Reversible hydrogen electrode

A reversible hydrogen electrode ( engl. Reversible hydrogen electrode , short RHE) provides a special electrode in electrochemical processes. It is a reference electrode of the type of hydrogen electrode and is different from the standard hydrogen electrode by the measured potential that does not match the pH changes so that it can be used directly in the electrolyte .

The name refers to the fact that the electrode is located in the actual measuring electrolyte. It is not separated by a salt bridge . The hydrogen ion concentration is therefore not 1, but corresponds to that of the measuring electrolyte; In this way it can be achieved that the measured potential does not change with the pH value . At 25 ° C, the potential of the RHE is adjusted according to the pH value:

{\ displaystyle E _ {\ mathrm {RHE}} = 0 ~ \ mathrm {V} -0 {,} 059 ~ \ mathrm {V} \ cdot {\ text {pH}}}

In general, the following applies to hydrogen electrodes on which the reaction:

{\ displaystyle 2 \; \ mathrm {H_ {3} O ^ {+}} \; + \; 2 \; \ mathrm {e ^ {-}} \ quad \ rightleftarrows \ quad \ mathrm {H_ {2}} \; + \; 2 \; \ mathrm {H_ {2} O}}

takes place, according to the Nernst equation, the following dependence of the equilibrium potential on the hydrogen pressure and on the activity of the oxonium ions : ${\ displaystyle E}$ ${\ displaystyle p \ mathrm {[H_ {2}]}}$ ${\ displaystyle a \ mathrm {[H_ {3} O ^ {+}]}}$

{\ displaystyle E = E ^ {0} + {\ frac {R \; T} {F}} \ left (\ ln \ left (a [\ mathrm {H_ {3} O ^ {+}}] \ right ) - {\ frac {1} {2}} \ ln \ left (\; p [\ mathrm {H_ {2}}] \ right) \ right)}

It is the standard potential (this is, by definition, equal to zero), is the universal gas constant , the absolute temperature and is the Faraday constant . ${\ displaystyle E ^ {0}}$ ${\ displaystyle R}$ ${\ displaystyle T}$ ${\ displaystyle F}$

During the electrolysis of water, overvoltages still occur, which means that the required cell voltage is higher than the equilibrium voltage due to kinetic inhibition. The overvoltage increases with increasing current density at the electrodes. The measurement of equilibrium potentials is therefore performed with as little current as possible.

Layout and function

The reversible hydrogen electrode has several advantages over other reference electrodes:

There is no contamination by Cl ^- or SO ₄^2-- ions , as in the silver chloride electrode possible
There are no diffusion potentials at the salt bridge,
Long-term measurements are possible because the salt bridge does not have to be constantly filled,
The temperature range is very broad (–20 ° C to 200 ° C) because the boiling liquid of the salt bridge is missing,
The electrode can have an internal hydrogen supply for max. one year of operation included,
A porous platinum / palladium electrode is much more resistant to contamination than a platinum-coated sheet metal.

Importance and handling

The reversible hydrogen electrode has meanwhile become established for measurements in strongly acidic or alkaline media. Here this electrode has replaced the mercury / mercury (II) oxide system. On the one hand because of the toxicity of the mercury, on the other hand because the usual glass bodies of the reference electrodes are attacked by the concentrated alkalis.

The figures show how the known characteristics - here a top layer diagram on platinum - shift in the potential position when the measurement is carried out with a reversible hydrogen reference electrode RHE or the more well-known calomel electrode. In sulfuric acid at pH 0 you can see that the difference between calomel and hydrogen electrodes is approx. 250 mV. However, the graphs against the reversible hydrogen potential are much clearer. You can immediately assign the hydrogen evolution, but also the occupancy of the different platinum surfaces.