Correlations of the excess Gibbs function and the activity coefficients of a binary, liquid‐phase system at constant temperature depend on reduction of P‐x‐y data taken for the system in vapor‐liquid equilibrium. It is shown that use of a full P‐x‐y data set is for all practical purposes based on just the P‐y values. Alternative procedures require only P‐x or P‐y data. In any case, effective use of a full P‐x‐y data set can be made only if redundent experimental values of Px,y or P are compared for consistency with calculated values. This cannot be accomplished by means of the popular area test for thermodynamic consistency.

The Rossini lecture affords the lecturer an opportunity to review his contributions to a subject and to reflect on them from the perspective of an extended career. The subject here is vapor/liquid equilibrium, and the topics reviewed include property relations, experimental measurements, consistency tests, and data reduction and correlation.In the first of these lectures Dr. Rossini remarked that the sponsoring group was named the Commission on Thermodynamics rather than the Commission on Chemical Thermodynamics so that engineering thermodynamics could be included. I am, however, the first engineer to give this lecture, a possible exception being Dr. Rossini himself. His B.S. degree was in fact in chemical engineering; however, he immediately turned to graduate work in chemistry, and certainly considered himself a chemist during his professional career. So it would seem that all of my predecessors have been chemists, very distinguished chemists, who have set a high standard indeed for this lecture. As the tenth Rossini lecturer I am expected to draw upon contributions I have made to thermodynamics. Of greatest practical interest no doubt are those relating to vapor/liquid equilibrium (VLE). I propose therefore to expound on the thermodynamic treatment of VLE data, with consideration limited to binary systems at low to moderate pressures, a scope suited to the occasion. This topic is more complex and less widely understood than might be supposed. PROPERTY RELATIONSCentral to the correlation of VLE data are the thermodynamic excess properties, which describe the behavior of the liquid phase. These properties, introduced during the mid-l930's, express the differences between actual property values of a solution and the values which would be exhibited by an ideal solution at the same temperature, pressure, and composition. The equation interrelating these properties was derived in one of my earliest papers on thermodynamics (1). Now known as the fundamental excess-property relation, it is an integral part of the thermodynamic structure that we lay before undergraduate students (2):where yi is the activity coefficient of species a in solution and g = GE/RT Implicit in Eq. (1) is the relation,

N. Y. 12181 A new apparatus for the measurement of vapor pressures of liquid solutions is described. Liquid solutions of known composition are prepared in a test cell by volumetric metering of degassed liquids from accurate piston-injectors. Once the injectors are charged, vapor pressures for a binary system at constant temperature over the entire composition range are measured in a day. Solution of the coexistence equation provides the isothermal vapor-liquid equilibrium relationship.

Through an empirical but rational approach to the development of equations to represent the composition dependence of the excess Gibbs function for binary liquid systems, it has been found possible to correlate VLE data precisely, even for highly nonideal systems. This makes possible the application of Barker's method to the reduction of VLE data on a routine basis. The validity of the method is demonstrated through application to several sets of data from the literature, and new experimental data are presented for six diverse binary systems in vapor-liquid equilibrium at 5OOC. M SCOPEIn Part I of this series of papers Van Ness et al. (1973) described the numerical methods by which one may accomplish the reduction of binary VLE data to yield a correlation for the excess Gibbs function of the liquid phase. Byer et al. (1973) in Part I1 demonstrated the effectiveness of the numerical procedure based on P-x data alone for 15 binary systems. There is an additional procedure for the reduction of P-x data, developed by Barker (1953), that was not pursued in these earlier papers. It is based on Equation (15) only one showed such large deviations from ideality that it could not be characterized by the four-suffix Margules equation. In fact, the data for this system, n-pentanol-n-hexane, defied representation within the precision of the data by all known equations. It was the existence of such highly nonideal systems that deterred us from the exploitation of Barker's method as a general procedure for the reduction of P-x data. However, Barker's method is a most attractive one-step fitting procedure, and this fact provides the incentive for development here of the means by which it can be made more generally applicable. CONCLUSIONS AND SIGNIFICANCEWe have found that highly nonideal systems, even those verging on instability, may be very precisely fit by one of two simple equations, the 5-suffix Margules equation or a new equation called the modified Margules equation. Both are basically four-parameter equations that reduce to the three-and four-suffix Margules equations as special cases. Thus they retain all of the advantages associated with the Margules equations, including the capability of predicting limited liquid-liquid miscibility. Applied to data from the literature for the methanol-carbon tetrachloride and chloroform-ethanol systems these equations are shown to yield very precise correlations of the P-x-y data when regression is carried out by Barker's method, which is based on just the P-x data. From this we conclude that reliable P-x data only are required to provide reliable VLE relationships. Moreover, it is shown that use of the reported y values along with the P-x data in the datareduction process distorts the correlation of both the P-x and y-x relationships. Barker's method in conjunction with the two types of Margules equations mentioned earlier are applied in the correlation of new VLE data at 5OoC for the binary systems acetone-chloroform, acetone-methanol, chloroform-methanol, chloroform-ethanol, chlo...

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