Standard reference material proposed for enthalpy-of-sublimation measurements. A comparative study of the standard molar enthalpy of sublimation of Fe( c-C 5H 5) 2(ferrocene) by calorimetry and knudsen-effusion techniques

Abstract

The standard molar enthalpy of sublimation of Fe( c-C 5H 5) 2, bis(η-cyclopentadienyl)iron(II), ferrocene, has been experimentally determined by the Knudsen-effusion method using a quartz-crystal microbalance. The results from two series of experiments were found to be Δ sub H ∘ m(298.15K) = (74.34 ± 0.41)kJ·mol −1and Δ sub H ∘ m(280.74K) = (74.60 ± 0.79)kJ·mol −1. This property has also been calorimetrically determined at the temperature T= 298.15K, and the mean value from 12 experiments were found to be Δ sub H ∘ m= (73.19 ± 0.66)kJ·mol −1. All the literature results for the standard molar enthalpy of sublimation reported to date which have been determined by indirect methods were critically evaluated, and the more reliable values retained. A global analysis including these selected values as well as our effusion and calorimetric results leads to a weighted average value of Δ sub H ∘ m= (73.42 ± 0.59)kJ·mol −1for the standard molar enthalpy of sublimation of Fe( c-C 5H 5) 2at T= 298.15K. From this study we propose the use of ferrocene as a standard reference material for measurements of enthalpy of sublimation.