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I have read in textbooks and websites that hydrogen gas needs to be bubbled into the SHE (Standard Hydrogen Electrode). I understand why for oxidation of the half cell - the gas is oxidised into H+ (aq) and the electrons move into the other half cell. Anions from the salt bridge then move to maintain cell neutrality.

However, I fail to understand why hydrogen gas needs to be bubbled into the cell during reduction, as hydrogen intake is not needed. As H+ (aq) gets reduced into gas, it will just bubble out whereever it is formed, not necessarily through the tube. Cations from the salt bridge move to maintain neutrality.

I suspect that it is something to do to maintain stability/neutrality. Can anyone please help?

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    $\begingroup$ What potential would SHE have if you did not bubble H2 through it? $\endgroup$
    – Poutnik
    Commented Jun 29, 2023 at 4:31
  • $\begingroup$ You use a salt bridge only to close the electric circuit (i.e., charge transport). It is filled e.g. with diluted aq. $\ce{KCl}$,* and plugged by diaphragms to hinder the travel of $\ce{K+}$ / $\ce{Cl-}$ into either one half cell across this boundary. You do not want a travel of matter (e.g., $\ce{Cu^{2+}}$) across this connection into the other half-cell. <br/> * An example of a strong electrolyte (i.e., complete dissociation) of ions with similar mobility which are not subject to electrochemical reaction in the difference of electrode potentials of your intended half-cell reaction(s). $\endgroup$
    – Buttonwood
    Commented Jun 29, 2023 at 8:35

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The SHE is not used as a working electrode with passing current, it would affect its potential.

It is used as the third, reference electrode with negligible current. The potentials of the working anode and/or cathode with eventually passing significant current are then measured against SHE.

Similarly are used other, more practical calomel or argentchloride reference electrodes $\ce{Hg|Hg2Cl2|KCl}$ or $\ce{Ag|AgCl|KCl}$. The latter is often integrated with pH glass electrode, forming the compact 2-electrode pH sensor.

Note that the real potential of SHE with deviations of $a_{\ce{H+}}$ or $p_{\ce{H2}}$ is:

$$E_\mathrm{SHE} = E^{\circ}_\mathrm{SHE} + \frac{RT}{2F} \ln{\left(\frac{a_{\ce{H+}}^2}{p_{\ce{H2}}}\right)}$$

The SHE requires $a_{\ce{H+}}=1$ and $p_{\ce{H2}}=\pu{1 atm}$, with the solution saturated by hydrogen. This can be hardly done other way than by bubbling hydrogen through the solution, what is BTW the part of SHE definition.

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