Osmotic concentration

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The osmotic concentration c osm (outdated osmolarity ) indicates the molar concentration (outdated molarity) of the osmotically active particles in a solution .

In the case of a solution, the volume of which corresponds approximately to the volume of its solvent, the osmotic concentration can be calculated by multiplying it by the mass density of the water from the osmolality b osm :

The unit of measurement for the osmotic concentration is osmol / m³, in clinical chemistry one writes m osmol / l instead . Usually, however, osmolality is used in medicine , e.g. B. in mosmol / kg.


If a solution has a higher osmotic concentration than a comparison solution, it is referred to as hyperosmolar , in the opposite case as hypoosmolar . If solutions are separated by a membrane that is only permeable to the solvent ( semipermeable membrane ), this migrates from the location of the lower osmotic concentration to the location of the higher osmotic concentration. This is why older literature also speaks of the " osmotic value or suction value of the solution".

The osmotic pressure , which quantifies the driving force for the movement of the solvent, is proportional to the osmolarity:


The size or type of the particles do not play a role in the osmotic pressure , since it is not a chemical but a physical phenomenon. Only the number of particles (dissolved atoms and ions , but also molecules such as sugar , proteins , ethanol ) is decisive, therefore osmotic pressure is a colligative property .


In the laboratory, the osmotic concentration is determined with an osmometer . The measuring principle is the measurement of the lowering of the freezing point ( cryoscopy ), since the number of particles dissolved in a solvent lowers the freezing point of the solution (or increases the boiling point). (This principle is also used in winter by means of salt spreading.)

Alternatively, the osmotic concentration can also be determined via the pressure difference between two chambers which are separated from one another by a semipermeable membrane. One chamber is filled with a defined comparison solution, the other with the solution to be examined. Since the particles cannot penetrate the membrane, the solvent has to diffuse into the chamber with the higher concentration until the resulting hydrostatic pressure has equalized the osmotic pressure. The increased liquid level can easily be measured.

Difference from molarity

The difference between molarity and osmotic concentration can be illustrated by an example:

  • Molarity: A 100 millimolar sodium chloride solution contains 0.1 mol NaCl per liter ( c = 0.1 mol / l = 100 mmol / l).
  • Osmotic concentration: In the solution , the common salt dissociates into the ions Na + and Cl - , so that 0.2 mol of osmotically active particles are dissolved ( c osm = 0.2 osmol / l = 200 mosmol / l). The actual osmotic concentration is slightly lower because not all particles dissociate and the solubility is temperature dependent.

Individual evidence

  1. Entry on osmotic concentration . In: IUPAC Compendium of Chemical Terminology (the “Gold Book”) . doi : 10.1351 / goldbook.O04343 .
  2. Strasburger: Textbook of Botany , 29th edition, Gustav Fischer, Stuttgart 1967, p. 210