Schematic structure of a calomel electrode with a salt bridge
A calomel electrode consists of mercury which is coated with poorly soluble calomel (Hg 2 Cl 2 ) and immersed in a saturated potassium chloride solution. Contact can be established with the analysis solution via a diaphragm ( semipermeable membrane) or a liquid film in the ground-glass stopper.
The cell notation of the saturated calomel electrode is:
Working principle
The electrode reaction that determines the potential is the equilibrium between metal in the electrode and metal ions in the KCl solution:
The potential of the electrode, E, is described by the Nernst equation , where E ° is the standard potential of the electrode reaction, R the general gas constant, T the thermodynamic temperature, and F the Faraday constant:
The following applies to the solubility equilibrium:
The equation of the solubility product can be used for the mercury concentration to get the potential as a function of the chloride concentration alone:
Like the standard potential, the K L term only depends on the temperature. So it makes sense to include it in a newly defined standard potential, that of the calomel electrode:
literature
Matthias Otto: Analytical Chemistry. 3rd, completely revised and expanded edition. Wiley-VCH, Weinheim 2006, ISBN 3-527-31416-4 , p. 366.