According to this theory there are certain non-diffusible fixed anions present in the inner side of the cell membrane. In such a situation the equilibrium can only be established by the addition of cations in order to balance the negative charge of the anions present in the cell sap. Here the Donnan equilibrium is obtained only when the products of the anions and cations in the inner side becomes equal to the products of the anions and cations in the external medium.
