DC Potentials, No Current Flow

With zero DC current in the electrode wire, the electrode is at equilibrium. A polarization still existing must therefore be endogenic. Even with zero current flow in the electrode, wire local currents may flow caused by inhomogeneities of the metal surface or contact with a nonisoelectric tissue area. Endogenic polarization is important and manifests itself by a polarization immittance measured with low AC currents so that the system is linear (see Section 8.4). 

With an external DC power supply connected to the electrolytic cell, the applied voltage that gives no DC current flow in the external circuit corresponds to the equilibrium potential of the half-cell (or actually the cell). It is the same voltage as read by a voltmeter with very high input resistance and virtually no current flow (pH meter). In electrochemistry, potentiometry is to measure the potential of an electrode at zero current flow, which is when the cell is not externally polarized. To understand the equilibrium potential with zero external current, we must introduce the concept of electrode reaction 

A pure redox system presupposes an inert electrode metal with no metal ion transfer to the solution. At the electrode the redox system is  

However, an element that is oxidized loses electrons but the element is referred to as the reducing agent. Memory rule OIL—Oxidation Is Loss (of electrons). 

Likewise, an element fliat is reduced gains electrons and is referred to as the oxidizing agent. 

---