Title: osmotic pressure
Long Title: IUPAC Gold Book - osmotic pressure
DOI: 10.1351/goldbook.O04344
Status: current
Index: quantity

Definition
Excess pressure required to maintain osmotic equilibrium between a solution and the pure solvent separated by a membrane permeable only to the solvent: \[\mathit{\Pi} = -\frac{R\ T}{V_{\rm{A}}}\ \ln a_{\rm{A}}\] where \(V_{\mathrm{A}}\), \(a_{\mathrm{A}}\) are the partial molar volume and activity of solvent \(\rm{A}\) for an incompressible fluid. For ideal dilute solutions, \(\mathit{\Pi} = c_{\rm{B}}\ R\ T=\rho _{\rm{B}}\ \frac{R\ T}{M_{\rm{B}}}\), where entities \(\rm{B}\) are individually moving solute molecules, ions, etc., regardless of their nature, \(c_{\rm{B}}\), \(\rho _{\mathrm{B}}\) are the amount and mass concentration of the solutes, and \(M_{\mathrm{B}}\) is the mass average molar mass of the solutes. The amount is sometimes expressed in osmol (meaning a mole of osmotically active entities), but this usage and the corresponding term osmolarity are discouraged.

Related Terms
- membrane: https://dev.goldbook.iupac.org/terms/view/MT06878
- mole: https://dev.goldbook.iupac.org/terms/view/M03980
- osmolarity: https://dev.goldbook.iupac.org/terms/view/O04343

Sources
- Green Book, 2nd ed., p. 51 (https://goldbook.iupac.org/files/pdf/green_book_2ed.pdf)
- Green Book, 3rd ed., p. 59 (https://doi.org/10.1039/9781847557889)

Other Outputs
- html: https://dev.goldbook.iupac.org/terms/view/O04344/html
- json: https://dev.goldbook.iupac.org/terms/view/O04344/json
- xml: https://dev.goldbook.iupac.org/terms/view/O04344/xml

Citation: Citation: 'osmotic pressure' in IUPAC Compendium of Chemical Terminology, 5th ed. International Union of Pure and Applied Chemistry; 2025. Online version 5.0.0, 2025. 10.1351/goldbook.O04344
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Accessed: 2026-06-13T02:55:18+00:00