An Apparatus for Measuring the Vapor Pressure of Liquids by the Gas Saturation Method

Abstract

In a projected study of acid and base equilibria in non-aqueous solvents, it was found desirable to determine the vapor pressures of weak bases from these solutions, for example, solutions of ammonia in benzene and ethyl alcohol. Since there are no data in the literature for these systems it was decided that an apparatus should be constructed which would give these values with a high degree of accuracy. |The vapor pressure of a liquid may be measured by either the static or dynamic method. The static methods are of three types; the direct, the indirect, and the differential. The direct method is one by which the vapor pressure of a liquid may be measured directly by placing a portion of it above the mercury in the vacuum of a barometer tube, heating to the desired temperature, and observing the depression of the mercury column. This method is open to the objection that the liquid causes too great depression of the mercury column. Another objection is that there is no assurance that the concentration will remain constant throughout the solution. The indirect method, introduced by Ramsay and Young, determines the vapor pressure by the changes of boiling points with varying barometric pressures. The objections to this method are: the concentration of the solution may change due to evaporation; the solute and solvent may be superheated; and the solute may decompose at high temperatures. The differential method is one which gives directly the difference between the vapor pressure of a solvent and that of a solution. The objections to this method are: the apparatus is too complicated and inflexible; and the time required to make a determination renders it impractical for any but the most refined work. The dynamic method, of which there have been several modifications, is a gas saturation method. Although this method has several disadvantages, as will be discussed later, it is quite accurate and fairly rapid. For this reason the dynamic method was decided upon.