In electrochemical testing, the reference electrode is the potential standard. Many beginners easily choose the wrong one, which can cause potential drift and sample contamination. Below is a summary of 5 of the most commonly used reference electrodes.
📌Standard Hydrogen Electrode (SHE/NHE)
Composition: Platinum-plated black platinum sheet, H⁺ activity 1 mol/L, continuously supplied with 1 atm hydrogen gas.
Features: Potential reference, specified potential at any temperature is 0.000 V.
Applications: Used only for theoretical calibration; cumbersome operation, flammable hydrogen gas, and highly toxic platinum; rarely used in routine experiments.
📌Saturated calomel electrode (SCE)
Composition: Hg, Hg₂Cl₂, saturated KCl solution.
Features: The potential at 25℃ is about +0.241 V vs SHE, the potential is stable.
Applicable: Neutral and acidic aqueous solutions.
⚠Avoid pitfalls
Contains highly toxic mercury, unstable above 80℃; leaks Cl⁻, chloride ion-sensitive systems are disabled, and will be gradually replaced by Ag/AgCl.
📌Silver/Silver Chloride Electrode (Ag/AgCl)
Composition: Silver wire coated with AgCl, immersed in KCl solution (saturated / 3 M / 1 M).
Features: Potential of approximately +0.197 V vs SHE in a saturated KCl system at 25℃; non-toxic, easy to prepare, and highly stable.
Applications: Aqueous solution systems; can also be modified for some non-aqueous systems; currently the most mainstream reference electrode.
📌Mercury/Mercury Oxide Electrode (Hg/HgO)
Composition: Hg, HgO, alkaline electrolyte (1 M KOH/NaOH).
Features: Potential at 25℃ (1 M KOH) approximately +0.098 V vs SHE; excellent stability in alkaline environments.
Applications: Ideal for alkaline electrocatalytic systems, preferred for HER and OER testing, reducing liquid junction potential interference. ⚠Contains mercury; take precautions.
📌Mercury/mercurous sulfate electrode (Hg/Hg₂SO₄)
Composition: Hg, Hg₂SO₄, saturated K₂SO₄ solution.
Features: The potential at 25°C is about +0.615 V vs SHE, and the system has no chloride ions.
Suitable for: acidic sulfate system. It is preferred when the sample is easy to react with chloride ions to avoid Cl⁻ contamination.
💡Summary
For routine acid and neutral tests, prioritize Ag/AgCl; for alkaline OER/HER, choose Hg/HgO; for chloride ion-sensitive systems, choose Hg/Hg₂SO₄;
SCE is being phased out; SHE is only used as a potential scale. Always clearly label the reference type during experiments to avoid errors in potential conversion.
