New concept of membrane equilibrium, and anomalous potentials arising in coupled transport of ions

A generalized concept of membrane equilibrium is proposed which is based upon the condition of standstill of ionic transport across the membrane. The Nernst-Donnan equilibrium is not at variance with the new concept but can be regarded as the special case where transport of ions through the membrane comes to a standstill when their electrochemical potentials in the two solutions are equal. This equilibrium must be reached when the ions permeate the membrane independently. In this case the two equilibrium conditions, that of a standstill of transport and that of equality of electrochemical potentials of the ions in the two solutions are equivalent. In more complicated forms of transport when several types of ion are transferred conjunctively across the membrane, ionic transport may come to a standstill without equality of the corresponding electrochemical potentials in the solutions.