Torsion measurement of orpiment vapor pressure

Total vapor pressures of orpiment, As2S3, were measured by the torsion effusion method, and their dependence was found to fit the following equation: log(p/kPa) = (14.14 +/- 0.20) - (9160 +/- 200)center dot(T/K) in the temperature range (501 to 583) K. The vapor molecular weight, evaluated at some temperatures in the covered range using the Knudsen equation on the vapor pressure measurements, was found to be equal to 250 +/- 20 g center dot mol(-1), which confirmed that, as reported in the literature, the primary sublimation processes of orpiment are As2S3(s) = As2S3(g) and As2S3(s) = (1/2)As4S4(g) + (1/2)S-2(g). Increasing the temperature does not change the importance of both reactions so that the second-law enthalpies associated with both reactions are comparable and equal to Delta H degrees(542 K) = 175 +/- 4 kJ center dot mol(-1).