Electrode potential

Electrode potential refers to the measurable voltage developed at the interface between an electrode and an electrolyte. It represents the tendency of a chemical species to be reduced or oxidised and forms the basis of all electrochemical measurements. In practical terms, the electrode potential is the voltage of a galvanic cell constructed from a standard reference electrode and a working electrode whose behaviour is being characterised. The International Union of Pure and Applied Chemistry (IUPAC) formalises its definition and associated conventions.
A widely used conventional reference is the standard hydrogen electrode (SHE), assigned an electrode potential of 0 V at all temperatures. Standard electrode potentials are therefore tabulated relative to this universal reference, providing a consistent framework for comparing the redox tendencies of different chemical species.

Origin and Interpretation of Electrode Potential

Electrode potential arises from processes occurring at the electrode–electrolyte interface. These include:

• transfer of charged species across the boundary
• specific adsorption of ions
• chemisorption and orientational effects of polar molecules, including solvent molecules

Each electrode in an electrochemical system—cathode and anode—possesses its own independent potential. The observable voltage of the cell is given by:

• Ecell_{\text{cell}}cell​ = Ecathode_{\text{cathode}}cathode​ – Eanode_{\text{anode}}anode​

This potential difference reflects the thermodynamic or kinetic state of the electrode. Depending on conditions, the electrode potential may correspond to:

• a reversible equilibrium potential, with no net current flowing
• a corrosion or mixed potential, with nonzero net reaction but zero current
• a dynamic working potential, when current flows as in galvanic corrosion or voltammetric measurements

Reversible potentials can be related to standard electrode potentials via extrapolation to standard-state conditions.
Under nonequilibrium conditions, electrode potentials depend on the phase composition at the interface and the kinetics of charge transfer, often described by the Butler–Volmer equation.

Measurement Techniques

Electrode potential is typically measured using a three-electrode system, comprising:

• a working electrode (electrode of interest)
• an auxiliary (counter) electrode
• a reference electrode (SHE or alternative)

To minimise IR (Ohmic) drop, the reference electrode is placed close to the working electrode, often using a Luggin capillary. Supporting electrolytes of high ionic conductivity are used to reduce resistive effects in the solution.
Measurements are made with the positive terminal of the electrometer connected to the working electrode and the negative terminal to the reference electrode.

Sign Conventions

Two historical sign conventions have existed:

• the Nernst–Lewis–Latimer (American) convention
• the Gibbs–Ostwald–Stockholm (European) convention

In 1953, IUPAC recommended the Stockholm/Gibbs convention (Convention 2) for defining electrode potentials, where reported potentials maintain a consistent electrostatic sign and do not change if a half-reaction is reversed. Under this definition, all potentials are expressed as reduction potentials.
Both conventions agree that the potential of the SHE is 0 V, and both assign the same sign to reduction half-reactions. However, they differ when the direction of a half-reaction is reversed. In the American convention the sign of E° also reverses; in the Stockholm convention it remains the same.
The rationale behind the American approach is maintaining the correct sign relationship with the Gibbs free energy equation:

• ΔG = –nFE

Since ΔG changes sign upon reversal of a reaction, proponents argue E should also change sign. Advocates of the Stockholm convention prioritise consistency with the electrostatic sense of potential differences.

Cell Voltage and Electrode Potentials

The potential of a cell assembled from two electrodes is calculated using:

• ΔVcell_{\text{cell}}cell​ = Ered, cathode_{\text{red, cathode}}red, cathode​ – Ered, anode_{\text{red, anode}}red, anode​or equivalently:
• ΔVcell_{\text{cell}}cell​ = Ered, cathode_{\text{red, cathode}}red, cathode​ – Eox, anode_{\text{ox, anode}}ox, anode​

This follows IUPAC’s definition, which states that the cell’s electric potential difference is the potential of the electrode on the right minus that of the electrode on the left when the cell diagram is written in standard form. A positive cell potential corresponds to spontaneous electron flow from the left electrode (anode) to the right electrode (cathode).

Conceptual Significance

Electrode potential links microscopic interfacial phenomena with measurable macroscopic voltage. It explains:

• why particular redox reactions occur
• how galvanic cells generate electricity
• how corrosion processes proceed
• how applied potentials drive electrolysis
• how voltammetric techniques probe reaction kinetics and mechanisms