Types of electrodes

A metal rod immersed in the aqueous solution of its salt or a gaseous non-metal in contact with the aqueous solution of its anion in the form of salt (or acid or alkali) function as single electrodes.

Metal electrodes:

  • Zinc electrode : Zn| ZnSO4 aqueous solution

  • Copper electrode: Cu |CuSO4 aqueous solution

  • Magnesium electrode: Mg|MgSo aqueous solution

  • Silver electrode: Ag | AgNO3 aqueous solution

  • Iron electrode : Fe | FeSO4 aqueous solution; etc.

Non-metal electrodes:

  • Hydrogen electrode: H+(aq) / H2(g), Pt

  • Chlorine electrode: Pt; Cl2(g) / Cl– (aq)

  • Oxygen electrode: Pt; O2(g) / OH– (aq)

  • Bromine electrode: Pt; Br2(g) / Br– (aq)

Single electrode potentials:

When a metal or non-metal is in contact with the solution of its ions tends to develop electric potential at the metal/ metal ion or non-metal/non-metal ion interface. It is called single electrode potential.

The magnitude of the potential (E) depends on

  • Chemical nature of the metal or the non–metal

  • The number of electrons transferred (n) in the half cell reaction namely

    • Mn+ + ne → M (M = metal)

    A + ne → A (A = non metal)

  • The concentration of the ions (C) of Mn+ or An– in the solution.

  • The temperature of the system (T)

E˚ = standard reduction potential of the metal measured at standard conditions, using (Mn+) as unit concentration. (25° C or 298K, 1 atmosphere pressure are called standard conditions.

The reduction potential of an electrode is exactly equal in magnitude but opposite in sign to its oxidation potential.

Single electrode potential cannot be determined experimentally by taking single electrode. Therefore, this single electrode is combined with a second single electrode and the emf of the cell is determined by using a potentiometer.

The second electrode is an electrode of known potential and is called as reference electrode.

The hydrogen electrode is the earliest reference electrode,

Hydrogen electrode consists of H2 gas at 1 atmosphere pressure in contact with H+ ions of unit activity

Pt, H2(g)/ H+ (aq)(a = 1). The potential of hydrogen electrode is assumed as zero volts.

Hydrogen electrode is also called as Normal Hydrogen Electrode (NHE) or standard Hydrogen Electrode (SHE)

A small platinum foil coated with platinum black is used as electrode in Hydrogen electrode.

The Pt electrode is partially immersed in the solution and is partially exposed to the atmosphere of H2 gas

Hydrogen electrode is a reversible electrode.

Nernst equation is applicable only for reversible electrodes.

R.P of Hydrogen electrode = –0.059 × PH

O.P of Hydrogen electrode = +0.059 × PH

Calomel electrode:

A saturated calomel electrode : Hg/Hg2Cl2 (S), KCl (solution) is now used as a secondary reference electrode.

The potential of saturated calomel electrode is –0.2422 volts (SRP).

Calomel electrode is a type of half cell in which the electrode is mercury coated with calomel (Hg2Cl2) and the electrolyte is a solution of potassium chloride and saturated calomel. In the calomel half cell the overall reaction is

Hg2Cl2(s) + 2e– → 2Hg(l) + 2Cl–