Membrane permeation in systems far from equilibrium

A method is discussed which makes possible an approximate integration of the Nernst‐Planck equations of motion with the additional consideration of a convective term. Only systems at constant temperature and in a stationary state without the occurence of chemical reactions are treated. Comparison of the approximate solution with the exact integral shows satisfactory agreement. The driving potentials in the membrane are expressed in terms of measurable potential differences between the outer phases. An explicit approximate formula for the so‐called “diffusion potential” is derived. The conditions necessary for the separation of different types of ions are formulated. In the appendix the exact integral for the bi‐ionic case is presented in parametric form.