A technique used for finding the concentration of solute particles is called vapor-phase osmometry.  The elevation of the boiling point and the depression of the freezing point both have the disadvantage that the temperature at which the measurement is made must be near the normal boiling or freezing point of the solvent. Vapor-phase osmometry does not have this limitation and may be used at any temperature within the liquid range of the solvent.

The apparatus used in a vapor-phase osmometry experiment consists of a chamber that is held at a constant temperature by a thermostat.  Within the chamber, a reservoir of pure solvent, at the temperature of the thermostat, establishes a constant partial pressure of the solvent within the chamber.   Two small electrical devices called thermistors are suspended in the chamber.   The electrical resistance of a thermistor changes as the temperature changes, and the apparatus can detect temperature differences as small as 1 x 10^-4 °C.

A small drop of solvent is suspended from one of the thermistors, and a drop of solution is suspended from the other one.  The vapor pressures of the solvent for both drops must be the same when equilibrium is established with the solvent molecules in the vapor phase.  This can be true only if the temperature of the drop of solution is higher than that of the drop of the solvent.  The temperature difference between the two thermistors is measured electrically.  Vapor-phase osmometers are calibrated with solutions of known concentration.

The apparatus used for vapor-phase osmometry.

How is this technique useful?  At 25 °C, a 0.050 m solution in water has a vapor pressure only about 0.021 torr lower than that of pure water (23.77 torr).  In a vapor-pressure osmometer, a temperature difference of 1.51 x 10^-2 °C is generated.  The difference in vapor pressure is almost undetectable, but the temperature difference can be measured to three significant digits.

Reger/Goode/Mercer:  Chemistry Principles and Practice,  2/e,  p. 497