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.

SCOPEThe apparatus described by Gibbs and Van Ness (1972) has been used systematically to measure solution vapor pressures as a function of liquid composition at 3OoC for 15 binary systems in vapor-liquid equilibrium (VLE). Data reduction by the method described in the preceding paper for P-x data yields correlating equations for the excess Gibbs free energy and the activity coefficients, from which the P-x-y relationship at 3OoC may be calculated. The systems studied are CC&, CHCl3, and CH2C12, each with tetrahydrofuran and with furan; tetrahydrofuran with furan; CH2C12 with methyl acetate, with acetone, and with l,4-dioxane; CHC13 with 1,4-dioxane; pyridine with acetone, with CHC13, and with CH2C12; and n-pentanol with n-hexane. Heat-of-mixing data, which are available in the literature for most of the systems reported, allow extension of these results to other conditions. CONCLUSIONS A N D SIGNIFICANCEThe rapidity with which P-x data may be taken commends the use of such data as the basis for formulation of VLE relationships for binary systems. These are useful in themselves, and they serve also in predictive schemes for multicomponent systems, for which experimental determination is much more tedious.Routine methods of data reduction are available, as reviewed in the preceding paper, and the reliability of the method depends solely on the reliability of the P-x data.Gibbs and Van Ness (1972) demonstrated for the ethanoln-heptane system agreement between results obtained with the present apparatus and data from the literature based on measurements with an equilibrium still. The results presented here are for systems representing a wide range of behavior, from large positive to large negative deviations from ideality.The experimental apparatus used in this work is fully described by Gibbs and Van Ness ( 1972). Liquid solutions of known composition are prepared in a thermostated test cell by volumetric metering of degassed liquids from accurate piston-injectors. Direct measurement of pressure in the cell provides for the solution vapor pressure. The raw data' therefore consist of pairs of values for pressure and liquid mole fraction at constant temperature and cover the full composition range from x1 = 0 to x1 = 1.As indicated by the discussion of P-x data in the preceding paper, the raw data are fit by the least-squares spline-fit procedure of Klaus and Van Ness (1967), and this provides the input for the numerical procedure of Mixon et al. (1965) which yields the g = GE/RT versus x1 relationship in accord with Equation (9b) of the preceding paper. The resulting numerical values of g/xlx2 or of ln(yl/yz) are then fit with respect to x1 by an appro-

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