Osmotic pressure

Osmotic pressure is the minimum pressure which needs to be applied to a solution to prevent the inward flow of its pure solvent across a semipermeable membrane. It is also defined as the measure of the tendency of a solution to take in a pure solvent by osmosis. Pure solvent is drawn into the solution until the osmotic pressure is counteracted by the hydrostatic pressure of the accumulated solution.

Overview

The phenomenon of osmotic pressure arises from the tendency of a pure solvent to move into a solution containing a solute to which the membrane is impermeable. This process is of vital importance in biology as the cell's cell membrane is selective towards many of the solutes found in living organisms.

When a cell is placed in a hypotonic environment (a solution that is less concentrated than the cell contents), water rushes into the cell and the cell swells to become turgid. This process is halted due to the wall pressure exerted by the cell wall. The turgidity of cells has no effect on the osmotic pressure because osmotic pressure is a property that is applicable to dilute solutions and is not a property that is applicable to systems on the verge of precipitation, gelation, etc.

Measurement

Osmotic pressure can be measured using an osmometer. Modern osmometers work by applying pressure to a solution and measuring the change in flow rate. The osmotic pressure is then calculated from the applied pressure and the flow rate.

Applications

Osmotic pressure has several real-world applications. It is used in kidney dialysis to filter waste from the blood, in reverse osmosis to purify water, and in the food industry to preserve foods.

See also

• Osmosis
• Semipermeable membrane
• Cell membrane
• Hypotonic solution
• Osmometer
• Kidney dialysis
• Reverse osmosis

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  Diagram illustrating the process of osmosis
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  Historical illustration from Pfeffer's osmotic investigations
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  Diagram showing the effect of osmotic pressure on blood cells