In an earlier paper concerning membrane phenomena it was shown that in isotonic systems the driving force of the flux of water is the electro-osmotic pressure and not the electric potential. As the electro-osmotic pressure vanishes in the point of the inversion where c_D=c_F, i.e., the concentration of the Donnan ions equals the concentration of the fixed ions, the flux of water vanishes independently of the electric potential. This behaviour was proved experimentally in the case of electro-osmosis. Moreover it was shown, that the diffusion potential in binary isotopic osmosis and the external electric potential in electro-osmosis lead to the same result.

The different behaviour of the transport phenomena in a solution and a membrane results from constraints which give raise to a modification of the Onsager-coefficients. The transformation principles of these coefficients are derived.