Abstract: The thermodynamic properties of system HCl-NaCl-C2H6O2-H2O was studied by EMF measurement in the cells without liquid junction: Pt, H2 (105 Pa)|HCl (m), C2H6O2 (w), H2O (1－w) | AgCl-Ag (A) Pt, H2 (105 Pa)|HCl (mA), NaCl (mB), C2H6O2 (w), H2O（1－w）| AgCl-Ag (B) at constant total ionic strength I=0.5, 1.0, 1.5 and 2.0 mol•kg－1, from 5 to 45 ℃, where mA and mB are the molalities of HCl and NaCl, respectively, w is the mass fraction of glycol in the mixed solvent, which is 10％ during all measurement. The standard electrode potential of Ag-AgCl in the mixed solvent has been determined from cell (A). The activity coefficients of HCl γA in the mixed solvent system have been determined from cell(B). The results show that the activity coefficients of HCl in HCl-NaCl solutions still obey Harned’s Rule. The standard transfer Gibbs free energies of HCl have been calculated. The primary, secondary and total medium effects of HCl have been calculated and discussed.

Key words: Activity coefficients, Harned’s rule, EMF, Medium effect, Glycol