The indirect measurement of the aqueous vapour-pressure of capillary systems by the freezing-point depression of benzene

Abstract

In the study of the soil-water system a knowledge of the variation of vapour- pressure with water content is of prime importance. The water content of soils in equilibrium with atmospheres of varying humidity can be readily obtained by prolonged exposure over sulphuric acid—water mixtures, preferably in vacuum desiccators. Indeed, the application to soils of the methods used by van Bemmelen (1) in his classical work on silica gel might have prevented much of the confusion that has arisen in certain soil studies. For certain purposes interpolation from other water contents is undesirable, but the direct measurement of vapour pressure on soils of defined water content is either laborious or inaccurate. Tensimetric methods are even more unreliable than with salt hydrates or gels, whilst the saturation of an air current requires large amounts of carefully-prepared soil and is extremely difficult with samples of low humidity. Several investigators have had recourse to the measurement of other properties thermodynamically connected with the vapour-pressure. The determation of the freezing-point of moist soils is practicable only at relative vapour-pressures greater than about 80 per cent. to 90 per cent., whilst the datometric method involves measurements at widely different temperatures for fairly dry soils.