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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, $\text{Ed}$on the surface of which a layer of calomel (mercury (I) chloride,${\text{Ed}}_{2}{\text{Cl}}_{2}$) 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:

$$\begin{array}{ccc}\hfill +2\phantom{\rule{.2em}{0ex}}{\text{e}}^{-}& \rightleftharpoons & 2\phantom{\rule{.2em}{0ex}}\text{Ed}\hfill \end{array}$$

The concentration of mercury (I) ions,${\text{Ed}}_{2}{\text{Cl}}_{2}$, results from the solubility product of calomel according to the following equation:

$$\begin{array}{ccc}\hfill {\text{Ed}}_{2}{\text{Cl}}_{2}& \rightleftharpoons & +2\phantom{\rule{.2em}{0ex}}{\text{Cl}}^{-}\hfill \end{array}$$

The potential difference to the standard hydrogen electrode (potential: ± 0.0 $\text{V}$) is 0.241$\text{V}$ (at saturated $\text{K}\text{Cl}$-Solution).

The name "calomel" (Greek: *nice black*) results from the black coloration when pouring over ${\text{Ed}}_{2}{\text{Cl}}_{2}$ 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):

$$\begin{array}{ccc}\hfill {\text{Ed}}_{2}{\text{Cl}}_{2}+2\phantom{\rule{.2em}{0ex}}\text{N}{\text{H}}_{3}& \to & \text{Ed}+\text{Ed}\left(\text{N}{\text{H}}_{2}\right)\text{Cl}+\text{N}{\text{H}}_{4}\text{Cl}\hfill \end{array}$$

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 (ed_{2}Cl_{2}) 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 *K*_{L.}-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:

## Calomel

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