Vapor Pressure Osmometer

The Model 833 Vapor Pressure Osmometer is an effective, easy to use tool for the determination of number average molecular weights of any non-volatile solute in the range of 100-25,000 Dalton.

Principles of Operation

The vapor pressure osmometer operates on the principle of differential vapor pressure between a pure solvent and a solution. Two carefully matched thermistors are placed in a chamber saturated with solvent vapor. When solvent is placed on both thermistors, they assume the same temperature. If a solution is placed on one of the thermistors, condensation heats the thermistor until the vapor pressure is raised to that of the pure solvent. The change in temperature causes a resistance change in the thermistor. This change in resistance is measured by a sensitive bridge circuit and displayed on a panel meter, strip chart recorder and/or computer terminal. A calibration curve relates the change in resistance to the molal concentration of the solution

The Model 833 VPO incorporates many improvements over other designs to simplify and improve molecular weight determinations. The cell and thermistor geometry are designed to minimize dilution of the sample by solvent condensation. This results in much slower “fall-off” and allows a much longer time window for readings. The syringes on the Model 833 are mechanically driven to insure more precise fluid delivery and are independently heated to minimize any equilibrium upset when introducing sample. This results in a faster equilibration time (typically 2-5 minutes), better reproducibility and greatly reduced cycle time per sample.