The electrode with the Calomel saturated with KCl , or ECS , is a electrode of reference very much used in practice.

Composition

The electrode with the Calomel saturated (in KCl) is made up of metal mercury (Hg) in contact with Calomel $Hg\_2Cl\_\; \{2\_\; \{(S)\}\}$ itself balances some with a potassium chloride solution (KCl) saturated .

It is symbolized by $Hg,\; Hg\_2Cl\_\; \{2\_\; \{(S)\}\}\; |Cl^-$ (KCl solution saturated).

It utilizes the following reactions :

$2Hg\; \backslash rightarrow\; Hg\_2\; ^\{2+\}\; +2e^-$

$Hg\_2\; ^\; \{2+\}\; +\; 2Cl^-\; \backslash \; let\; us\; rightleftharpoons\; Hg\_2Cl\_\; \{2\_\; \{(S)\}\}$

That is to say:

$2\; Hg\; +\; 2Cl^-\; \backslash \; let\; us\; rightleftharpoons\; Hg\_2Cl\_\; \{2\_\; \{(S)\}\}\; +2e^-$

It thus utilizes the redox cell $Hg\_2Cl\_\; \{2\_\; \{(S)\}\}/Hg$.

Interest

The potential taken by the electrode depends only on the concentration in ions chlorides. The potential of the calomel electrode is thus fixed by the concentration of the solution of KCl in which it bathes: concentration molar, décimolaire, or saturated.

The potential of the ECS with 25 °C compared to the normal electrode with hydrogen is: $E\; =\; 0,246\; V$