Which electrode is commonly used as a reference electrode in potentiometry?

• A. Mercury-Mercury (Calomel)
• B. Silver-silver
• C. Copper-copper
• D. Glass electrode

Answer: (A)

In potentiometry, the reference electrode must provide a stable, known potential that doesn’t drift with changes in the sample. The calomel electrode, which uses the mercury/mercury(I) chloride redox couple in a saturated chloride solution, yields a highly stable and well-defined potential at room temperature. This stability comes from a predictable equilibrium that remains largely unaffected by the analyte or its ionic strength, making it a reliable baseline against which the working electrode’s potential can be measured. Because of these qualities, the calomel reference has been a standard choice in many laboratories and instruments.

A silver-silver chloride reference electrode is also used in many setups and offers similar stability, but the calomel electrode has a long history of well-characterized potentials. The glass electrode is an ion-selective sensor, not a reference, and copper-copper does not provide the stable, well-defined potential required for a reference electrode.