Reference electrodes

In electrochemistry, the standard hydrogen electrode (SHE) is the absolute reference electrode; its potential is by definition equal to zero at all temperatures. Its practical realization is the normal hydrogen electrode (NHE) made with a platinum wire dipped in an acid solution with a pH=0 (normal solution in H ₃ O ⁺  ) in which hydrogen gas is bubbled under 1 bar. However, this reference electrode is never used by the corrosionnists who prefer using stronger electrodes and easier to handle:

The copper sulfate – copper reference electrode is made with a wire or a bar of copper dipped in an aqueous saturated copper sulfate solution. The potential of this reference electrode is, at 25°C, equal to +316 mV compared to the NHE.

Like all the reference electrodes, the electrolytic junction with the aqueous environment is carried out with a porous inner surface (sintered).

The silver - silver chloride reference electrode is made with a silver wire covered with silver chloride, dipped in a chloride solution. The potential of this electrode depends on the concentration (activity) of the chloride ions: in the presence of saturated potassium chloride it is +195 mV compared to the NHE, at 25°C. In sea water, it is +0,25 V/NHE (a higher precision would require the precise knowledge of the chloride content of the considered sea water).

In this electrode, mercury is in contact with mercurous chloride, Hg ₂ Cl ₂ (calomel) in the presence of potassium chloride (KCl). The potential of this electrode in saturated KCl environment is +241mV compared to the NHE, at 25°C.