For solutions in protic solvents, the universal reference electrode for which, under standard conditions, the standard electrode potential (H⁺ / H₂) is zero at all temperatures. The absolute electrode potential of the hydrogen electrode under standard conditions can be expressed in terms of thermodynamic quantities by applying a suitable Born–Haber cycle, thus: \[E^{\,\unicode{x26ac}}\left(\text{H}^{+}/\text{H}_{2}\right)\left(\text{abs}\right)=\Delta _{\text{at}}G^{\,\unicode{x26ac}}+\Delta _{\text{ion}}G^{\,\unicode{x26ac}}+\frac{\alpha _{\text{H}^{+}}^{\text{o,S}}}{F}\] where Δ at G° and Δ ion G° are the atomization and ionization Gibbs energies of H₂, αH+o,S is the real potential of H₂ in solvent S and F is the Faraday constant. The recommended absolute electrode potential of the hydrogen electrode is: \[E^{\,\unicode{x26ac}}\left(\text{H}^{+}/\text{H}_{2}\right)^{\text{H}_{2}\text{O}}\left(\text{abs}\right)=(4.44\pm 0.02)\ \text{V}\quad \text{at}\quad 298.15\ \text{K}\]