Osmose (osmos = griech. Impact, thrust) is on one side arranged (= selective) diffusion of a solvent by a semipermeable medium. The solvent diffuses of the material solved from ranges with lower concentration into ranges with higher concentration of the solved material. The semipermeable (better: selectively permeable) medium is for the solvent permeable, not however for the solved material. With the semipermeable medium it concerns thereby frequently a diaphragm or a septum (also diaphragm called).

The osmose runs always in such a way that an osmotic pressure develops. Particles of the solvent diffuse from the solution of smaller concentration (hypotonische solution) into the solution of higher concentration (hypertonische solution), so that the concentrations adapt themselves each other (isotonische solutions). The volumes of the solutions change however, so that with reaching the osmotic equilibrium all solutions separated by the semipermeable diaphragms have the same concentration, but possibly a larger or smaller volume than before, since the solvent particles left the range and/or came (see illustration). A special case is present if in the left U-glass-thigh pure water is from the outset and the semipermeable diaphragm only for water is permeable. Then it can come never to a concentration reconciliation, because on the right side of the U-glass is with ever more largely becoming dilution still particles of the solved material, to the left of the semipermeable diaphragm however no solved materials. Because of the existing concentration gradient the volumes of the solutions in this case also, but only so long, up to other way an osmotic equilibrium change are reached: The higher liquid column resulted from water influx into the right U-glass-thigh produces now a hydrostatic pressure, which back-presses the water particles diffusing by the semipermeable diaphragm again. In addition, in this case a smaller develops and on the right of a liquid volume larger of it in the represented U-glass by the osmose to the left of the diaphragm.

Similarly as with the temperature equalizing with the osmose no energy is needed freely or, but the entropy rises. Thus the process is irreversible (speak: Runs off only toward concentration reconciliation.)

Osmotic pressure

In a system in the thermodynamic equilibrium at given pressure and given temperature the Gibbs energy is minimal and constant. It applies:

\ mbox {D} G = V \ cdot \ mbox {D} p - S \ cdot \ mbox {D} T + \ sum^N_ {i=0} \ mu_i \ cdot \ mbox {D} n_i = 0

With the given boundary conditions arises:

0 = V \ cdot \ mbox {D} p + \ sum^N_ {i=0} \ mu_i \ cdot \ mbox {D} n_i.

The pressure term and the term of the chemical remain remaining are it a component by the diaphragm refuse the whole area to fill out, then the pressure must change over the diaphragm around the equation to fulfill.

The osmotic pressure \ pi results thus with the molecular densities c_i from the change of all chemical potentials \ mu too:

\ mbox {D} \ pi = - \ sum^N_ {i=0} \ mu_i \ cdot \ mbox {D} c_i.

Here all mixture effects are considered. Since such a exact computation is not possible usually for lack of measured values, mixture effects of the solved salts and the concentration of the solvent are sometimes neglected:

\ pi = - blank \ cdot \ sum^ {N} _ {i=1} \ delta \ left (c_i \ \ LN cdot a_i \ right).

Far approximation is the neglect of the mixture effect of the solved salt with the solvent. Here the activity is set a_i the solved materials equal 1 and it arises as rough approximation:

\ pi = - blank \ cdot \ sum^ {N} _ {i=1} \ delta c_i.

This rough computation can contain however easily an error of more than 50%. If the solved materials are present however only in small concentration and on both sides of the diaphragm in similar form (few mixture effects), are this approximation quite usually.

Examples

• When cooking a one uses salt, in order to prevent an osmotic water influx into (easily salzhaltige) the vegetable and thus taking place swelling with taste loss. Turned around one reaches with a beef-tea by omitting the salt that the taste materials of the meat are out-washed by osmotic penetration of water into the

• The dialysis of blood is possible also only owing to the osmotic pressure. Here is a urea-free, but salzhaltiges Dialysat on the side, the blood on the other side of a semipermeable diaphragm. The urea enriched in the blood diffuses by the diaphragm on the secondary winding. Further blood components, which likewise do not lie below the pore diameter of the diaphragm diffuse, since the dialysis solution is adjusted to an identical concentration. Thus the Abreichern of the blood components is prevented.

• Plants carry liquids from the root range into the points. After the realizations of a team of the university Nordarizona trees can become only maximally 200 meters high, since then the osmotic pressure as well as the capillary forces is not sufficient any longer to overcome the force of gravity. (Source: Nature of volume. 428 April 2004)

• The blow-out of ripe cherries after a downpour is to be due to the osmotic admission of water by the cherry surface. Turned around a salad put on with salad sauce loses its rigidity after relatively short time. It receives these by the water of which existing in the cells due to the osmose to the Salatsauce is normally transferred.

• Removing from alcohol from beer for the production of alcohol-free beer is based also on the osmotic effect.

See also

• Colloid-osmotic pressure
• Osmometer
• Osmotic value
• Osmose power station
• Turgor
• Reverse osmosis

Articles in category "Osmose"

We found here 13 articles.