Osmotic pressure

Measure of the tendency of a solution to take in pure solvent by osmosis Osmosis in a U-shaped tube 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. Potential osmotic pressure is the maximum osmotic pressure that could develop in a solution if it was not separated from its pure solvent by a semipermeable membrane. It is the difference between hydrostatic pressures of the solution and the pure solvent. Osmosis occurs when two solutions containing different concentrations of solute are separated by a selectively permeable membrane. Solvent molecules pass preferentially through the membrane from the low-concentration solution to the solution with higher solute concentration. The transfer of solvent molecules continues until osmotic equilibrium is attained. Theory and measurement A Pfeffer cell used for early measurements of osmotic pressure Jacobus van 't Hoff derived thermodynamically a quantitative relationship between osmotic pressure and solute concentration, expressed in the following equation, where x w {\displaystyle x_{w}} is the water activity: Π = − ( R T / V m