Abstract: The T-P-Xdata of water-acetonitrile-(KCl, CaCl₂, KI) system, at 26.66-93.33 kPa and in the salt concentration range of 0.1-0.7 mol·kg^-1, were measured by using the CP Iebul-liometer.The Clausius-Clapeyron-type equation of binary mixtures containing salt was derived by using the thermodynamical theory.Under the particular condition, this equation may be reduced to several formula presented in literatures.The theoretical equation was proved to be correct by the experimental results.It has been shown from the thermodynamical theory and the experimental results that: (1) The relation between Pand Tis satisfied with the Clausius-Clapeyron-type equation, when the concentration of the salt and the relative composition of the acetonitrile aqueous solutions are fixed.In other words, plotted IgPagainst 1/T, a series of straight lines can be obtained.(2) When the relative composition of binary solution and the pressure of the system are fixed, the boiling point of the systems is decreased by the addition of KCIand CaCl₂, and the extent of decrease of CaCl₂ is larger than that of KCI.However, under the same condition, the boiling point of the systems is increased by the addition of KI.(3) For each of the salts, the change tendency is increased with the increase of the concentration of acetonitrile in the water-acetonitrile mixtures.The reason for the unusual phenomena of the decrease of boiling point may be attributed to the difference of interactions between salt-water and salt-acetonitrile in the salted binary systems.The effect of salt on the properties of binary systems depends on the net result of interaction between the salts and the mixed solvents.

Key words: Salt effect, Interaction, Linear relationship