Each half-reaction has a standard electrode potential (E), which is equal to the potential difference or voltage at equilibrium under standard conditions of an electrochemical cell in which the cathode reaction is the half-reaction considered, and the anode is a standard hydrogen electrode where hydrogen is oxidized:
:H → H + e
The electrode potential of each half-reaction is also known as its reduction potential (E), or potential when the half-reaction takes place at a cathode. The reduction potential is a measure of the tendency of the oxidizing agent to be reduced. Its value is zero for H + e → H by definition, positive for oxidizing agents stronger than H (e.g., +2.866 V for F) and negative for oxidizing agents that are weaker than H (e.g., −0.763V for Zn).
For a redox reaction that takes place in a cell, the potential difference is:
:E = E – E
However, the potential of the reaction at the anode is sometimes expressed as an oxidation potential:
:E = –E
The oxidation potential is a measure of the tendency of the reducing agent to be oxidized but does not represent the physical potential at an electrode. With this notation, the cell voltage equation is written with a plus sign
:E = E + E