Thermodynamics Research Center / ThermoML | Journal of Chemical Thermodynamics

ABSTRACT

The vapor pressures of the solid and liquid 2,4- and 3,4-dinitrobenzoic acids were determined by torsion effusion and thermogravimetry under both isothermal and non-isothermal conditions, respectively. From the temperature dependence of vapor pressure derived by the experimental torsion effusion and thermogravimetry data the molar enthalpies of sublimation Dg crH m (hTi) and vaporization Dgl H m (hTi) were determined, respectively, at the middle hTi of the respective temperature intervals. The melting temperatures and the molar enthalpies of fusion of these compounds were measured by d.s.c. Finally, the results obtained by all the methods proposed were corrected at the reference temperature of 298.15 K 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. Therefore, the averages of the standard (p = 0.1 MPa) molar enthalpies, entropies and Gibbs free energies of sublimation at 298.15 K, have been derived: Compound Dg crH m o298:15 KP/ (kJ mol 1) Dg crS m o298:15 KP/ (J mol 1 K 1) Dg crG m o298:15 KP/ (kJ mol 1) 2,4-Dinitrobenzoic acid 134 +- 3 244 +- 8 61 +- 5 3,4-Dinitrobenzoic acid 129 +- 3 234 +- 8 59 +- 5 2009 Elsevier Ltd. All rights reserved. 1. Introduction In the last decade, the 3,5-dinitrobenzoic acid was studied for its ability to form adducts with several compounds having very interesting supramolecular assemblies [1,2], while the structures of the adducts between the 2,4- and 3,5-dinitrobenzoic acids with pyridine and 3,5-dimethylpyridine respectively, have been examined with a particular view to the N H O hydrogen bonding interaction [3,4]. 2,4-Dinitrobenzoic acid is contained in slushing compounds for anticorrosion protection of metals [5,6] or coated with a Cr-laminated glass support when mixed with crosslinkable buthylmetacrylate and then heated under defined condition [7]. Aqueous emulsions of some dinitrobenzoic acids, picric acid and 2,4,6 trinitrotoluene were used in order to render steel sheet and strip C-smut free after batch annealing [8]. Crystals of 3,4-dinitrobenzoic acid with a bitter taste and melting temperature of 439 K were used in quantitative sugar analysis [9]. Despite a great interest has been devoted to the different research areas involving the 3,5-dinitrobenzoic acid, the only thermochemical data available in the literature on nitrobenzoic derivatives is concerning the mono- and di-substituted nitro benzoic acids [10 11], and, to the best of our knowledge, no relevant studies were published on dinitrobenzoic acids (DNBAs), except for two early papers written by Lebedeva et al. [12] and Verkade [13]. Previous studies carried out in our laboratories by coupling thermogravimetry (TG) with torsion effusion techniques substantially confirms the reliability of the former for vapor pressure measurements in the range from 10 to 103 Pa. [14,15]. Therefore, as a follow-up of previous studies concerning the vapor pressure of pesticides measured by a thermobalance in a flowing inert gas atmosphere [14 17], in this work we focused our attention on the 2,4- and 3,4-dinitrobenzoic acids (2,4-DNBA and 3,4-DNBA, respectively) (figure 1) aiming the measurements of their vapor pressures and standard (p = 0.1 MPa) molar enthalpies of sublimation (and vaporization) from the temperature dependence of vapor pressure determined above the solid (and liquid) using effusion (and thermogravimetry) techniques.