Estimating vapor enthalpies of sublimation by rising-temperature absorbance spectroscopy

Determining the enthalpies of sublimation of low volatile materials in the vapor phase is favored. Although benzoic acid, ferrocene, and naphthalene are used as reference materials in instrumentation calibration for indirect determination of vapor pressure of low volatile materials, the discrepancy of the values of their enthalpies of sublimation is considerably large (∼10-15 kJ mol^-1). Time is a crucial factor in determining thermodynamic parameters for hazardous materials. In this article, we illustrate the use of UV-Vis absorbance spectroscopy in determining the enthalpies of sublimation of these materials in the vapor phase non-isothermally. The enthalpies of sublimation of benzoic acid, ferrocene, and naphthalene are found to be 86.11 ± 0.6, 72.40 ± 0.8, and 69.13 ± 0.3 kJ mol^-1, respectively. The determined values of enthalpies of sublimation are in good agreement with the mean value of those reported in the literature and those recommended by the kinetic committee of ICTAC. The results suggest that UV absorbance spectroscopy is a reliable tool in non-isothermally measuring the thermodynamic properties of materials in the vapor phase.