The role of the electric potential in the description of transport processes in electrochemical systems is critically analyzed. Since the electrostatic potential drop between two parts of a system which differ in temperature, pressure, or concentration cannot be measured, an observable electric potential (OEP) is defined from the potential difference measured between the electrodes 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 different definitions of the activity coefficient gamma-. It is concluded that the OEP formalism presents clear benefits with respect to the NPE formalism.