Reversible hydrogen electrode

A reversible hydrogen electrode (RHE) is a

The name refers to the fact that the electrode is directly immersed in the actual electrolyte solution and not separated by a salt bridge. The hydrogen ion concentration is therefore not 1 mol/L, or 1 mol/kg, but corresponds to that of the electrolyte solution. In this way, it is possible to achieve a stable potential with a changing pH value. The potential of the RHE correlates to the pH value:

${\displaystyle E=0.000-0.059\times \mathrm {pH} }$

In general, for a hydrogen electrode in which the reduction of the hydronium ions (H₃O⁺) occurs:

${\displaystyle {\ce {{2H3O+}+{2e^{-}}<=>{H2}+{2H2O}}}}$

or, more often commonly written simply with  H⁺ denoting H₃O⁺:

${\displaystyle {\ce {{2H+}+{2e^{-}}<=>{H2}}}}$

with,

${\displaystyle K={\frac {p{{\ce {H2}}}}{{\ce {(aH+)^2}}}}}$

the equilibrium potential E depends on the hydrogen pressure pH₂ and the activity aH⁺ as follows:

${\displaystyle E=E^{\ominus }-{\frac {RT}{zF}}\ln K}$

${\displaystyle E=E^{\ominus }-{\frac {RT}{2F}}\ln {\frac {p{{\ce {H2}}}}{{\ce {(aH+)^2}}}}}$

${\displaystyle E=E^{\ominus }-{\frac {RT}{F}}\left({\tfrac {1}{2}}\ln p{{\ce {H2}}}-\ln {\ce {(aH+)}}\right)}$

${\displaystyle E=E^{\ominus }+{\frac {RT}{F}}\left(\ln a{\ce {H+}}-{\tfrac {1}{2}}\ln p{\ce {H2}}\right)}$

Here, ${\displaystyle E^{\ominus }}$ is the

The benefit of that electrode is that no salt bridge is needed:

• There is no contamination of the electrolyte by chlorides or sulfates.
• There are no diffusion potentials at the electrolyte bridge (liquid junction potential). This is important at temperature different to 25 °C.
• Long-time measurements are possible (no electrolyte bridge means no maintenance of the bridge)

See also

• Dynamic hydrogen electrode
• Palladium-hydrogen electrode

References