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Partition equilibrium

Summary

Partition equilibrium is a special case of chemical equilibrium wherein one or more solutes are in equilibrium between two immiscible solvents[1]. The most common chemical equilibrium systems involve reactants and products in the same phase - either all gases or all solutions. However, it is also possible to get equilibria between substances in different phases, such a liquid and gas that do not mix (are immiscible). One example is gas-liquid partition equilibrium chromatography, where an analyte equilibrates between a gas and liquid phase[2]. Partition equilibria are described by Nernst's distribution law[3]. Partition equilibrium are most commonly seen and used for Liquid–liquid extraction.

The time until a partition equilibrium emerges is influenced by many factors, such as: temperature, relative concentrations, surface area of interface, degree of stirring, and the nature of the solvents and solute.  

Example edit

 
Process of establishing partition equilibrium in a separatory funnel for ammonia in water and chloroform.

For example, ammonia (NH3) is soluble in both water (aq) and the organic solvent trichloromethane (CHCl3) - two immiscible solvents. If ammonia is first dissolved in water, and then an equal volume of trichloromethane is added, and the two liquids shaken together, the following equilibrium is established:

Kc = [NH3 (CHCl3)]/[NH3 (aq)]        (where Kc is the equilibrium constant)

The equilibrium concentrations of ammonia in each layer can be established by titration with standard acid solution. It can thus be determined that Kc remains constant, with a value of 0.4 in this case.

Partition coefficient edit

This kind of equilibrium constant measures how a substance distributes or partitions itself between two immiscible solvents. It is called the partition coefficient or distribution coefficient.

Partition equilibrium chromatography edit

See: Partition chromatography, Gas chromatography

Partition equilibrium chromatography is a type of chromatography that is typically used in gas chromatography (GC) and high performance liquid chromatography (HPLC). The stationary phase in GC is a high boiling liquid bonded to solid surface and the mobile phase is a gas.[4] In gas-liquid chromatography, analyte from the mobile gas phase equilibrates with the liquid phase. Molecules more soluble in the liquid phase will remain longer in the column, allowing for separation using partition equilibriums.[4]

See also edit

References edit

  1. ^ García-Alvarez-Coque, María Celia (2005). Encyclopedia of Analytical Science (2nd ed.). Elsevier. pp. 164–172. ISBN 978-0-12-369397-6.{{cite book}}: CS1 maint: date and year (link)
  2. ^ Ettre, Leslie (2001-05-01). "The Birth of Partition Chromatography". Milestones in Chromatography. LCGC North America-05-01-2001. 19.
  3. ^ Atkins, Peter (2006). Physical chemistry (8th ed.). New York, NY 10010: W. H. Freeman and Company. pp. 162–190. ISBN 0-7167-8759-8.{{cite book}}: CS1 maint: date and year (link) CS1 maint: location (link)
  4. ^ a b Simmons, M. C.; Snyder, L. R. (1958-01-01). "Two-Stage Gas-Liquid Chromatography". Analytical Chemistry. 30 (1): 32–35. doi:10.1021/ac60133a007. ISSN 0003-2700.