Donnan equilibrium (which can also be referred to as the Gibbs-Donnan equilibrium) describes the equilibrium that exists between two solutions that are separated by a membrane. The membrane is constructed such that it allows the passage of certain charged components (ions) of the solutions. The membrane, however, does not allow the passage of all the ions present in the solutions and is thus a selectively permeable membrane.

Donnan equilibrium is named after Frederick George Donnan, who proved its existence in biological cells. J. Willard Gibbs had predicted the effect some 30 years before.

The impermeability of the membrane is typically related to the size of the particular ion. An ion can be too large to pass through the pores of the membrane to the other side. The concentration of those ions that can pass freely though the membrane is the same on both sides of the membrane. As well, the total number of charged...