Reference electrodes

Ideally, SHE (Standard Hydrogen Electrode) should be used as a reference in any electrochemical measurement. However, the use of SHE is limited due to some obstacles. First, SHE is not an option for non-aqueous systems. In corrosion studies we normally work with water systems, so SHE can be used in them. However, there is another problem: an SHE requires gaseous hydrogen, which is flammable, not too safe and not too convenient in a laboratory. For that reason alternative reference electrodes are used instead of SHE. Two most popular alternatives are the SCE (Saturated Calomel Electrode) and Ag|AgCl electrode. The former can be written as
Cl-(4 mol/L) | Hg2Cl2 (s) | Hg (l) | Pt

and uses the reaction Hg2Cl2 + 2e- ↔ 2Hg + 2Cl-.

The latter is

Cl-(3-4 mol/L) | AgCl (s) | Ag (s)

and the reaction in it is AgCl + e- ↔ 2Ag + Cl-.

Which reference electrode is better? Both are the same. SCE is a little bit better in the sense of measurement noise, but it contains mercury, which may present a safety problem. Besides this, traces of mercury may cause fast corrosion of aluminum (because mercury prevents aluminum surfaces of forming protective oxide films), therefore SCE may be a risk for some metal parts of equipment. Ag|AgCl makes a little bit more noise, which is most often not observable in practice, except some very sensitive measurements, but its constituents are friendlier to users and to the equipment.

Both SCE and Ag|AgCl have to be stored in the apprpriate solutions (3M or 4M or saturated KCl in accordance with the specification of each electrode) between uses. They must be rinsed in distilled water after each use and their potential should be periodically calibrated.