Mass-Spectrometric and Vapor Pressure Studies on the Sublimation of Realgar (As4S4)

Abstract

The nature of the sublimation process for the molecular solid realgar (As₄S₄) has been investigated using mass spectrometric and vapor pressure techniques. Vapor originating from the congruent sublimation of this compound is composed predominantly of As₄S₄(g) molecules, with smaller concentrations of other species. Of these latter species, those of As₄S₃, As₄, and As₂ are believed to originate from partial decomposition of solid As₄S₄. The amount of these decomposition products relative to As₄S₄(g) in the vapor phase was found to decrease with increasing temperature. The mass spectra of the vapors resulting from the sublimation of synthetic and natural samples of realgar were essentially the same. The expression for the equilibrium sublimation of realgar in the temperature range 452–534°K is $\log \mathrm{\hspace{0.17em}P(}\mathrm{atm}{\text{)= (8.021±0.200)\hspace{0.17em}−\hspace{0.17em}(0.619±0.010)10}}^4\mathrm{/T}$ . On the assumption that realgar sublimes primarily to As₄S₄(g) molecules, the second law heat and entropy for sublimation at 493°K were calculated to be $\text{28.3±0.5}\mathrm{kcal}/\mathrm{mole}$ and $\text{36.7±0.9}\mathrm{eu}$ , respectively. From free surface sublimation studies the sublimation coefficient at 455°K was found to be 0.47.