The High Temperature Vapour Pressure Curve and the Critical Point of Liquid Selenium

The vapour pressure curve of liquid Se has been measured up to the critical point by observing the discontinuous jumps of the conductivity when the liquid is evaporated. The critical point was found at Tcrit = 1590 ± 20°C and Pcrit = 380 ± 30 bar. The highly corrosive Se was contained in a cell of sintered alumina and graphite allowing conductivity measurements to be made up to 1750°C and more then 1200 bars. The measurements were performed in an internally heated autoclave. Although the conductivity of liquid Se increases with increasing temperature at constant pressure to nearly metallic values close to the critical point, an analysis of the vapour pressure data in terms of “correspondence state theory” led to the suggestion that Se is essentially a molecular fluid in the vicinity of the critical point rather than a metallic one.