The use of isothermal heat-conduction calorimetry in direct measurements of solvent vapor pressure

Principles for applying isothermal heat-conduction calorimetry for direct s tudies of vapor/liquid equilibria are presented. The ideas have been tested by measurements of the vapor pressure of water over aqueous NaCl(aq) solut ions at 25 degrees C. Seven different stock solutions were used with a comp osition ranging from 0.0879 mol-kg(-1) to saturated solution. The imprecisi on and inaccuracy were not dependent on the composition and were found to b e about +/-2 Pa over the entire composition range. The sample solution was placed in a container separated from the pure solvent which was kept in the calorimetric cell. Solvent vapor was transported isothermally between the container and the cell by means of an inert carrier gas. The vapor pressure was evaluated by measuring the heat-flow rate associated with the process where the vapor equilibrated with a NaCl(aq) solution is fully saturated by passing through pure solvent. Corrections for treating vapor phase imperfe ctions are presented. The method was found to be fast, accurate, and easy t o use. The concept developed here can easily be applied in any commercial h eat-conduction calorimeter of modular design.