Abstract: The conductance bridge was used to measure the molar conductances for H_2O-C_2H_5OH-NaCl system and the headspace gas chromatograph was used to measure the activity of water and ethanol in H_2O-C_2H_5OH-(NaCl, KCl, KBr) systems. The associ-ation constant, limiting molar conductance and distance parameter in H_2O-C_2H_5OH-MX (NaCl, KCl, CsCl, KBr) systems are regressed from Lee-Wheaton model. The results show the orders of association ability of ions are Na~+>K~+>Cs~+ and Cl~->Br~-, and these four salts form ion-pair separated by solvent in ethanol. The improved Pitzer-Li equation is applied to calculate the short-range term of activity coefficient. The association of ions is taken into account and the results show that the procedure of our calculation is reasonable. When the concentration of salt increases, the activity of water decreases, but the activity of C_2H_5OH increases. It means that these salts present a salting-in effect for water but salting-out effect for C_2H_5OH.

Key words: Ion association, Molar conductance, Activity, Salting-in effect, Salting-out effect