Rare-earth sulfides at high temperature. I. On the existence of equilibrium in Knudsen cells

Two different approaches have been used to investigate whether gaseous equilibrium exists above solid mixtures and to study whether measurements of equilibrium constants can be accomplished inside a Knudsen cell. Both were mass spectrometric studies of isomolecular exchange reactions of rare-earth sulfides. The first was a study of a single reaction generated in different chemical systems containing different condensed reacting phases and thus yielding different sulfur chemical potentials. The second was a study of a reaction in the presence and absence of a third condensed substance. The results demonstrated that gaseous equilibrium was achieved in the cell for these particular systems. The opinion is expressed that similar studies should be performed on chemical systems being used for studies of isomolecular exchange reactions. In order that measurements on such reactions can be reliably made, the solids should have comparable volatility, and the gases should have comparable dissociation energies.