Answer:
A calomel electrode is a reference electrode that is based on reactions between mercury (I) chloride (calomel) and elemental mercury. 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 the solution that contains the other electrode.
Its structure consists of an outer glass tube that is fitted with a frit at the bottom. This permits electrical contact with the solution on the outside. Another tube is fitted on the inside of the first tube. The bottom of this inner tube has glass wool at the bottom to allow for further connection of electricity between the contents of both tubes.
In calomel electrodes, mercury paste is packed on the inner tube, with mercurous chloride being dispersed in a saturated potassium chloride solution. This can be represented as:
Hg|Hg2Cl2KCl (xM) saturated
The electrode reaction is:
Hg2Cl2 + 2e Hg2Cl2 == 2Hg + 2Cl-
When the electrode is immersed in a solution, there is an electric contact made between the electrolyte and the sample at a certain 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 include the porous ceramic junctions used for routine applications. Cracked bead junctions are used for samples that require slow electrolyte flow, while sleeve junctions require fast electrolyte flow. The polymer-bodied types of electrodes include electrodes such as the liquid-filled with a junction, ceramic junction and the gel-filled with porous polymer junction. The permanent gel-filled type do not require a lot of maintenance and can be applied to numerous routine applications.
The electrode potential is reported with the concentration of potassium chloride, because the electrode potential depends on the potassium chloride concentration. The common reference voltage for a saturated calomel electrode is +0.244 V. Mercurous chloride can disproportion to mercury and mercuric chloride if supplied with a temperature of above 80°C, thus restricting the operation of the calomel electrode below 80°C. Its design and thermal characteristics make the rate of change of progress slow. They can be used in pH measurements and general aqueous electrochemistry.
