The distribution of diffusible ions on either side of a semipermeable membrane in the presence of macro-ions that are too large to pass through the membrane. The Donnan equilibrium thus is the result of (1) external constraints (boundary conditions) that enforce an unequal distribution of mobile ions and (2) a corresponding electrical potential on the membrane as a balance. This equilibrium is named for F. G. Donnan, the first to describe it and formulate the theory.