The equilibrium characterized by an unequal distribution of diffusible ions between two ionic solutions (one or both of the solutions may be gelled) separated by a @[email protected] which is impermeable to at least one of the ionic species present, e.g. because they are too large to pass through the pores of the @[email protected] The @[email protected] may be replaced by other kinds of restraint, such as @[email protected], the field of gravity, etc., which prevent some ionic components from moving from one phase to the other, but allow other components to do so.