Thermodynamic properties of pyridine .1. Vapor pressures, high-temperature heat capacities, densities, critical properties, derived thermodynamic functions, vibrational assignment, and derivation of recommended values

Measurements leading to the calculation of standard thermodynamic properties for pyridine (Chemical Abstracts registry number 110-86-1) are reported for temperatures from the triple-point temperature T-tp=231.51 K to T=560 K. The standard state is defined as the ideal gas at the pressure p=p degrees=101.325 kPa. Vapor pressures were measured by comparative ebulliometry for the pressure range 5 kPa to 270 kPa (temperature range: T=319 K to T=426 K). Two-phase (liquid + vapor) heat capacities were measured for the temperature range 300 <(T/K) < 580 with a differential scanning calorimeter (d.s.c.), and saturation heat capacities C-sat were derived. Densities for the liquid phase were measured for the temperature range 323 ( (T/K) < 523. The critical temperature T-c was determined experimentally by d.s.c., and the critical pressure acid critical density were derived. Enthalpies of vaporization were calculated. Literature values for enthalpy increments and entropies by adiabatic heat-capacity calorimetry for the temperature range 10 < (T/K) < 360 and the molar energy of combustion were combined with the present results to derive standard entropies, enthalpy increments, and Gibbs free energies of formation at temperatures between T-tp=231.51 K and T=560 K. A new assignment of the fundamental vibrational wavenumbers for pyridine is reported. Comparisons with standard entropies calculated statistically provide corroboration for the calorimetrically derived values. The results are used to test the effectiveness of several common vapor-pressure formulations in the representation of broad pressure ranges. (C) 1996 Academic Press Limited