Osmotic concentration

The osmotic concentration ( osmolarity outdated ) is the product of osmolality and the mass density of water:

It indicates the number of moles of osmotically active particles in moles per liter of solution and is therefore a measure of the osmotic pressure.

In medical analysis osmolarity osmol / L or OSM / L is used as a unit, and at lower concentrations mosmol / L or mOsm / L (case also osmol / L or mOsmol / l). Typically, the osmolality is in the medical field, however, used, such as in mol / g

Has a solution a higher osmolarity than a comparison solution, so it is referred to as hyperosmolar, it has a lower osmolarity than hypoosmolar.

Explanation

Size or nature 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 ( atoms and dissolved ions, but also molecules such as sugars, proteins, ethanol ) is critical, therefore, the osmotic pressure is a colligative property.

Measurement

In the laboratory, the osmolarity is determined with an osmometer. The measurement principle is used to measure the freezing point depression ( cryoscopic ), since the number of particles dissolved in a solvent, the freezing point of the solution decreases (or increases the boiling point ). ( This principle is also used in winter by salting. )

Alternatively, the osmolarity can be determined using the pressure difference between two chambers separated by a semipermeable membrane. A chamber is thereby filled with a defined reference solution, the other with the solution to be examined. Because the particles do not pass through the membrane, the solvent must diffuse into the chamber as long as the higher concentration to the resulting hydrostatic pressure has equalized the osmotic pressure. The increased fluid level can be easily measured.

Difference to the molarity

The difference between molarity and osmolarity can be illustrated with an example:

• Molarity: A 100 millimolar sodium chloride solution containing 0.1 mole of NaCl per liter
• Osmolarity: in the solution dissociates the saline into the ion Na , and Cl -, so that 0.2 moles of osmotically active particles are released The actual osmolarity is slightly less because there is not completely dissociate any particles and the solubility is dependent on temperature.