It works by taking advantage of the decrease in vapor pressure that occurs when solutes are added to pure solvent.
[2] A typical vapor phase osmometer consists of: (1) two thermistors, one with a polymer-solvent solution droplet adhered to it and another with a pure solvent droplet adhered to it; (2) a thermostatted chamber with an interior saturated with solvent vapor; (3) a liquid solvent vessel in the chamber; and (4) an electric circuit to measure the bridge output imbalance difference between the two thermistors.
[3] The voltage difference is an accurate way of measuring the temperature difference between the two thermistors, which is a consequence of solvent vapor condensing on the solution droplet (the solution droplet has a lower vapor pressure than the solvent).
is the polymer-solvent solution concentration It is necessary to calibrate a vapor phase osmometer and it is important to note that K is found for a particular solvent, operational temperature, and type of commercial apparatus.
The equation above can then be used to calculate K. Vapor phase osmometry is well suited for the analysis of oligomers and short polymers while higher polymers can be analyzed using other techniques such as membrane osmometry and light scattering.