Reference Electrode:

The two most popular types of reference electrode are the saturated calomel and silver/silver chloride systems. Both types of reference electrodes exhibit many of ideal characteristics, those include maintaining fixed potential over time and temperature that will having long-term stability and returning to the initial potential after being subject to small currents.

i) The saturated calomel electrode (SCE) is composed of metallic Hg, solid mercurous chloride (Hg₂Cl₂) and a saturated solution KCl an equilibrium. Consequently, the potential of the SCE (+0.241/2 V versus the standard hydrogen electrode) remains constant even if some of the liquid evaporates over time. The SCE is most popular and within a constant temperature bath, the error caused through fluctuating temperature could be eliminated. The SCE could be used as a reference electrode in a sample that does not exceed 80° C.

ii) The Ag/AgCl electrode includes a silver wire, coated on one end along with the insoluble AgCl salt. While the electrode is immersed within a saturated KCl solution, its potential at 25° C depends only on the Cl^- concentration and is +0.192V versus the standard hydrogen electrode. The Ag/AgCl electrode should not be used in the solutions that contains species that can precipitate or complex with silver.

Reference electrode should be prepared and maintained so that the level of the internal liquid is kept above sample solution to avoid infusion of sample into reference electrode. This is a commonly used precautionary measure to avoid any contamination of the sample by Cl^- , Ag^- or Hg ²⁺ ions.