What Does Saturated Calomel Electrode (SCE) Mean?
A saturated calomel electrode (SCE) is a reference electrode based on the reaction between elemental mercury and mercury (I) chloride. The aqueous phase in contact with the mercury and the mercury (I) chloride (Hg2Cl2, virtually insoluble salt, also known as calomel) is a saturated solution of potassium chloride in water. The electrode is normally linked via a porous frit to the solution in which the other electrode is immersed. This porous frit is a salt bridge.
The calomel electrode is very practical and very robust, and is one of the most common electrodes used in corrosion studies. The SCE is used in pH measurement, cyclic voltammetry and general aqueous electrochemistry. The SCE met the criteria of ideal reference electrodes; therefore, SCE is the most popular reference electrode for use in aqueous solutions.
Corrosionpedia Explains Saturated Calomel Electrode (SCE)
A saturated calomel electrode is a reference electrode consisting of mercury and mercury-chloride molecules. This electrode can be relatively easier to make and maintain compared to the standard hydrogen electrode (SHE). It is composed of a solid paste of Hg2Cl2 and liquid elemental mercury attached to a rod that is immersed in a saturated KCl solution. It is necessary to have the solution saturated because this allows for the activity to be fixed by the potassium chloride and the voltage to be lower and closer to the SHE. This saturated solution allows for the exchange of chlorine ions to take place. The potential of the calomel electrode depends upon the concentration of the potassium chloride solution. When potassium chloride solution is saturated, the electrode is known as saturated calomel electrode (SCE). All this is usually placed inside a tube that has a porous salt bridge to allow the electrons to flow back through and complete the circuit.
The SCE has the advantage that the concentration of Cl-, and, therefore, the potential of the electrode, remains constant even if the KCl solution partially evaporates. A significant disadvantage of the SCE is that the solubility of KCl is sensitive to changes in temperature. At higher temperatures (above 80°C) the concentration of Cl- increases, and the electrode’s potential decreases. Electrodes containing unsaturated solutions of KCl have potentials that are less temperature dependent, but experience a change in potential if the concentration of KCl increases due to evaporation. Sometimes it is inconvenient to use a liquid metal in a reference electrode, and therefore an alternative reference electrode—the silver/silver chloride electrode—is often employed instead of SCE. The calomel electrode contains mercury, which poses much greater health hazards than the silver metal used in the Ag/AgCl electrode.