QS Study

Secondary Standard Electrodes

An electrode with an invariant potential. In electrochemical methods, where it is essential to monitor, determine, or organize the potential of another electrode (denoted indicator, test, or working electrode), it is required to use a reference electrode, which maintains a potential that remains almost unchanged during the course of an electrochemical measurement.

The hydrogen electrode is not a convenient reference electrode to use in measurement for the following reasons:

(i) One has to prepare a Standard Hydrogen Electrode (SHE) every time it is needed; the procedure is cumbersome.

(ii) Maintaining a stream of hydrogen at 1 atm takes careful arrangement,

(iii) Preparation of a solution of an acid in which the concentration of H+ ion is exactly 1.0 mol L-1 is time consuming.

(iv) It is not easy to prepare pure hydrogen.

(v) Hydrogen has to be handled very carefully as it is inflammable.

In order to avoid these difficulties other electrodes that can be prepared easily and whose electrode potentials are constant under certain conditions have been devised. Such electrodes are known as secondary standard electrodes. Two such electrodes arc:

  1. Calomel Electrodes
  2. Silver Electrodes

Calomel electrode: Calomel electrodes represented as- Pt│Hg.Hg2 Cl2(s).KCl (salt)(aq)

The electrode reaction is- Hg2 Cl2(s) + 2e = 2Hg (l) + 2Cl (aq)

The potential of saturated calomel electrode is 0.244 V.

Silver Electrodes: This electrode is prepared by dipping a silver rod into a saturated solution of potassium chloride over a paste of AgCl and is represented as-

Ag│AgCl(s), Cl (salt)(aq)

The electrode reaction is- AgCl(s) + e ↔ Ag (s) = Cl (aq)