The standard hydrogen electrode

The SHE consists of:

• a small sheet of platinum (about 1 cm²) covered with a layer of finely divided, spongy "platinum black" (this surface adsorbs hydrogen gas and also acts as an inert conductor);
• hydrogen gas at a constant pressure of 1 atmosphere bubbling over the platinum;
• the platinum dips into an acid solution in which the concentration of H⁺ is 1 mol/L.

Under these standard conditions the electrode is the standard hydrogen electrode, with E° = 0.00 V. The half-reaction is

2H⁺(aq) + 2e⁻ ⇌ H₂(g)

• Because the electrode potential depends on the H⁺ concentration and on the pressure of hydrogen, the potential of a hydrogen electrode changes if you alter the acid concentration or the gas pressure (or both).
• That is why both conditions (1 mol/L and 1 atm) must be fixed to call it "standard".