STANDARD REFERENCE MATERIAL PROPOSED FOR ENTHALPY-OF-SUBLIMATION MEASUREMENTS - A COMPARATIVE-STUDY OF THE STANDARD MOLAR ENTHALPY OF SUBLIMATION OF FE(C-C5H5)(2) (FERROCENE) BY CALORIMETRY AND KNUDSEN-EFFUSION TECHNIQUES

The standard molar enthalpy of sublimation of Fe(c-C5H5)(2),bis(eta-cy clopentadienyl)iron(II), ferrocene, has been experimentally determined by the Knudsen-effusion method using a quartz-crystal microbalance. T he results from two series of experiments were found to be Delta(sub)H (m)(o)(298.15 K) = (74.34 +/- 0.41) kJ . mol(-1) and Delta(sub)H(m)(o) (280.74 K) = (74.60 +/- 0.79) kJ . mol(-1). This property has also bee n calorimetrically determined at the temperature T = 298.15 K, and the mean value from 12 experiments were found to be Delta(sub)H(m)(o) = ( 73.19 +/- 0.66) kJ . mol-1. All the literature results for the standar d molar enthalpy of sublimation reported to date which have been deter mined by indirect methods were critically evaluated, and the more reli able values retained. A global analysis including these selected value s as well as our effusion and calorimetric results leads to a weighted average value of Delta(sub)H(m)(o) = (73.42 +/- 0.59) kJ . mol(-1) fo r the standard molar enthalpy of sublimation of Fe(c-C5H5)(2) at T = 2 98.15 K. From this study we propose the use of ferrocene as a standard reference material for measurements of enthalpy of sublimation. (C) 1 995 Academic Press Limited.