Vapor pressure osmometry
The vapor pressure osmometry is a technique for determining the molecular weight of macromolecular substances , on the lowering of partial vapor pressure of the solvent of the polymer solutions in comparison with that of the pure solvent based.
functionality
In a closed, thermostated room saturated with solvent vapor (vapor pressure osmometer) there are two highly sensitive semiconductor temperature sensors , the electrical resistance of which changes with the temperature. These are wetted differently. One thermistor with pure solvent, the other with the polymer solution to be measured. The vapor pressures of solution and pure solvent are different. However, they are in contact with one another via the vapor phase and strive for a state of equilibrium . Over time, equilibrium between the drops is established, as solvent vapor condenses on the thermistor with the sample drop, since the vapor pressure of the solvent in the polymer solution is lower than that of the pure solvent. When the solvent condenses , heat of condensation is released, as a result of which the temperature of the solution drop rises. The temperature difference between the sample and solvent drops is measured after equilibrium has been established (certain waiting time). Since the temperature differences are small (<0.1 K) and therefore a high level of measurement accuracy is required, the two thermistors, when wetted with the same solvent, are compared using a Wheatstone bridge before the actual measurement . This means that even small changes in resistance of 5 × 10 −4 % can be measured.
The temperature difference between the two drops is a measure of the decrease in vapor pressure caused by the dissolved substance and, if the concentration is known, also for its molar mass. Before this, the device constant of the measuring arrangement must be determined with calibration substances (with known concentration and molar mass) in order to be able to calculate the molar mass from the temperature difference measured for the sample.