Vapor pressure osmometry

The vapor pressure osmometry, a technique for determining the molecular weight of macromolecular substances, based on lowering of partial vapor pressure of the solvent of the polymer solution in comparison to that of the pure solvent.

Operation

In a closed, thermostated with solvent vapor saturated chamber ( vapor pressure osmometer ) are two highly sensitive semiconductor temperature sensors whose electrical resistance changes with temperature. These are wetted different. A thermistor with pure solvent and the other to be measured with the polymer solution. The vapor pressures of solution and pure solvent are different. However, they are available via the vapor phase in contact with each other and strive for a state of equilibrium. With time, provides a balance between the drop by the solvent vapor condenses on the thermistor with the sample drop as the vapor pressure of the solvent in the polymer solution is lower than that of the pure solvent. When condensing the solvent condensation heat is released as a result the temperature of the solution droplet increases. The temperature difference is measured after equilibration ( wait for some time) between sample and solvent droplets. Because the temperature differences are small (<0.1 K) and therefore a high measurement accuracy is required, the two thermistors are matched when wetted with the same solvent prior to the actual measurement via a Wheatstone bridge. Thus, even small changes in resistance of 5 × 10-4 % are measurable.

The temperature difference between the two drops is a measure of the vapor pressure lowering by the solute and by knowing the concentration and for their molecular weight. Before this the instrument constant of the measuring system are determined ( with known concentration and molar mass ) with calibration substances in order to calculate above from the measured for the sample temperature difference, the molar mass can.