometric method. The conductance data show that the stoichiometry of the complexes formed between DA15C5 and UO22+cation in most cases is 1:1 [M:L], but in some solvent systems also a1:2 [M:L2] complex is formed in solution. The results obtained in this study show that the mechanism of complex formation reactions between the macrocyclic ligands and the metal cations may change with the nature and even the composition of the mixed binary solvents. The values of stability constant of (DA15C5.UO22+) complex which were obtained from conductometric data, show that the stability of the complex is affected by the nature and also the composition of the solvent system and in all cases, a non-linear behaviour is observed for the variation of log Kf of the (DA15C5.UO22+) complex versus the composition of the binary mixed solvents. The stability order of (DA15C5.UO22+) complex in pure studied solvents was found to be: PC > H2O > DMF~ MeOH.

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