Reference electrodes are half-cells with a stable and well-defined electrode potential.
These electrodes are used to provide a constant potential against which the potential of other electrodes can be measured.
Common reference electrodes include the Standard Hydrogen Electrode (SHE), Silver-Silver Chloride Electrode (Ag/AgCl), and Calomel Electrode.
1) Standard hydrogen electrode:
Construction:
The SHE consists of a platinum (Pt) electrode that is coated with platinum black to increase its surface area.
This electrode is immersed in a solution containing H⁺ ions (usually a 1 M HCl solution) and hydrogen gas (H₂) is bubbled over the platinum electrode at a pressure of 1 atm.
The entire system is maintained at a temperature of 25°C.
Working:
The SHE is based on the half-reaction:
2H+ (aq) + 2e- ⇌ H2 (g)
At the platinum electrode, hydrogen gas is either oxidized (loses electrons) or reduced (gains electrons), depending on the direction of the electrochemical process.
The potential of the SHE is arbitrarily assigned as 0.00 V under standard conditions (25°C, 1 atm, 1 M H⁺). It serves as the universal reference point to which other electrode potentials are compared.
2) Silver Chloride Electrode (Ag/AgCl electrode):
Construction:
The Ag/AgCl electrode consists of a silver wire coated with silver chloride (AgCl).
This wire is immersed in a solution of potassium chloride (KCl) of a known concentration.
The electrolyte is saturated with KCl, and the reference potential is dependent on the concentration of chloride ions (Cl⁻).
Working:
The electrode operates based on the equilibrium:
AgCl (s) + e- ⇌ Ag (s) + Cl- (aq)
The potential of this electrode depends on the chloride ion concentration in the solution. The potential is well-known and constant for specific chloride concentrations.
For a saturated KCl solution, the standard potential of this electrode is around +0.197 V versus SHE.
3) Calomel electrode:
Construction:
The calomel electrode consists of mercury (Hg) in contact with mercury(I) chloride (calomel, Hg₂Cl₂).
This assembly is immersed in a potassium chloride (KCl) solution of known concentration (saturated or unsaturated).
A porous salt bridge connects the electrode with the solution being measured to maintain electrical neutrality.
Working:
The calomel electrode works based on the half-reaction:
Hg2Cl2 (s) + 2e- ⇌ 2Hg (l) + 2Cl- (aq)
The potential depends on the concentration of the KCl solution. For a saturated KCl solution, the electrode has a potential of approximately +0.244 V versus SHE at 25°C.
This electrode is widely used because of its stable potential and ease of preparation.