Reversible Electrode

Reversible Electrode

A galvanic cell is an electric device which generates electricity at the expense of redox reaction. The redox cell reaction is an overall reaction which consists of oxidation and reduction half cell reactions. The oxidation half cell reaction occurs at anode while the reduction half cell reaction occurs at cathode. Anode and cathode are commonly called as electrode. Electrode which can act both as an anode and cathode is called reversible electrode.
Cd|Cd⁺² electrode acts as an anode if it is connected with a Ag|Ag⁺ cathode-
Cd|Cd⁺² || Ag⁺|Ag
But, Cd|Cd⁺² electrode acts as cathode if connected to a Zn|Zn⁺² anode-
Zn|Zn⁺² || Cd⁺²|Cd
Hence, Cd|Cd⁺² electrode ia a reversible electrode.

Types of Reversible Electrode

There are four types of reversible electrodes according to their compositions.
1. Metal-metal ion electrode
2. Metal-sparingly soluble salt electrode
3. Gas electrode
4. Redox electrode (Oxidation reduction electrode)

Metal-Metal Ion Electrode

This electrode is set up by dipping a metal in a solution containing it's own ions, e.g. Zn rod dipped into ZnSO₄ solution containing Zn⁺² ions of concentration C. It is represented as-
Zn⁺²(aq) | Zn(s)
The reduction reaction at the electrode is-
Zn⁺²(aq) + 2e → Zn(s)

Metal-Sparingly Soluble Salt Electrode

This electrode consists of a metal coated with one of its sparingly soluble salts and immersed in a solution containing an electrolyte having a common anion as that of the salt.
Example-Silver electrode coated with sparingly soluble AgCl dipped in KCl solution with common anion Cl^-. This electrode is represented as-
Cl^-(aq) | AgCl(s) | Ag(s)
The reduction reaction at the electrode is-
AgCl(s) + e → Cl^-(aq) + Ag(s)

Gas electrode

This is developed by bubbling pure and dry gas around a platinised platinum foil dipped in the solution containing ions (of the gas) reversible with respect to the gas bubbled. The gas is adsorbed on the surface of platinum foil and establishes an equilibrium with its ions in the solution. Pt electrode provides electrical contact and also acts as a catalyst.
Hydrogen gas electrode-
H⁺(aq) | H₂ | Pt
The reduction reaction at the electrode is-
H⁺(aq) + e → 1/2H₂(g)

Redox electrode (Oxidation reduction electrode)

This electrode consists of a platinum wire dipped in a solution containing the ions of the same metal (or a substance) in two different oxidation states.
Fe⁺² - Fe⁺³, Sn⁺² - Sn⁺⁴ etc.
A platinum electrode which provides an electrical contact and acts as catalyst aquires an equilibrium between two ions in the solution, due to their tendency to undergo a change from one oxidation state to another. The electrodes are represented as-
Fe⁺²(aq), Fe⁺³(aq) | Pt
The reduction reaction at the electrode is-
Fe⁺³(aq) + e → Fe⁺²(aq)

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