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Osmotic pressure is the minimum pressure which needs to be applied to a solution to prevent the inward flow of water across a semipermeable membrane. It is also defined as the measure of the tendency of a solution to take in water by osmosis.
The phenomenon of osmotic pressure arises from the propensity of a pure solvent to move through a semi-permeable membrane and into a solution containing a solute to which the membrane is impermeable. This process is of vital importance in biology as the cell's membrane is selective toward many of the solutes found in living organisms.
In order to visualize this effect, imagine a U-shaped tube with equal amounts of water on each side, separated by a water-permeable membrane made from dialysis tubing at its base that is impermeable to sugar molecules. Sugar has been added to the water on one side. The height of the liquid column on that side will then rise (and that on the other side will drop) proportional to the pressure of the two solutions due to movement of the pure water from the compartment without sugar into the compartment containing the sugar water. This process will stop once the pressures of the water and sugar water on both sides of the membrane become equal (see osmosis).
Consider the system at the point it has reached equilibrium. The condition for this is that the chemical potential of the solvent (since only it is free to flow toward equilibrium) on both sides of the membrane is equal. The compartment containing the pure solvent has a chemical potential of , where is the pressure. On the other side, in the compartment containing the solute, the chemical potential of the solvent depends on the mole fraction of the solvent, . Besides, this compartment can assume a different pressure, . We can therefore write the chemical potential of the solvent as . If we write , the balance of the chemical potential is therefore:
Here, the difference in pressure of the two compartments is defined as the osmotic pressure exerted by the solutes. Holding the pressure, the addition of solute decreases the chemical potential (an entropic effect). Thus, the pressure of the solution has to be increased in an effort to compensate the loss of the chemical potential.
Derivation of osmotic pressure
In order to find , the osmotic pressure, we consider equilibrium between a solution containing solute and pure water.
We can write the left hand side as:
where is the activity coefficient of the solvent. The product is also known as the activity of the solvent, which for water is the water activity . The addition to the pressure is expressed through the expression for the energy of expansion:
where is the molar volume (m³/mol). Inserting the expression presented above into the chemical potential equation for the entire system and rearranging will arrive at:
If the liquid is incompressible the molar volume is constant, , and the integral becomes . Thus, we get
The activity coefficient is a function of concentration and temperature, but in the case of dilute mixtures, it is often very close to 1.0, so
For aqueous solutions, when determining the mole fraction of water, it is necessary to take into account the ionisation of salts. For example 1 mole of NaCl ionises to 2 moles of ions, and the mole fraction of water reduces accordingly.
Historically chemists have found it time consuming to calculate natural logs, so they used molal concentrations for dilute solutions as shown below. With spreadsheets the equations above are easy to use and offer much greater accuracy over a wider range of concentrations.
- i is the dimensionless van 't Hoff factor
- M is the molarity
- R=.08205746 L atm K−1 mol−1 is the gas constant
- T is the thermodynamic (absolute) temperature
This equation gives the pressure on one side of the membrane; the total pressure on the membrane is given by the difference between the pressures on the two sides. Note the similarity of the above formula to the ideal gas law and also that osmotic pressure is not dependent on particle charge. This equation was derived by van 't Hoff.
- Hypertonicity is the presence of a solution that causes cells to shrink.
- Hypotonicity is the presence of a solution that causes cells to swell.
- Isotonicity is the presence of a solution that produces no change in cell volume.
When a biological cell is in a hypotonic environment, the cell interior accumulates water, water flows across the cell membrane into the cell, causing it to expand. In plant cells, the cell wall restricts the expansion, resulting in pressure on the cell wall from within called turgor pressure.
Osmotic pressure is the basis of filtering ("reverse osmosis"), a process commonly used to purify water. The water to be purified is placed in a chamber and put under an amount of pressure greater than the osmotic pressure exerted by the water and the solutes dissolved in it. Part of the chamber opens to a differentially permeable membrane that lets water molecules through, but not the solute particles. The osmotic pressure of ocean water is about 27 atm. Reverse osmosis desalinates fresh water from ocean salt water.
Osmotic pressure is necessary for many plant functions. It is the resulting turgor pressure on the cell wall that allows herbaceous plants to stand upright, and how plants regulate the aperture of their stomata. In animal cells, which lack a cell wall, excessive osmotic pressure can result in cytolysis.
For the calculation of molecular weight by using colligative properties, osmotic pressure is the most preferred property.
Potential osmotic pressure
Potential osmotic pressure is the maximum osmotic pressure that could develop in a solution if it were separated from distilled water by a selectively permeable membrane. It is the number of solute particles in a unit volume of the solution that directly determines its potential osmotic pressure. If one waits for equilibrium, osmotic pressure reaches potential osmotic pressure.