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In this chapter, the topic of Electrodes is explained through the concept of hydrogen electrodes. This electrode is built by bubbling H2 gas in acid (such as HCl, H2SO4, or another mineral acid).

The equilibrium in this case is represented as-

H+(aq)   +  e– ↔ 1 / 2 H2(g)

In this case, the electrode is reversible with respect to the H+ ion. H2 gas is non-conducting in nature hence noble metal like Pt is used for making electrical contact in the circuit and therefore the electrode is represented as-

Pt ; H2(g) | H+

and the electrode reaction is written as

2H+ (aq)   +  2e–—————–> H2(g)

If the acid in Pt foil dipped contains 1-mole concentration of (H+) ion at 25°C then the electrode is called hydrogen electrode.

Normal hydrogen electrode (NHE) or standard hydrogen electrode(SHE) is used as the reference electrode. However, it is not convenient to use normal hydrogen electrodes as reference electrode because of the following difficulties:

(i) Gases are difficult to handle

(ii) It is difficult to maintain the H+ ion concentration as 1M and the pressure of the gas at 1 atm.

(iii) The electrode is poisoned even by the presence of traces of impurities.

(2) Calomel electrode Hg- Hg2 Cl2 electrode:

Calomel electrode is simple and reliable. It is also used as reference electrode for determining electrode potentials and pH of the supplied solutions. This electrode consists of Hg, solid mercurous chloride; Hg2Cl2, and a saturated solution of KCl. This electrode is represented as-

Hg(I), Hg2Cl2(s); KCl (solution)

This electrode is reversible with respect to Cl– ions, i.e. when it acts as anode, the electrode reaction is written as

2Hg (1)  +  2 Cl (aq)——————————  Hg2Cl2(s)  +  2 e–

and when it acts as the cathode, the reverse reaction takes place. If the electrode reaction involves reduction, the Hg2(ous) ion derived by Hg2Cl2 would be discharged at the electrode, hence more and more of calomel would pass into the solution.