The Standard Hydrogen Electrode

The standard hydrogen electrode (SHE) is the universally agreed reference point against which every other electrode's standard potential is measured, chosen by international convention to have a standard reduction potential of exactly $0.00\ \text{V}$ at every temperature.

It consists of a platinum electrode, specially coated with finely divided platinum black (to provide a large surface area and catalyse the reaction efficiently), immersed in a $1\ \text{M}$ solution of $\text{H}^{+}$ ions, over which hydrogen gas is continuously bubbled at exactly $1\ \text{atm}$ pressure. The platinum itself does not react — it acts only as an inert conductor and catalytic surface — and the actual electrode reaction is $2\text{H}^{+}(aq, 1\ \text{M}) + 2e^{-} \rightleftharpoons \text{H}_2(g, 1\ \text{atm})$, which can proceed in either direction (reduction of $\text{H}^+$ to $\text{H}_2$, or oxidation of $\text{H}_2$ back to $\text{H}^+$) depending on which other electrode it is paired with.

To determine the standard electrode potential of any other half-cell — say, a zinc electrode in $1\ \text{M}\ \text{ZnSO}_4$ — that half-cell is connected to a standard hydrogen electrode to form a complete cell, and the resulting cell EMF is measured directly with a voltmeter. Since the SHE is defined to contribute exactly $0.00\ \text{V}$, whatever EMF the complete cell shows is, by definition, the standard electrode potential of the other half-cell (with the correct sign assigned based on which electrode the measured current shows to be the cathode). …