Thermodynamics Research Center / ThermoML | Thermochimica Acta

ABSTRACT

The vapor pressures of the liquid 2,4- 2,6- and 3,4-dihydroxybenzoic acids were determined by thermogravimetry (TG) under both isothermal and non-isothermal conditions, respectively. From the temperature dependence of vapor pressure derived by the experimental data the molar enthalpies of vaporization deltalgHmdeg (less thanTgreater than) were determined, respectively, at the middle less thanTgreater than of the respective temperature intervals: 2,4-dihydroxybenzoic acid (l): ln(p/Pa) = (26.52+-0.67)- (9495+-350)(K/T) (511-534 K) 2,6-dihydroxybenzoic acid (l): ln(p/Pa) = (28.20+-0.49)- (9891+-230)(K/T) (454-477 K) 3,4-dihydroxybenzoic acid (l): ln(p/Pa) = (32.78+-0.80)- (12651+-400)(K/T) (485-506 K) The melting temperatures and the molar enthalpies of fusion of these compounds were measured by d.s.c.. The vapor pressures of these compounds in the solid state, measured by torsion-effusion technique, were compared with recently published data, while the corresponding molar sublimation enthalpies were determined. In order to validate the vapor pressure results determined by TG the experimental molar enthalpies of fusion were compared with those calculated by subtracting the molar vaporization enthalpies to the sublimation ones, both adjusted to their respective melting temperatures. Finally, the standard (pdeg =0.1 MPa) molar enthalpies, entropies and Gibbs energies of sublimation, corrected at the reference temperature of 298.15 K, have been calculated using the estimated heat capacity differences between gas and liquid for vaporization experiments and the estimated heat capacity differences between gas and solid for sublimation experiments.