A Significant Error in the Determination at Low Pressures of the Virial Coefficients of Vapours

Abstract

THE differential method¹ which we have developed arid which has been applied successfully to the accurate measurement at low pressures of the deviations from ideality of a number of gases has, with vapours, been found to give unreliable results. Usually, with these the PV/P isothermals show a curvature concave towards the pressure axis. This effect was noticed by Casado2 with benzene vapour, and the literature contains references to curved low-pressure isothermals with several easily liquefiable gases. Hamann and Pearse3 noticed this behaviour with methyl chloride and methyl bromide. Since, on theoretical grounds, it is very unlikely that a third virial coefficient could produce an effect of the magnitude observed even in benzene, it is clear that an experimental error of some significance is involved. This error is due to adsorption, which hitherto has been neglected by other workers in this field.