DONNAN POTENTIAL2

"The concept of the Donnan potential is analogous to that of the equilibrium electrode potential. Consider two table salt (sodium chloride) solutions of different concentrations separated by a cation-exchange membrane. The concentration difference will set up a diffusional force driving the sodium chloride from the higher concentration solution into the lower concentration solution through the membrane. However, the ion-exchange membrane will permit only the passage of the positively charged sodium cations. Consequently, excess positive electrical charges will accumulate on the low concentration solution side of the membrane, while excess negative electrical charges will accumulate on the high concentration side because of the negatively charged chloride anions that are left behind. This charge separation will induce an electrical potential difference that will drive the electromigration of the sodium ions in the direction opposite to that of the diffusion. The overall result will be that the net movement of the sodium ions into the lower concentration solution will slow and eventually stop when the two opposing forces are equal end the two opposing fluxes are equal. In this so called ""Donnan equilibrium,"" the diffusional flux of the sodium ions in one direction will be equal to the electromigrational flux in the opposite direction, resulting in net zero mass transport and zero charge transport. The electrical potential difference across the membrane under these equilibrium conditions is the ""Donnan potential."" "