In relation to reference electrodes, the saturated calomel electrode (SCE) is widely used in labs because of stability. What is its approximate potential relative to the standard hydrogen electrode at 25°C?

• A. It is defined as 0 V by convention
• B. It is around +0.000 V vs SHE
• C. It is around -0.241 V vs SHE
• D. It is around +0.241 V vs SHE

Answer: (D)

Reference electrodes have fixed, reproducible potentials against which measurements are made. The saturated calomel electrode derives its potential from the Hg2Cl2/Hg redox couple in saturated KCl, and at 25°C this couple places the electrode about 0.241 volts above the standard hydrogen electrode. So the SCE sits at a positive offset of roughly +0.241 V relative to SHE. This is why the correct choice is the one stating around +0.241 V; other options suggest zero or a negative offset, which don’t match the established potential of the SCE.