The numerical method of orthogonal collocation is used within a thermodynamically consistent framework to calculate equilibrium vapor compositions from P-T-x data for binary systems. The method may be used for isothermal or isobaric data in both the normal liquid and in the critical region, and it applies to any choice of standard states.

To use the method it is necessary to have experimental or estimated values of vapor phase fugacity coefficients for both components in the mixture and liquid molar volumes (isothermal case) or heats of mixing (isobaric case). If vapor composition data are available, each set of P-T-x-y data may be checked for thermodynamic consistency by comparing y (experimental) with y (calculated). Illustrative calculations are given. The method is shown to provide an excellent procedure for obtaining Henry's constants from P-x data for systems with one noncondensable component.