Calomel electrode

Calomel electrode

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Area of ​​Expertise - Analytical chemistry, electrochemistry

(also calomel half-cell). Electrode of the 2nd type, often used as a reference electrode in electrochemistry, consisting of mercury, Edon the surface of which a layer of calomel (mercury (I) chloride,Ed2Cl2) is located. A potassium chloride solution of precisely adjusted activity serves as the electrolyte (often a saturated solution). The electrical discharge takes place via a platinum wire that is immersed in the mercury.

The reaction is decisive for the electrochemical potential of the electrode:


The concentration of mercury (I) ions,Ed2Cl2, results from the solubility product of calomel according to the following equation:


The potential difference to the standard hydrogen electrode (potential: ± 0.0 V) is 0.241V (at saturated KCl-Solution).

The name "calomel" (Greek: nice black) results from the black coloration when pouring over Ed2Cl2 with ammonia solution as a result of a disproportionation of the mercury (I) to finely divided, black metallic mercury, Couldn't accept input NUMBER ["0"] Hg (0), and mercury (II):


Even when exposed to light, mercury (I) chloride turns dark, similar to the silver halides, due to photochemical decomposition with deposition of the metal.

Learning units in which the term is dealt with

Electrodes 2nd type15 minutes.

ChemistryGeneral ChemistryElectrochemistry

Description of the electrodes type 2.

Potentiometric pH measurement30 min.

ChemistryGeneral ChemistryElectrochemistry

Introduction to potentiometric pH measurement and description of the glass electrode.

EMK, electromotive force45 min.

ChemistryPhysical chemistryElectrochemistry

The learning unit describes the cause, the size and the dependencies of the electromotive force in different electrode systems. The standard reduction potentials and various half-cells (gas electrode, metal electrode, silver-silver chloride electrode, calomel electrode, redox electrodes and glass electrode) are explained.

Calomel electrode

the Calomel electrode (GKE, Saturated Calomel Electrode, SCE) is an electrode of the second kind.

A calomel electrode consists of mercury, which is mixed with poorly soluble calomel (ed2Cl2) is coated and immersed in a saturated potassium chloride solution. Contact can be made 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:

The potential-determining electrode reaction 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 ° the standard potential of the electrode reaction, R. the general gas constant, T the thermodynamic temperature and F. the Faraday constant is:

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:

Of the KL.-Term, like the standard potential, only depends on the temperature. So it makes sense to include it in a newly defined standard potential, that of the calomel electrode:


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Video: Calomel Electrode (August 2022).