Measurement of vapour-liquid equilibrium data of liquids with low vapour pressure

A semimicrostill for measuring vapour-liquid equilibrium data suitable especially for the determination at pressures about 100 Pa is described. The still operates isothermally on the principle of differential distillation. The composition of equilibrium phases is determined chromatographically. The error caused by unstationariness of the apparatus was studied. As an example of its use, the vapour-liquid equilibrium data of the N-methylpyrrolidone-ethylene glycol solution at 50 °C are reported. The dependence of excess Gibbs energy on composition was expressed by means of the fourth-order Redlich-Kister and NRTL equations. The system exhibits negative deviation from ideal behaviour. Thermodynamic treatment proves a good consistency of the equilibrium data measured.