when a semipermeable membrane or its equivalent (e.g., a solid ion exchanger) separates a nondiffusible substance, such as protein, from diffusible substances, the diffusible anions and cations are distributed on the two sides of the membrane so that 1) the products of their concentrations are equal, and 2) the sum of the diffusible and nondiffusible anions on either side of the membrane is equal to the sum of the concentrations of diffusible and nondiffusible cations; the unequal distribution of diffusible ions thus produced creates a potential difference across the membrane (membrane potential). SYN: Gibbs-Donnan equilibrium. [Frederick G. Donnan]