Mercury covered by a layer of mercurous chloride in contact with saturated potassium chloride solution is a description of which of the following types of electrodes?

• A. Sodium
• B. Calomel
• C. Calcium
• D. Silver/silver chloride

Answer: (B)

This describes the saturated calomel electrode, a classic reference electrode. The inner element is mercury covered with mercurous chloride (Hg2Cl2), which is in contact with a saturated potassium chloride (KCl) solution. The key is the Hg2Cl2/Hg redox couple: Hg2Cl2 + 2 e− ⇌ 2 Hg + 2 Cl−. The saturated KCl fixes the chloride ion activity, so the electrode potential stays nearly constant regardless of the sample solution. Because of this stable potential, it serves as a reliable reference in measurements. The potential relative to the standard hydrogen electrode is about +0.244 V at 25°C. Other options either use a different chloride compound (like AgCl in silver/silver chloride) or are not based on mercurous chloride, so they don’t match this specific description.