Thermodynamic Equilibria in Xylene Isomerization. 1. The Thermodynamic Properties of p-Xylene

Measurements and calculations leading to the determination of thermodynamic properties for the gaseous and condensed phases of p-xylene (Chemical Abstracts registry number (supplied by the authors) [106-42-3]) are reported. All measurement results reported were obtained with a differential-scanning calorimeter (DSC). The critical temperature was measured by DSC. Saturation heat capacities for the liquid phase between 370 K and 550 K, the critical density and the critical pressure were derived with fitting procedures involving the new DSC results and literature vapor pressures and densities. Results were combined with heat capacities reported in the literature obtained with adiabatic calorimetry and the enthalpy of combustion to derive standard molar entropies, enthalpies, and Gibbs free energies of formation at selected temperatures between 250 K and 550 K. The standard state is defined as the ideal gas at the pressure p = p° = 101.325 kPa. Standard entropies are compared with those calculated statistically on the basis of assigned vibrational spectra. Results are compared with literature values. Literature vapor pressures, enthalpies of vaporization, virial coefficients, densities, and heat capacities for the condensed and gaseous phases are checked for consistency with the values used in this research.