A lab measurement uses a Ag/AgCl reference electrode with 3.5 M KCl; what is the approximate potential vs SHE?

• A. About +0.210 V (relative to SHE at 25°C).
• B. About -0.210 V.
• C. About +0.030 V.
• D. About -0.030 V.

Answer: (A)

The key idea is that an Ag/AgCl reference electrode has a stable potential defined by the AgCl(s) + e− ⇌ Ag(s) + Cl− equilibrium, and this potential shifts with how much chloride is in solution. The Nernst equation for this couple is E = E° − (RT/F) ln a(Cl−). At 25°C, that becomes E ≈ E° − 0.05916 log10(aCl−). The standard potential for this couple with unit chloride activity is about +0.222 V versus SHE. In 3.5 M KCl, the chloride activity is high, so the potential remains positive but is slightly reduced, landing near about +0.210 V vs SHE. That’s why +0.210 V is the commonly cited approximate potential for this electrode under these conditions.