For solutions in protic solvents, the universal reference electrode for which, under standard conditions, the standard electrode potential ($\ce{H^{+}}$/$\ce{H2}$) 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: \[E^{\,\unicode{x26ac}}\left(\rm{H}^{+}/\rm{H}_{2}\right)\left(\rm{abs}\right)=\Delta _{\rm{at}}G^{\,\unicode{x26ac}}+\Delta _{\rm{ion}}G^{\,\unicode{x26ac}}+\frac{\alpha _{\rm{H}^{+}}^{\rm{o,S}}}{F}\] where \(\Delta _{\rm{at}}G^{\,\unicode{x26ac}}\) and \(\Delta _{\rm{ion}}G^{\,\unicode{x26ac}}\) are the atomization and ionization Gibbs energies of $\ce{H2}$, \(\alpha _{\rm{H}^{+}}^{\rm{o,S}}\) is the real potential of $\ce{H2}$ in solvent $\ce{S}$ and \(F\) is the Faraday constant. The recommended absolute electrode potential of the hydrogen electrode is: \[E^{\,\unicode{x26ac}}\left(\rm{H}^{+}/\rm{H}_{2}\right)^{\rm{H}_{2}\rm{O}}\left(\rm{abs}\right)=(4.44\pm 0.02)\ \rm{V}\quad \rm{at}\quad 298.15\ \rm{K}\]