Central role of the observable electric potential in transport equations -art. no. 016122

Nonequilibrium systems are usually studied in the framework of transport eq uations that involve the true electric potential (TEP), a nonobservable var iable. Nevertheless another electric potential,the observable electric pote ntial (OEP), may be defined to construct a useful set of transport equation s. Tn this paper several basic characteristics of the OEP are deduced and e mphasized: (i) the OEP distribution depends on thermodynamic state of the s olution, (ii) the observable equations have a reference value for all other transport equations; (iii) the bridge that connects the OEP with a certain TEP is usually defined by the ion activity coefficient, (iv) the electric charge density is a nonobservable variable, and (v) the OEP formulation con stitutes a natural model for studying the fluxes in membrane systems.