Question

Describe the Construction & Working of Calomel electrode

Answer 1

Answer:

Calomel electrode is the mercury-mercurous cbloride electrode. It consists of glass vessel having bent side tube. Pure mercury is placed at the bottom of the tube. Which is covered with a paste of mercury- mercurous chloride (Hg+Hg2Cl2) i.e., calomel. The remaining portion of the cell is filled with a solution of normal (1N) or decinormal (0.1N) or saturated KCl. A platinum wire sealed into a glass tube is dipped into mercury layer is used to provide the external electrical contact. The side tube is used for making electrical contact with a salt bridge.  The electrode can be represented as

Hg(l)/Hg2Cl2 (s) / Cl-

The calomel electrode can act as anode or cathode depending on the nature of other electrode of the cell.

When it acts anode, the electrode reaction is

2Hg(l) → Hg22+ + 2e-

Hg22+ + 2Cl- → Hg2Cl2

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2 Hg + 2Cl- → Hg2Cl2 + 2e- (Oxidation reaction)  When it acts as cathode, the electrode reaction is,

Hg22+ + 2e-→ 2 Hg

Hg2Cl2 → Hg22+ + 2Cl-

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Hg2Cl2 + 2e- → 2Hg (l) + 2 Cl- [Reduction reaction]  The net reversible electrode reaction is,

Hg2Cl2(s) + 2e- <════> 2 Hg (l) + 2 Cl-

Electrode potential is given by

E = Eo – 2.303 RT/2F log [Cl-]2

= Eo -0.0591 log [Cl-] at 298 K

The electrode potential is decided by the concentration of chloride ions & the electrode is reversible with chloride ions at 298K, the electrode potentials are as follows.

0.1N KCl electrode (0.334V)

1 N KCl electrode (0.281 V)

Saturated KCl electrode (0.2422 V)

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