Reference electrodes are stable and well-defined electrodes used as a reference point in electrochemical measurements. They provide a known and constant electric potential against which other electrodes, particularly working electrodes, can be compared. Reference electrodes are essential in electrochemical techniques, ensuring accurate potential measurement and understanding the electrochemical behavior. Examples include the Standard Hydrogen Electrode (SHE), Silver/Silver Chloride Electrode (Ag/AgCl), and Saturated Calomel Electrode (SCE).
1. Standard Hydrogen Electrode (SHE)
Construction
The SHE consists of a platinum electrode immersed in a solution of hydrogen ions (H+) at 1 atmosphere pressure and a platinum wire.
Working Principle
The SHE defines its potential as 0 V and serves as the reference against which other electrodes measure their potential. It operates based on the half-reaction: 2H+ + 2e- → H2(gas).
2. Silver Chloride Electrode (Ag/AgCl)
Construction: The Ag/AgCl electrode consists of a silver wire coated with a layer of silver chloride (AgCl) immersed in a potassium chloride (KCl) solution.
Working Principle: The Ag/AgCl electrode maintains its well-defined and stable potential due to the reversible Ag/AgCl half-cell reaction: AgCl(s) + e- → Ag(s) + Cl-. It finds common usage as a reference electrode in potentiometry.
3. Saturated Calomel Electrode (SCE)
Construction: The SCE consists of a mercury pool at the bottom of the electrode, separated from a saturated potassium chloride (KCl) solution by a porous ceramic plug. A silver wire with mercury chloride (Hg2Cl2) connects to the mercury pool.
Working Principle: The potential of the SCE is maintained by the equilibrium between the mercury and mercury chloride. It has a stable potential due to the reversible half-cell reaction: Hg2Cl2(s) + 2e- ⇌ 2Hg(l) + 2Cl-.
