The vapor pressure method is suitable for the determination of the average molecular weight of polymers which are soluble in a variety of different organic solvents and for the determination of osmolality of aqueous solutions.
Two thermistors, connected for difference measurement, are suspended in a thermostated measuring cell, which is filled with the saturated vapor of the solvent. The measuring probes, which are first covered with solvent droplets, will adapt to the cell temperature.
There is no temperature difference between the two probes. Exchanging the solvent droplet of one of the probes by a droplet of the solution will lead to condensation of solvent vapor due to the lower vapor pressure of the solvent above the solution. The released condensation enthalpy increases the temperature of the solution droplet, which simultaneously leads to an increase of the vapor pressure.
After reaching the vapor pressure equilibrium of the solution droplet, a relative stable temperature is obtained at the solution droplet. This temperature is converted by the measuring system to a direct voltage signal and thereby as a measuring value at the user’s disposal. The resulting relative measuring value is nearly proportional to the osmolal concentration of the solutions, may be however affected by heat loss and non ideal behaviour of the polymer solutions.
By calibrating with solutions of known molality and osmolality and by means of a final statistical calculation of the measuring value, the nonlinear behaviour may be eliminated.