Selected standard reduction potentials

Every entry represents reduction, meaning electron gain, and is referenced to the standard hydrogen electrode, SHE. Its E° is 0 V by convention. An electrode potential is relative to a reference electrode, not a separately measured absolute voltage of a piece of metal.[1]

The tabulated value belongs to its stated chemical form and phase. Br₂(aq) and Br₂(l) are different references; a metal ion with complexing ligands is not automatically the free aqueous ion. Added entries apply at 25 °C, or 298.15 K, with standard activities and gas fugacity referenced to 1 bar where relevant.

A concentration of 1 mol/L corresponds to activity 1 only with the matching convention and idealization. Real electrolytes may depart appreciably. Conditions therefore specify activities rather than simply saying “all solutions 1 M”.[3]

Reduction occurs at the cathode and oxidation at the anode. For a spontaneous galvanic direction under standard conditions, the cathode has the higher compatible reduction potential. Positive cell voltage initially describes the thermodynamics of the written overall reaction.

In a galvanic cell, external electrons flow from anode to cathode; their signs are negative and positive respectively. Electrolysis can drive a reaction in reverse. Anode still means oxidation and cathode reduction, although their electrical signs differ.

To add half-reactions, equalize their electron counts by multiplying stoichiometric coefficients. Potential remains unchanged because it is energy per transferred charge. Doubling reaction extent and transferred charge doubles free energy but not their ratio.[2]

Reversing the overall reaction reverses cell-voltage sign. Halving or doubling the overall reaction in the same direction leaves E° unchanged while n and ΔᵣG° scale. Reverse a half-reaction to describe oxidation, or use unchanged reduction values in cathode-minus-anode. Do not mix the two sign methods.

Acidic permanganate reduction uses 8 H⁺ and 5 electrons to form Mn²⁺. The added alkaline form uses 3 electrons, produces MnO₂ and forms 4 OH⁻. Products, electron count and standard potential differ. The name “permanganate” alone is therefore insufficient.[1]

An acidic standard entry also requires standard H⁺ activity 1; an alkaline standard entry requires OH⁻ activity 1. An arbitrary acidic or alkaline laboratory solution does not automatically meet these conditions. Account for pH through the reaction quotient of the full half-reaction.

Acidic and alkaline forms cannot be treated as a unit conversion. For multiple half-cells, species and equilibria must match the actual chemical environment; relevant thermodynamically possible side reactions may need additional balancing.

Q has product activities in the numerator and reactant activities in the denominator, each raised to its stoichiometric exponent. Electrons are not inserted as dissolved species. Pure condensed phases in the matching reference contribute 1; gases contribute fugacity ratios and solutions their activities.

The factor 0.05915935 V belongs to 298.15 K. At another temperature, recalculate RT/F; E° itself may also vary with temperature. For a complete cell, use its overall Q or calculate both electrode potentials and subtract.

Reversible voltage is not automatically terminal voltage under load. Internal resistance, mass transport and activation overpotentials affect operation. A favourable reaction may be slow; surface oxides or passivation can further inhibit it. A tabulated potential alone gives neither current nor reaction rate.

Liquid-junction potentials and reference electrodes influence measurement. An Ag/AgCl electrode at a real chloride activity does not automatically have standard potential +0.22233 V. Converting a measurement requires the electrode filling, temperature and reference scale.

Lookup rows retain decimal places from their specifically named source. Different sources may tabulate slightly different values or conventions. Do not mix them as though they formed one high-precision data set. Existing iron and zinc entries use NIST values; added OpenStax entries are identified separately and do not overwrite them.[1][5]

Many arithmetic digits can document a calculation without proving small measurement uncertainty. Round the final result according to data quality and purpose. In equilibrium calculations, a small potential difference can produce a large change in the predicted constant; use a consistent, adequately documented data set.