Calomel Electrode

Published: | Updated: November 4, 2020

Definition - What does Calomel Electrode mean?

A calomel electrode is a type of reference electrode that is based on reactions between mercury (I) chloride (calomel) and elemental mercury. These electrodes are commonly used in used in voltmeters and pH meters.

A good non-polarizable calomel electrode is very robust, which is why is it used by many two-electrode systems where the supporting electrolyte is a non-reactive chloride salt.

The calomel electrode’s structure consists of an outer glass tube that is fitted with a frit at the bottom. This permits electrical contact with a solution outside the electrode. An inner tube is placed inside an outer tube. The bottom of the inner tube has glass wool at the bottom to provide for electrical contact between the contents of both tubes.

Mercury paste is packed on the innermost tube, with mercurous chloride being dispersed in a saturated potassium chloride solution.

Figure 1. Diagram of a calomel electrode.

Figure 1. Diagram of a calomel electrode.

The calomel electrode is a type of half-cell in which the electrode is mercury coated with calomel (Hg2Cl2) and the electrolyte is a solution of potassium chloride and saturated calomel. This can be represented as:

Hg|Hg2Cl2KCl (xM) saturated

The electrode reaction is:

Hg2Cl2 + 2e Hg2Cl2 == 2Hg + 2Cl-

Corrosionpedia explains Calomel Electrode

A good calomel electrode (especially a saturated calomel electrode) is similar to an ideal non-polarizable electrode with a stable potential. The large amount of mercury, calomel and chloride ions, with their constant concentrations, makes the potential stable at equilibrium if small amounts of current have passed through the electrode. This is advantageous because the electrode can then be used as a reference electrode across many experiments.

Reference electrodes have a stable and well-known electrode potential and can be used to measure the likelihood of corrosion in various aqueous solutions. An ideal non-polarizable electrode means that the electrode potential can be expected to be stable even if a current is passed across the electrode.

The high stability of the electrode potential is usually achieved by employing a redox system with constant concentrations of each participant of the redox reaction. The aqueous phase in contact with both the calomel and the mercury is a saturated solution of water and potassium chloride. The linking of the electrode is through a porous frit to a solution that contains the other electrode.

When the electrode is immersed in a solution, an electrical contact is made between the electrolyte and the sample at an opening located near the end of the electrode. This forms a conductive bridge between the sample, the reference electrode and the indicating electrode.

The glass body liquid-filled types of electrodes include a porous ceramic junction used for routine applications. Cracked bead junctions are used for samples that require a slow electrolyte flow, while sleeve junctions require fast electrolyte flow. The polymer-bodied types of electrodes include electrodes such as the liquid-filled type with a junction, the ceramic junction type and the gel-filled with a porous polymer junction. The permanent gel-filled type requires little maintenance and can be used for many applications.

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