A thermal method of measuring the vapour pressure of an aqueous solution

Abstract

The method described below of measuring the difference of vapour pressure between any two aqueous solutions, or between a solution and the solvent, arose from experiments on isolated surviving muscles, in which it was noticed that stimulation in nitrogen led to a large increase in the rate of resting heat-production. This increment in heat rate was traced finally to condensation of moisture on the muscle, due to the lowering of vapour pressure caused in the muscle by the accumulation of the products of activity. These experiments are described elsewhere. The unexpected sensitivity of the apparatus to a change of vapour pressure led to its trial with solutions of various concentrations held by strips of filter paper; the results were so promising that a special instrument was designed and constructed, which alone is referred to in the following pages. The method has various advantages: ( a ) it is direct and fairly exact, e. g ., the difference of vapour pressure between water and a not too dilute aqueous solution can be measured at any required temperature, within 1 to 2 per cent. ; ( b ) it is fairly rapid: a reading is obtained in 30 to 45 minutes and four or five measurements can be made at the same time, if desired, by a single observer; ( c ) it has a wide range, e. g ., one can measure, on the one hand, the difference of vapour pressure between 0⋅1 M. NaCl and 0⋅2 M. cane sugar, or, on the other, between 5 M. NaCl (5 g. molecules NaCl to 1000 g. H 2 O) and water, the latter difference being of the order of 500 times the former; ( d ) very small quantities of the solutions are required, enough namely to moisten 1 to 2 sq. cm. of filter paper, say 0⋅2 cc.