Osmomat 070 Vapor pressure osmometer

The Osmomat 070 is suitable for the determination of the average molecular weight (Mn) of polymers which are soluble in different organic solvents and for the determination of the osmolality of aqueous solutions.

The instrument can measure the molecular weights in the range from 40 to 50,000 in organic solutions and up to approximately 5000 g/Mol in water.

This cell unit must be operated by a Gonotec Control Unit SA or Control Unit B.


  • temperature difference measurement with two thermistors of high resolution
  • display of the temperature difference by means of the Wheatstone-bridge with a resolution of about 5 x 10-5 ºC
  • auto zero adjustment of the measuring system
  • cell is thermostated with a very high temperature constant
  • adjustable cell temperature in steps of 1 ºC between 30 ºC and 130 ºC thermistors are suitable for measuring electrically conducting liquids

Key data

Lowest measurable concentration
Toluene: 3.3 x 10-5 Mol/kg
Water: 1.7 x 10-4 Mol/kg
Molecular weight range
Toluene: 50 - 50.000 Dalton
Water: 50 - 5.000 Dalton
Sample volume
approximately 1 mL per solution, at least 3 solutions in graduated molal conconcentration
Sp. vp of the sample
<0.1% of the solvent used
Calibration substances

Measurement: Vapor pressure osmometer

Vapor pressure osmometer

The vapor pressure method is suitable for the determination of the average molecular weight of polymers which are soluble in a variety of different organic solvents and for the determination of osmolality of aqueous solutions.

Two thermistors, connected for difference measurement, are suspended in a thermostated measuring cell, which is filled with the saturated vapor of the solvent. The measuring probes, which are first covered with solvent droplets, will adapt to the cell temperature.

There is no temperature difference between the two probes. Exchanging the solvent droplet of one of the probes by a droplet of the solution will lead to condensation of solvent vapor due to the lower vapor pressure of the solvent above the solution. The released condensation enthalpy increases the temperature of the solution droplet, which simultaneously leads to an increase of the vapor pressure.

After reaching the vapor pressure equilibrium of the solution droplet, a relative stable temperature is obtained at the solution droplet. This temperature is converted by the measuring system to a direct voltage signal and is thereby as a measuring value at the user's disposal. The resulting relative measuring value is nearly proportional to the osmolal concentration of the solutions, may be however affected by heat loss and non ideal behavior of the polymer solutions.

By calibrating with solutions of known molality and osmolality and by means of a final statistical calculation of the measuring value, the nonlinear behavior may be eliminated.