NACE Resource Center

Standard Electrode Potentials

Standard potential differences are the actual cell potential differences measured in reversible cells under standard conditions. For solid or liquid compounds or elements, standard conditions are the pure compound or element; for gases they are 100 kPa pressure, and for solutes they are the ideal 1 molar (mol/liter) concentration.

Tables of standard electrode potentials can be obtained if any one electrode, operated under standard conditions, is designated as the standard electrode or standard reference electrode with which all other electrodes will be compared. This electrode is called the standard hydrogen electrode, abbreviated SHE The potential difference across a reversible cell made up of any electrode and a Standard hydrogen electrode (SHE) is called the reversible potential of that electrode, E.

If this other electrode is also being operated under standard conditions of pressure and concentration, then the reversible potential difference across the cell is the standard electrode potential E0 of that electrode. In many practical potential measurements, the standard hydrogen electrode cannot be used because hydrogen reacts with other substances in the cell or because other substances in the cell react with the catalytic platinum electrode surface upon which the H⁺/H₂ potential is established.

It is often much more convenient to use alternative electrodes whose potentials are precisely known with respect to the SHE Two of the electrodes most commonly used for this purpose are the Ag/AgCl electrode, //AgCl(s),Cl-/Ag(c) at E⁰ = +0.2224 V vs. SHE, and the saturated calomel electrode (SCE) at 0.241 V vs. SHE The effect of changing the reference electrode is to change the zero of a potential scale while leaving the relative positions of all of the potentials unchanged.

Equilibrium reaction of the main reference electrodes used in corrosion

The potential corresponding to these half-reactions can be calculated from basic thermodynamic data by ...

• first calculating the energy at a function of temperature for all chemical species involved

• then using Nernst equation to adjust for non standard activities or concentrations