Observable electric potential and electrostatic potential in electrochemical systems

The role of the electric potential in the description of transport processe s in electrochemical systems is critically analyzed. Since the electrostati c potential drop between two parts of a system which differ in temperature, pressure, or concentration cannot be measured, an observable electric pote ntial (OEP) is defined from the potential difference measured between the e lectrodes reversible to one of the ion constituents in equilibrium with the system. The OEP formalism is compared to the Nernst-Planck equations (NPE) formalism. The comparative study is applied to the mass transport process that occurs in a liquid junction of a binary electrolyte solution when the electric current density is zero. The potential profiles are shown for the OEP and the electrostatic potential, the latter being calculated fur differ ent definitions of the activity coefficient gamma-. It is concluded that th e OEP formalism presents clear benefits with respect to the NPE formalism.