How does osmotic pressure work

Osmotic pressure

1 definition

The osmotic pressure is the pressure that is caused by the molecules dissolved in a solvent on the more concentrated side and that drives the flow of the solvent (usually water) through a semipermeable membrane.

The solvent flows from the side with the lower particle concentration through the said membrane to the side with the higher osmotic pressure, that is to say to the higher particle concentration. The membrane is not passable for the particles themselves.

2 influencing factors

The osmotic pressure depends on the concentration ratios of the two solutions on the respective side of the membrane. The osmotic pressure is greater on the side with the higher concentration of solutes.

An influx of water from the interstitium into the cells takes place from the side with the lower concentration to the side with the higher concentration, until the internal pressure of the cells (turgor) is equivalent to the external osmotic pressure.

3 calculation

The same physical laws apply to dilute liquid solutions as to ideal gases. It applies here that the osmotic pressure

  • is proportional to the molar concentration of the substance dissolved in the liquid
  • depends solely on the number of particles in the dissolved substance
  • is proportional to the absolute temperature

The osmotic pressure of a solution of 1 mol of substance in 22.4 l of solvent is 101.325 kPa at 0 ° C (273.15 K). These relationships are formulated by van’t Hoff’s law, which, however, only applies to dilute solutions (<0.1 M).

  • Π = osmotic pressure (in Pascal)
  • c = molar concentration of the solution
  • R = universal gas constant
  • T = absolute temperature (in Kelvin)