Typically abbreviated as EC, this electrode replaces the standard hydrogen electrode (SHE) in many measurements as it is easier to construct, and less risky to handle (despite having mercury). Inside it contains a KCl solution as an electrolytic medium for the flow of electrons.
The calomel electrode can have different variants depending on its size, or more importantly, on the concentration of the KCl. When the KCl solution is saturated, we speak of a saturated calomel electro (ESC). The ESC is easier to prepare than the EC, but more sensitive to changes in temperature.
For the calomel electrode to work, the Hg-Hg 2 Cl 2 pair must react, either gaining or losing electrons.
When the reduction or gain of electrons occurs inside the calomel electrode, we have the following reactions:
Hg 2 Cl 2 → Hg 2 2+ + 2Cl – (Ionization)
Hg 2 2+ + 2e – → 2Hg (Reduction)
Hg 2 Cl 2 + 2e – → 2Hg + 2Cl – (Net reaction)
Therefore, Hg 2 Cl 2 gains electrons reducing to metallic mercury.
The potential E of the electrode when the reduction occurs is given by the equation:
E = Eº – 0.0591 Log [Cl – ]
Where it is observed that E depends exclusively on the concentration of Cl – ions , E ° being the standard reduction potential for this electrode measured against the standard hydrogen electrode.
Inside the electrode an oxidation process can also occur:
2Hg → Hg 2 2+ + 2e – (Oxidation)
Hg 2 2+ + 2Cl – → Hg 2 Cl 2 (Precipitation)
2Hg + 2Cl – → Hg 2 Cl 2 + 2e – (Net reaction)
That is, the mercury is oxidized to generate more Hg 2 Cl 2 .
The potential E in this case is given by:
E = Eº + 0.0591 Log [Cl – ]
And again, E depends on [Cl – ].
The general reaction for the calomel electrode is:
Hg 2 Cl 2 (s) + 2e – ⇌ 2Hg (l) + 2Cl –
The sense of balance above will depend on the medium where the electrode is contacted. The Cl – ion determines the solubility of Hg 2 Cl 2 , which in turn affects the formation or oxidation of Hg.
And the potential determined for a specified concentration of Cl ions – will be equal to:
E calomel = E red – E ox
E calomel being the potential that is reported as a reference in certain potential tables.
Calomel Electrode Features
Representation of the half cell
The half cell of the calomel electrode can be represented as follows:
Pt | Hg | Hg 2 Cl 2 | Cl – (xM)
Where only the concentration of Cl – ions is important , expressed in molarity or normality. The electrode potential E will vary if it is filled with solutions of different concentrations of KCl.
For example, an EC with 0.1 M KCl has an E equal to 0.3356 V at 25 ° C; while ESC, with saturated KCl, has an E equal to 0.2444 V at the same temperature.
Commercially, three types of calomel electrodes are available: saturated (ESC), nineteenth-normal (0.1 N or 0.1 M KCl) and normal (1 N or 1 M KCl). A 1 M KCl calomel electrode would be represented as:
Pt | Hg | Hg 2 Cl 2 | Cl – (1 M)
In the image above we show the main parts of an ordinary calomel electrode. It is made of glass, and consists of two containers: an external one, which is placed in electrochemical contact with the measurement medium and contains the KCl solution; and an internal one, where the Hg-Hg 2 Cl 2 mixture rests .
Internally, the calomel electrode contains liquid mercury, on which a paste of Hg 2 Cl 2 moistened with mercury adheres . This is the most active phase of the electrode. Porous glass is used to allow only Cl – ions to enter or exit , but not Hg 2 Cl 2 crystals or mercury droplets.
A platinum wire, through which electrons flow, is immersed in mercury, and is responsible for connecting the electrode with the voltmeter and the external circuit in question.
The KCl solution, which contains undissolved crystals of the salt, is poured through the filling hole. Meanwhile, at the bottom of the electrode we have a very small opening in a porous glass, which comes into direct contact with the measurement medium. The purpose of porous glass is to allow contact without undesirable exchanges of substances that contaminate the electrode or sample.
The calomel electrode has the following advantages over the standard hydrogen electrode:
-Easy to build and manipulate
-Its cell potential remains constant even if the water evaporates
-Does not need a salt bridge
The ESC is the easiest of the calomel electrodes to construct, since it is enough to dissolve KCl in water until its crystals form. The solution will then be saturated, and ready to pour into the electrode.
The calomel electrode, however, has the following disadvantages:
-By containing liquid mercury, it can have a negative impact on the environment
-Cannot be used for quantitative analysis in samples with a temperature higher than 60 ºC, since the Hg 2 Cl 2 begins to decompose, causing the electrode readings to fail
The ESC also has the disadvantage that it is very sensitive to changes in temperature.
The silver-silver chloride electrode has replaced the calomel electrode in many of the potentiometric determinations.
The calomel electrode is one of the many electrodes that are used daily in potentiometric determinations, making it possible to obtain the half-cell potentials of the analytes or the species of interest.
Also, the calomel electrode is used in pH measurements and cyclic voltammetry.