A method for correlating and extending vapour pressure data to lower temperatures using thermal data: Vapour pressure equations for some n-alkanes at temperatures below the normal boiling point

Abstract

Vapour pressures for many liquids are available in the range 50–760 torr but accurate determinations at lower pressures are frequently absent. Extrapolation based on purely empirical equations is hazardous and a convenient method is proposed for extrapolating such data to lower temperatures by utilizing readily available thermal data. The method is based on thermodynamically exact relationships. It has the advantage over previous such approaches that it makes greatest use of data, such as ideal gas and liquid heat capacities at around the temperature at which the vapour pressure is required, which are likely to be available. When applied to the n-alkanes C 6–C 16 from the normal boiling temperature down to a reduced temperature of 0·41 the method leads to a convenient vapour equation of the form ln p 0 = aln T + bT − ( d/T) + E. The constants in this equation have been obtained in a systematic way from the available thermal, volumetric and vapour pressure data for these alkanes. These constants, together with vapour pressures and latent heats at 25°C calculated from them, are listed. Agreement between the calculated quantities at 25°C and experimental values (where available) is good.