By Sunil Bhardwaj

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When a solution of an electrolyte containing a non diffusible ion is separated by means of a semi permeable membrane from a solution having both diffusible ions, then it is found that the electrolyte gets unequally distributed.

Consider that a NaCl solution is separated from a solution of NaR, where R is nondiffusible ion. The Na+ and Cl- ions can diffuse into the other compartment in pairs so that electrical neutrality is not disturbed.

If a solution of NaR is placed on one side of the membrane and on the other side water is placed, then Na+ ions cannot diffuse because it will disturb the electrical neutrality. This phenomenon of distribution of an electrolyte in the presence of nondiffusible ion was first predicted by Gibbs and later on was demonstrated and proved by Donnan. Hence such type of equilibrium is known as Donnan membrane equilibrium.

Consider that NaCl solution of concentration C1 is separated from the solution NaR having concentration C2 by means of semi permeable membrane. When the diffusion takes place consider that x gram ions of Na+ ions and x grams of Cl ions are diffused into the right hand compartment after the equilibrium is reached.