Spectroscopic determination of enthalpies of sublimation of organic materials in the vapor phase: Benzoic acid, ferrocene, and naphthalene

Techniques for determining the enthalpies of sublimation of low volatile materials in the vapor phase are quite rare. Although benzoic acid, ferrocene, and naphthalene are used as reference materials for indirect determination of vapor pressure of volatile materials, the discrepancy of the values of their enthalpies of sublimation is considerably large (∼10-15 kJ/mol). In this work, the enthalpies of sublimation of vapors of these materials are determined using absorbance spectroscopy in the ultraviolet region of the electromagnetic spectrum. This methodology is based on the linear proportionality between the density of the saturated gas of the material and the absorbance of the gas. The determined values of enthalpies of sublimation are in excellent agreement with the mean value of those reported in literature and those recommended by the ICTAC. This method is shown to be accurate and rapid.