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 @MT06878@ 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 @MT06878@. The @MT06878@ may be replaced by other kinds of restraint, such as @GT07621@, 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.