Reference Electrodes Guide: Ag/AgCl, SCE, Hg/HgO & RHE

Previously, in  How to Choose Electrodes? A Practical Guide to the Three-Electrode System, we called the reference electrode the voltage ruler of an electrochemical cell. This article looks at that ruler in detail: how each type works, how to convert between potential scales, and how to choose the right one for your electrolyte.

It matters because a reference electrode that has drifted by ten millivolts will not tell you. It simply shifts every peak, onset potential and overpotential you report, and the data still looks reasonable.

What Is a Reference Electrode?

A reference electrode holds a fixed, known and repeatable potential, giving the electrochemical workstation a stable zero point to measure the working electrode against. The working electrode hosts the reaction you care about. The counter electrode completes the current circuit. The reference electrode simply stays still.

It manages this because its internal redox couple has a very high exchange current density — the reaction is fast and highly reversible, so the tiny current drawn through the reference lead does not shift its potential. This is called non-polarizable behaviour. Its potential stays set by the Nernst equation for its own chemistry, not by what happens at the working electrode.

What makes a good reference electrode

• A stable, well-documented potential that is repeatable from unit to unit, so results can be compared across labs and across years.
• High exchange current density, so drawing a small reference current does not move the potential.
• Chemical compatibility with the test electrolyte. This is the most common cause of failure in practice.
• A low temperature coefficient, so the potential holds steady during long battery cycling or operando runs. This is not negligible: for a saturated-KCl Ag/AgCl electrode, a 10 °C swing is worth roughly 10 mV.
• Minimal ion leakage across the junction, since stray Ag⁺, Hg₂²⁺ or Cl⁻ can poison a catalyst or react with the working electrode.

Potential Scales and Conversion Formulas

All potentials below are given against the standard hydrogen electrode (SHE), also written NHE. It is defined as exactly 0.000 V at all temperatures and is the zero point of the whole electrochemical scale. It is rarely used in routine work — it needs a constant flow of hydrogen gas and the platinum surface poisons easily — but every practical reference electrode is calibrated against it.

Converting between scales is where errors creep into published work. The maths is easy; the mistake is usually the wrong filling solution or the wrong sign.

Example. A CO₂ reduction test in pH 7 electrolyte shows an onset at −0.80 V vs Ag/AgCl. On the RHE scale: −0.80 + 0.198 + (0.0592 × 7) = −0.188 V vs RHE. Always state which filling solution your electrode uses — a 3.5 M fill instead of saturated moves the answer by 7 mV.

A note on published values. Reference electrode potentials differ slightly between sources, and the mercury-based electrodes differ the most. For Hg/Hg₂SO₄ in saturated K₂SO₄, Bard and Faulkner give +0.64 V while the Handbook of Analytical Chemistry gives +0.65 V; for Ag/AgCl in saturated KCl, published figures run from about +0.195 V to +0.199 V. Spreads of 10–20 mV between textbooks are normal. Whichever you use, state the electrode, the filling solution and the source in your methods section so the conversion can be reproduced.

Types of Reference Electrodes

1. Silver/Silver Chloride Electrode (Ag/AgCl Electrode)

The Silver/Silver Chloride (Ag/AgCl) electrode is the most widely used reference electrode in electrochemistry today, and the default choice for most aqueous work. It is a silver wire coated with silver chloride, sitting in a chloride electrolyte — most commonly saturated KCl, sometimes 3 M KCl or 3 M NaCl — connected to the test solution through a porous frit. The filling solution is normally pre-saturated with a small amount of AgCl as well, which stops the silver chloride coating on the wire from slowly dissolving away.

AgCl + e⁻ ⇌ Ag + Cl⁻

Beyond Battery's Ag/AgCl electrode is rated at +0.198 V vs SHE at 25 °C with a saturated KCl fill, which sits in the middle of the published range for this electrode. The potential is set by chloride activity, so it shifts with the electrolyte:

It leads the field because it is mercury-free, cheap to make consistently, stable in neutral and mildly acidic or alkaline water, usable well above the temperature ceiling of calomel (up to around 100 °C depending on construction), and small enough for compact cells and in-situ optical cells.

Correcting for temperature. The potential is temperature-dependent, and between 10 and 40 °C it is commonly estimated as:

• Saturated KCl: E (mV) = 199 − 1.01 × (T − 25)
• 3.5 M KCl: E (mV) = 205 − 0.73 × (T − 25)

where T is in °C. A saturated fill is more temperature-sensitive than an unsaturated one, because the KCl solubility itself changes with temperature. Published coefficients vary between sources, so for temperature-critical work measure the offset yourself rather than relying on a tabulated figure. For comparison, the SCE coefficient is around 0.65 mV/°C.

Single-junction, double-junction and semi-solid designs

Single-junction models put the electrolyte in direct contact with the sample through one frit — simple, low resistance, fast. Good for routine aqueous voltammetry.

Double-junction models add a second bridge chamber between the internal element and the sample. This blocks silver and chloride from reaching the test solution, at the cost of slightly higher resistance. Choose it for trace analysis, biosensing, silver or lead electrochemistry, chloride-sensitive catalysts, and long runs where leakage builds up.

Semi-solid (gel) models take a different approach: instead of a free liquid, they use a saturated KCl gel — the same chloride electrolyte, thickened into a gel so it stays in place. The chemistry is identical and chloride activity still sets the potential, but there is nothing to spill, leak or refill. These designs are also described as solid-state, leak-free or no-refill reference electrodes, and suit compact cells, in-situ optical windows and long unattended measurements. Note that "electrolyte-less" is industry shorthand for this construction: the electrolyte is still there, just in gel form rather than liquid.

All seven models below use the same Ag/AgCl couple and are rated at +0.198 V vs SHE. What differs is the body material, the size, and how the electrolyte is contained:

Body material matters as much as size. Glass is fine for most neutral aqueous work, but it is attacked by strong alkali and by fluoride-containing electrolytes, and it breaks. For aggressive media, choose a polymer body: R2060 in PTFE for standard-size cells with a refillable liquid fill, or the semi-solid PK-1020 and PK-1038 in PEEK, which pair the same chemical resistance with a saturated KCl gel — nothing to leak into the cell, nothing to refill between runs, and an unbreakable body. At 2.0 mm, PK-1020 is also the narrowest electrode in the range for tight cell fittings. Model dimensions are listed in the Ag/AgCl electrode user manual.

If you are planning to use a semi-solid electrode inside a controlled-atmosphere glovebox, talk to us first. A saturated KCl gel still contains its solvent, so both the moisture specification of your box and the working life of the gel in a dry atmosphere need to be checked against your setup before you commit.

Applications: aqueous CV and LSV, EIS, corrosion and coating tests, biosensors, aqueous zinc-ion and sodium-ion battery electrodes, CO₂ electroreduction in MEA and gas diffusion electrode flow cells, and in-situ Raman, UV-Vis and XRD spectroelectrochemistry.

2. Saturated Calomel Electrode (SCE)

The Saturated Calomel Electrode uses a mercury / mercurous chloride couple in saturated KCl and reads about +0.241 V vs SHE at 25 °C.

Hg₂Cl₂ + 2e⁻ ⇌ 2Hg + 2Cl⁻

For most of the twentieth century it was the standard reference in analytical and corrosion work, and a large body of literature is calibrated to its scale. Two things are retiring it. Its mercury content is now restricted for shipping, handling and disposal in many institutions. And calomel disproportionates on heating (Hg₂Cl₂ → Hg + HgCl₂), which destabilises the potential. Sources put the practical ceiling between about 50 and 80 °C, so treat the SCE as a room-temperature electrode and use Ag/AgCl for anything heated. It also adds chloride to the sample.

Applications: legacy corrosion protocols and published standards that specify the SCE scale.

3. Mercury/Mercuric Oxide Electrode (Hg/HgO Electrode)

The Hg/HgO reference electrode exists because chloride-based electrodes do not survive long in strong base — the AgCl coating on an Ag/AgCl electrode is progressively converted to silver oxide, and calomel is likewise unstable at high pH. Filled with 1 M or 20% KOH or NaOH, it reads about +0.098 V vs SHE in 1 M KOH at 25 °C — the value conventionally used in the electrocatalysis literature for RHE conversion. Published figures for this electrode vary more than most, so check the value quoted for your specific fill concentration.

HgO + H₂O + 2e⁻ ⇌ Hg + 2OH⁻

The real benefit is the liquid junction potential. Because the filling solution matches the alkaline test electrolyte, the junction potential stays small and steady. Put a neutral-filled SCE or Ag/AgCl into 1 M KOH instead and you create a sharp pH gradient at the frit, which causes acid-base drift in every reading.

Applications: alkaline water electrolysis, OER and HER catalysis in base, AEM electrolysers, alkaline fuel cells, and Ni-MH and Ni-Cd battery electrodes.

4. Mercury/Mercurous Sulfate Electrode (Hg/Hg₂SO₄ Electrode)

The Mercury Sulfate reference electrode replaces chloride with sulfate chemistry. In saturated K₂SO₄ it reads about +0.64 V vs SHE at 25 °C. Note that the figure often quoted as +0.615 V is the standard potential at unit sulfate activity, not the value for a saturated fill — the two are frequently confused.

Hg₂SO₄ + 2e⁻ ⇌ 2Hg + SO₄²⁻

Its whole purpose is to be chloride-free. If the working electrode reacts with Cl⁻ — silver and lead electrochemistry are the usual cases — a leaking SCE or Ag/AgCl junction stops being a passive reference and starts contaminating the cell.

Applications: lead-acid battery research, sulfuric acid and sulfate electrolytes, and chloride-free corrosion studies. It is not suitable for strongly alkaline solutions — use Hg/HgO there instead.

5. Silver/Silver Ion Electrode (Ag/Ag⁺ Electrode)

Aqueous reference electrodes do not belong in a dry cell. Put a standard Ag/AgCl into acetonitrile, propylene carbonate, DMSO or a lithium-ion electrolyte and water and chloride will cross into the cell, spoiling both the electrolyte and the reference potential.

The Silver/Silver Ion (Ag/Ag⁺) electrode uses a silver wire in a non-aqueous AgNO₃ solution made up in the same solvent as the cell, usually with a supporting electrolyte such as TBAPF₆ or TBAP.

Ag⁺ + e⁻ ⇌ Ag

Its potential depends on solvent, supporting electrolyte and Ag⁺ concentration, so it is not quoted as a fixed value vs SHE. Standard practice is to calibrate it in the cell against the ferrocene/ferrocenium (Fc/Fc⁺) couple, as IUPAC recommends, and report potentials on that scale.

Applications: lithium-ion and post-lithium electrolyte studies, organic electrosynthesis, molecular electrochemistry, and non-aqueous flow batteries.

6. Reversible Hydrogen Electrode (RHE)

Every electrode above is designed to stay still. The RHE does the opposite on purpose: its potential moves with pH at about 59 mV per pH unit at 25 °C, the same slope followed by most proton-coupled reactions.

That is the point. On the RHE scale, HER, OER and CO₂RR results measured at pH 1 and pH 13 sit on one comparable axis, with hydrogen evolution fixed at 0 V. This is why electrocatalysis papers report overpotentials vs RHE. In practice most labs do not run a true RHE, which needs continuously bubbled hydrogen over platinum. They measure against a stable fixed electrode — Ag/AgCl for neutral and acidic media, Hg/HgO for alkaline — and convert using the formulas above.

Reference Electrode Comparison Table

How to Choose the Right Reference Electrode

Match the reference chemistry to the electrolyte first. Everything else — body material, size, cost — comes second.

• Neutral or mildly acidic aqueous: Silver/Silver Chloride (Ag/AgCl). Well characterised, mercury-free, and available in the widest range of sizes and body materials.
• Strong base (KOH, NaOH), alkaline electrolysis, OER and HER: Mercury/Mercuric Oxide (Hg/HgO), then convert results to RHE.
• Sulfate media, lead-acid, chloride-sensitive systems: Mercury/Mercurous Sulfate (Hg/Hg₂SO₄).
• Organic electrolytes, lithium-ion and post-lithium research: Silver/Silver Ion (Ag/Ag⁺), calibrated against Fc/Fc⁺.
• CO₂ electroreduction: Ag/AgCl for neutral and bicarbonate electrolytes, Hg/HgO if the catholyte is strongly alkaline; report vs RHE.
• Aggressive or corrosive electrolytes: a polymer-bodied Ag/AgCl — PTFE (R2060) if you want a refillable standard-size electrode, or semi-solid PEEK (PK-1020, PK-1038) for a leak-free, no-refill option in a narrower body.
• Trace analysis, biosensing, silver or lead electrochemistry: double-junction Ag/AgCl (R8060).

Placement: Salt bridge/Luggin Capillaries and iR Drop

A well-calibrated electrode in the wrong place still gives the wrong answer. The measured potential includes the uncompensated solution resistance between working and reference electrodes — the iR drop, which grows with current and distance.

• Get close, but not too close. Place the tip, or a Luggin capillary, about two capillary diameters from the working electrode. Closer than that and it shields the surface, distorting the current distribution.
• Stay out of the current path between working and counter electrodes, where the potential field is steepest.
• Use a Salt bridge/Luggin capillary for high-current work such as battery electrode testing, electrolysis and electrocatalysis.

Maintenance and Troubleshooting

A reference electrode is a consumable instrument, closer to a pH probe than to glassware. Most unexplained drift traces back to neglecting it.

• Top up with the correct filling solution on liquid-filled models — saturated KCl pre-saturated with AgCl, not plain KCl and never deionised water. Topping up with plain KCl slowly strips the AgCl coating off the internal wire. Semi-solid models cannot be refilled; when the gel degrades, the electrode is replaced.
• Store wet, never dry. Keep liquid-filled electrodes in filling or storage solution between uses. Letting the frit dry out is the fastest way to ruin a good electrode. Gel models also need their recommended storage condition; a dried-out gel will not recover.
• Inspect and clean the frit. Clogging raises resistance, adds noise and slows the response.
• Never use it as a counter electrode. Passing real current through it causes permanent damage.

How to check one. Put the electrode under test and a fresh electrode of the same type in the same electrolyte, and measure the potential between them with a high-impedance voltmeter or your potentiostat. Agreement within a few millivolts is fine; more than that means drift. For non-aqueous cells, calibrate against Fc/Fc⁺ instead. Keep one reference electrode aside purely as a calibration standard — it is the cheapest quality control you can run.

Shop Reference Electrodes at Beyond Battery

Beyond Battery supplies every reference electrode covered here, in body materials and dimensions to suit standard, compact and corrosive-media cells:

• Silver/Silver Chloride (Ag/AgCl) Electrode — glass, PTFE, double-bridge and semi-solid PEEK bodies (R1038, R1060, R2060, R8060, PK-1020, PK-1038)
• Saturated Calomel Electrode (SCE)
• Hg/HgO Reference Electrode
• Mercury Sulfate Reference Electrode (Hg/Hg₂SO₄)
• Silver/Silver Ion (Ag/Ag⁺) Electrode

Browse the full reference electrodes collection, or explore our electrodes and electrochemical cells. When cells and electrodes are ordered together, electrodes are matched to the cell dimensions.

Not sure which one suits your electrolyte or cell? Contact our technical team — we work with researchers in over 40 countries and are glad to advise on electrode selection, filling solutions and maintenance.

Frequently Asked Questions

What does a reference electrode do?

It holds a fixed, known potential so the potentiostat has a stable zero point to measure the working electrode against. Almost no current passes through it, so its own potential does not move during the experiment.

What is the potential of a Silver/Silver Chloride (Ag/AgCl) reference electrode?

+0.198 V vs SHE at 25 °C for Beyond Battery's electrode in neutral aqueous solution with saturated KCl fill. It shifts with chloride activity: about +0.205 V in 3.5 M KCl, +0.210 V in 3.0 M KCl, and +0.222 V at unit chloride activity.

How do you convert Ag/AgCl to the RHE scale?

E(vs RHE) = E(vs Ag/AgCl) + 0.198 + 0.0592 × pH at 25 °C. So −0.80 V vs Ag/AgCl at pH 7 becomes −0.188 V vs RHE.

What is a gel or semi-solid reference electrode?

One that uses a saturated KCl gel instead of a free liquid filling solution. The chemistry is unchanged — chloride activity still sets the potential — but there is nothing to spill, leak or refill. Also called solid-state, leak-free or no-refill designs, and often listed as "electrolyte-less", although the electrolyte is still present in gel form.

What is the difference between Ag/AgCl and SCE?

Ag/AgCl (+0.198 V vs SHE) is mercury-free, smaller and stable at higher temperatures. SCE (+0.241 V vs SHE) contains mercury and becomes unstable on heating, with sources placing the ceiling between about 50 and 80 °C. Ag/AgCl is now the default in most labs.

Which reference electrode works in alkaline electrolyte?

Hg/HgO. Its KOH or NaOH filling solution matches the test electrolyte, keeping the liquid junction potential small and avoiding the acid-base drift you get from a neutral-filled electrode in strong base.

Can Ag/AgCl be used in organic electrolytes?

Not directly — water and chloride cross the junction and destabilise both the electrolyte and the reference potential. Use a Silver/Silver Ion (Ag/Ag⁺) electrode calibrated against Fc/Fc⁺, or a double-junction Ag/AgCl with a compatible bridge solution.