Combustion Calorimetry of Rhombohedral Sulfur in Fluorine: A Question of Impurities. The Standard Molar Enthalpies of Formation of SF6(G) and SO2-4(Aq) at the Temperature 298.15 K

Document Type


Publication Date



A specially prepared sample of rhombohedral sulfur (USBM-P1b) used in several earlier thermodynamic investigations has been shown to contain mass fraction ≈ 310 · 10-6 of hitherto unsuspected impurities. Among the studies that may possibly be affected by this discovery is a determination by one of us of the energy of combustion of sulfur in fluorine from which was deduced the standard molar enthalpy of formation of sulfur hexafluoride ΔfHom(SF6, g, 298.15 K) (J. Chem. Thermodynamics 1985, 17, 349). Very recently, much purer sulfur, containing mass fraction <1 >· 10-5 of contaminants, has become available. The research reported in the present paper deals with the measurement, and comparison, of the energies of combustion, corrected for impurities, of the two samples of sulfur. There is good agreement between the results. Those obtained with the high-purity material are more precise, due, in large part, to the uncertainty associated with the impurity corrections, and they are used further to calculate ΔfH°m. Consequently, ΔfHom(SF6, g, 298.15 K) was adjusted by 0.5 kJ · mol-1 from our previous determination. The revised value is -(1221.7±0.3) kJ · mol-1. Similarly, for the reason that USBM-P1b was also used in three earlier oxygen-bomb calorimetric investigations which yielded ΔfH°m(H2SO4 · vH2O), we have recalculated the results of those experiments as well. In addition, three determinations of the enthalpy of oxidation of SO2 to H2SO4(aq) have been reexamined. In summary, on the basis of the six reassessed values for ΔfHom(H2SO4 · vH2O), we conclude that the CODATA-selected ΔfH°m(SO2-4, aq) should be adjusted very slightly, but still within the assigned uncertainty limits. © 1992 Academic Press Limited All rights reserved.

Journal Title

The Journal of Chemical Thermodynamics





First Page


Last Page




First Department

Chemistry and Biochemistry