Half-Cell Potential Calculator
The half-cell potential calculator determines the actual electrode reduction potential under non-standard ion concentrations using the Nernst equation applied to a single half-reaction. Half-cell potentials are important in corrosion science (predicting whether a metal will corrode in a given environment), analytical chemistry (ion-selective electrodes measure concentration through potential), electrochemical biosensors, and understanding metabolic redox reactions in biochemistry. Enter the standard reduction potential, number of electrons, and concentrations of the oxidized and reduced forms to find the actual half-cell potential.
Half-cell Nernst equation
Ox + ne- to Red
E = E_std - (R*T / (n*F)) * ln([Red]/[Ox])
At 25 degC: E = E_std - (0.05916/n) * log10([Red]/[Ox])
E(vs SCE) = E(vs SHE) - 0.241 V
E(vs Ag/AgCl) = E(vs SHE) - 0.197 V
Reference electrode conversions
- SHE (standard hydrogen): 0.000 V by definition.
- SCE (saturated calomel): +0.241 V vs SHE.
- Ag/AgCl (3.5 M KCl): +0.205 V vs SHE.
- Ag/AgCl (saturated): +0.197 V vs SHE.
- Convert to SHE: add the reference electrode potential.
Half-cell potential: frequently asked questions
What is a half-cell potential?
A half-cell potential (also reduction potential) is the voltage of an electrode half-reaction measured relative to the standard hydrogen electrode (SHE = 0.000 V). Standard values (E_standard) are tabulated at 1 M concentration, 1 atm, 25 degC. Non-standard values are calculated using the Nernst equation: E = E_std - (0.05916/n) * log10([Red]/[Ox]) at 25 degC.
What is the standard hydrogen electrode?
The standard hydrogen electrode (SHE) is the reference: 2H+(aq, 1M) + 2e- to H2(g, 1atm) with E = 0.000 V by definition. All other standard reduction potentials are measured relative to the SHE. In practice, the SHE is difficult to use, so secondary reference electrodes are common: saturated calomel electrode (SCE, +0.241 V vs SHE) and silver-silver chloride electrode (Ag/AgCl, +0.197 V vs SHE).
How do I convert between reference electrodes?
E(vs SHE) = E(vs SCE) + 0.241 V = E(vs Ag/AgCl) + 0.197 V. To convert the other way: E(vs SCE) = E(vs SHE) - 0.241 V. Always state the reference when reporting potentials. Misidentifying the reference electrode is a common source of error in published electrochemical data.
What is the effect of pH on electrode potentials?
For reactions involving H+ or OH-, the Nernst equation includes a pH-dependent term. For example, the oxygen reduction reaction in acidic solution (O2 + 4H+ + 4e- to 2H2O): E = 1.229 - 0.05916*pH at 25 degC. The potential changes by -59.16 mV per pH unit per electron (or -59.16/n mV per pH unit for n electrons involving one H+ per electron).
What is formal potential?
The formal potential (E_formal) is the measured potential at equal concentrations of oxidized and reduced species in a specific medium, accounting for activity coefficients and complexation. It replaces E_standard for practical calculations in real solutions. For example, Fe3+/Fe2+ has E_standard = +0.771 V but E_formal = +0.68 V in 1 M H2SO4 due to activity effects.
Official sources
- NIST: NIST Chemistry WebBook - Electrochemical Data.
- IUPAC: IUPAC Recommendations on Reference Electrodes.
Reviewed by the CalculatorHub team, edited by James Graham, 14 June 2026. See our methodology.