Ag/AgCl vs SCE vs Hg/HgO: Which Reference Electrode Should You Use?
Choose by electrolyte. Use Ag/AgCl or a saturated calomel electrode in neutral and acidic solutions, Hg/HgO in alkaline solutions such as KOH, Hg/Hg₂SO₄ where chloride must stay out of the cell, and Ag/Ag⁺ in organic solvents.
Reference electrodes compared
| Electrode | Potential vs SHE (25 °C) | Best for | Avoid in |
|---|---|---|---|
| Ag/AgCl (saturated KCl) | +0.197 V | Neutral and acidic aqueous solutions | Strong alkali; cells where chloride harms the work |
| Saturated calomel (SCE) | +0.241 V | Aqueous work and corrosion testing | Hot solutions; labs avoiding mercury |
| Hg/HgO (1 M KOH) | +0.098 V | Alkaline solutions such as KOH and NaOH | Acidic solutions |
| Hg/Hg₂SO₄ (saturated K₂SO₄) | +0.64 V | Sulfuric acid; chloride-free work | Alkaline solutions |
| Ag/Ag⁺ | Depends on solvent and salt | Organic solvents such as acetonitrile | Aqueous solutions |
Three questions that decide it
- What is the pH? Alkaline electrolytes call for Hg/HgO. Acidic and neutral ones can use Ag/AgCl or SCE.
- Can chloride reach your cell? If a trace of chloride would harm the catalyst or the metal under test, use Hg/Hg₂SO₄ or Hg/HgO.
- Is the electrolyte water? For organic solvents, use a non-aqueous Ag/Ag⁺ reference, so that no water enters the cell.
Reporting your potentials
Papers on electrocatalysis usually report potentials against the reversible hydrogen electrode (RHE), so that results at different pH can be compared. At 25 °C, E(RHE) = E(measured) + E(reference vs SHE) + 0.0592 × pH. The potential converter does this sum for you.
Always state the reference electrode and its filling solution. An Ag/AgCl electrode in 3 M KCl sits at about +0.210 V, not +0.197 V, so leaving this out makes a result hard to reproduce.
Common mistakes
- Using Ag/AgCl in strong KOH, where it is not the best choice.
- Letting the tip dry out. Store it in its filling solution.
- Leaving an air bubble at the tip, which breaks the connection and makes the reading noisy.
- Never checking it. Compare it now and then with a reference kept only for that purpose: a shift of more than a few millivolts means it needs attention.
Frequently asked questions
Which reference electrode is best for KOH?
Hg/HgO. It is built for alkaline electrolytes, and it is the usual reference for alkaline water electrolysis and AEM studies.
Is Ag/AgCl better than a calomel electrode?
For most aqueous work the two are interchangeable. Ag/AgCl contains no mercury and is simpler to handle, while SCE has a long history in corrosion work.
Which reference electrode has no chloride?
Hg/Hg₂SO₄ and Hg/HgO use no chloride in their filling solutions, so they suit chloride-sensitive work.