Potentiostat

date2026-07-24tags:chem:

The potentiostat is the instrument that makes every technique on this wiki possible. It enables OCP, LPR, CV, and EIS. This page is about the instrument itself. It covers why the instrument needs three electrodes instead of two. It covers what the instrument does internally. It covers how to hook up a real cell safely.

Why three electrodes

Two electrodes work for a simple first idea. A two electrode setup combines a working electrode and a counter reference electrode. It is how a battery or simple electrolysis cell is wired. The problem is that any current flowing through an electrode polarizes it. Polarization pushes the potential away from equilibrium. The reference electrode must not move. Forcing current through it shifts its potential. Your known reference point is no longer known.

The three electrode cell splits the two jobs:

Working electrode (WE): This is the sample. The reaction you care about happens here.
Reference electrode (RE): See Reference Electrodes. It stays essentially current free. Its potential stays fixed.
Counter electrode (CE): Also called auxiliary electrode. It carries the current instead. It is usually an inert high surface area electrode like platinum wire or mesh or graphite. It is chosen so that whatever half reaction happens at its surface does not matter to your measurement. It just needs to sink or source electrons freely.

The potentiostat control loop measures the potential difference between WE and RE. It does this through a high impedance amplifier that draws negligible current from the reference. Ideal current is less than nanoamps. The instrument adjusts the current it drives between WE and CE until the WE versus RE potential matches the experiment program. Conventional current flows in the external circuit between WE and CE. The RE just watches.

You cannot casually swap electrodes mid experiment. A two electrode measurement works as a valid simplification only when the counter electrode polarization is small enough or symmetric enough to not matter. This is true for some battery and symmetric cell work. It is not true for a corrosion cell where you need an accurate WE potential.

iR drop and compensation

The reference does not sit exactly at the working electrode surface even though it carries no current. Electrolyte always exists between the reference tip and the working electrode. That gap has real ohmic resistance Ru, the uncompensated resistance. The working electrode current i flowing through the solution toward the counter electrode causes a potential drop across that path:

E_applied = E_true + iRu

The potentiostat amplifier controls E_applied. It does not directly control E_true. At low current or in highly conductive electrolyte this error is negligible. In resistive electrolytes like organic solvents or thin films or coatings, or at high current density, it causes a large systematic distortion of your voltammogram or polarization curve. Peaks shift. Tafel slopes look wrong. LPR derived Rp is inflated by the Ru added in series.

The Luggin capillary and the shielding artifact

The obvious fix is to bring the reference tip as close as possible to the WE. This has its own problem. A reference or Luggin capillary placed too close physically blocks the current path from the WE to the CE. It disturbs the current distribution on the WE surface. The portion of the WE under the capillary tip sees less than its share of the total current. The measured polarization curve underestimates the true current density. This is the Luggin capillary shielding artifact. It appears in the corrosion literature. See Mansfield, Corrosion 1982, 38, 556-559.

The practical guideline: bring the capillary tip to within one capillary diameter of the WE surface. Do not go closer. Position it off center, not directly in front of the WE center. This disturbs the current distribution less. A drawn glass Luggin capillary with a 0.5 mm tip needs about 0.5 mm from the surface. A reference electrode in a bulkhead fitting has no capillary. The electrode body limits the closest approach to a few mm at best. The increased Ru is the price for avoiding the shielding artifact.

Two practical fixes:

Minimize it physically: Bring the reference or Luggin capillary tip as close to the working electrode as practical. Use a more conductive electrolyte or supporting electrolyte. Use a larger working electrode to counter electrode geometry.
Compensate it electronically: Most modern potentiostats offer positive feedback iR compensation. The instrument estimates Ru. It often does this via a high frequency current interrupt or impedance measurement. The instrument adds a corresponding correction into its control loop. The reported and applied potential better reflect E_true. Over compensation makes the control loop unstable. Oscillation and ringing appear. Dial it in and watch for noise. It is not a one click fix. See PalmSens iR compensation application notes at https://www.palmsens.com/knowledgebase/.

Compliance voltage

The compliance voltage is the maximum voltage the potentiostat can put across the WE CE loop. It forces the requested current or potential. If the cell resistance is high enough, reaching the target WE versus RE potential requires exceeding that maximum. The instrument hits compliance. It saturates. The loop can no longer track your program. The reported potential is no longer the one you asked for. The current or potential trace flatlines or clips.

The rule of thumb: the compliance voltage must exceed the sum of the maximum current times the cell total resistance and the full potential window you sweep. Multiply by a safety margin of about 1.5.

V_comp > I_max * R_cell + E_window

If your CV sweeps from minus 2 V to plus 1 V vs SCE, the window is 3 V. The cell resistance is 10 k ohm at a peak current of 1 mA. You need V_comp > 1.5 * (0.001 * 10000 + 3) = 6 V. Any portable potentiostat handles that. If the same sweep runs in a resistive organic electrolyte with R_cell = 100 k ohm, the required compliance jumps to 1.5 * (0.001 * 100000 + 3) = 154.5 V. This exceeds most benchtop instruments and all portable units. Fix the issue by reducing cell resistance. Use a bigger or better counter electrode. Add more supporting electrolyte. Shorten the electrode spacing. Do not expect the instrument to push through.

Potentiostat vs galvanostat

The same instrument usually does both. It uses different control modes.

Potentiostatic mode: Hold the WE versus RE potential at a set value. Program it if needed. A CV triangle wave is an example. An LPR step is another. Measure the resulting current. This is the default mode for OCP, LPR, CV, and most EIS.
Galvanostatic mode: Hold the current between WE and CE at a set value. Measure the resulting WE versus RE potential. Used for constant current charge discharge and battery testing. Also used for galvanostatic EIS or any experiment where you force a fixed reaction rate.

The three electrode wiring is identical either way. The difference is purely which variable, E or i, is the control loop target and which is the measured output.

Portable vs benchtop instruments

Portable and USB potentiostats: Examples include PalmSens EmStat and PalmSens4 lines and Pine Research WaveNow and WaveDriver series. These units are compact and computer or battery powered. They are increasingly capable. They handle routine CV, LPR, and basic EIS in teaching or field settings. Their compliance voltage, bandwidth, and current ranges are more limited than a benchtop instrument. Their iR compensation and EIS frequency range can be narrower.
Benchtop instruments: Examples include Gamry, Bio Logic, Metrohm Autolab, and Pine WaveDriver benchtop lines. They offer higher compliance voltage, wider current and frequency range, more sophisticated iR compensation, and multi channel operation. They cost more, take more bench space, and are larger.

For an instrument agnostic primer on the potentiostat control loop and electrode roles, PalmSens maintains a public knowledgebase at https://www.palmsens.com/knowledgebase/. It has short articles on working reference and counter electrode roles, compliance voltage, iR compensation, and two three and four electrode configurations. Pine Research YouTube channel at https://www.youtube.com/channel/UCkaSux3EzBER0CU8P_9IOsw has recorded webinars covering the same ground with more circuit level detail.

Four electrode measurements

The three electrode cell is standard for most electrochemistry. A four electrode configuration exists and is worth knowing about. A four electrode setup uses two separate reference electrodes. One sits near the working electrode. The other sits near the counter electrode. The potentiostat measures the potential difference between them. This is the standard configuration for:

Membrane and separator studies: A fuel cell or electrolyzer membrane study wants the potential drop across the membrane itself. It does not want that drop plus both electrode overpotentials. The second reference isolates the membrane contribution.
Ionic conductivity measurements of thin films.
Two electrode cells with a known counter electrode: Battery half cells where the counter potential is a known reference like Li/Li+.

Most benchtop potentiostats support four electrode mode. They label it as four wire or four electrode with separate RE1 and RE2 leads. Portable and USB units rarely support it.

Sciencemadness notes: building and repairing your own potentiostat

The sciencemadness wiki and forums contain practical notes on building a simple potentiostat from op amp components. This is not a replacement for commercial instrument stability, bandwidth, or compliance voltage. It is a useful starting point for understanding the instrument loop at the component level. Recurring themes include:

The control amplifier: A standard design uses a single op amp like an OPA604 or similar JFET input amplifier for high input impedance. The reference electrode connects to the non inverting input. The working electrode sits at virtual ground through a current to voltage converter or transimpedance amplifier on the WE lead. The electrochemical cell sits in the feedback loop of the control amplifier. This is the classic three electrode potentiostat topology from the literature. See Bard and Faulkner chapter 15 for the circuit level treatment.
Current ranges: Home built designs switch between feedback resistors on the transimpedance amplifier to change current ranges. A fixed resistor per decade with a range switch is the simplest approach. Switching the resistor while the cell is polarized can generate a transient that saturates the amplifier. Most designs use relays or JFET switches with a brief hold period during the switch.
Noise and shielding: The reference electrode high impedance path is a noise antenna. Sciencemadness designs emphasize short shielded cables, a grounded Faraday cage around the cell, and a separate ground plane for the analog electronics. Commercial instruments address this with a guard shield around the RE input.
Compliance voltage: A home built potentiostat compliance voltage is limited by the op amp supply rails. Typical values are plus or minus 12 to plus or minus 15 V for a standard JFET op amp. This handles a small cell in conductive electrolyte. It does not handle a high resistance organic electrolyte or a large cell. The discussion of compliance voltage above matters even before you build anything.

The PalmSens knowledgebase articles on the operational amplifier view of the potentiostat and the Pine Research webinars are more reliable than variable quality forum posts. The sciencemadness notes are a starting point for the circuit topology. Do not follow them blindly.

Connecting a real cell

A typical bench setup works like this:

WE lead to your sample. Mount it so only the intended area is exposed. See Standard Operating Procedure for polishing and masking.
RE lead to the reference electrode. Position it close to the WE surface without touching it.
CE lead to the counter electrode. Use Pt mesh or wire. Size it comparably to or larger than the WE to avoid current limitation.
Immerse all electrodes in the electrolyte in a single cell. Separate by a frit or salt bridge if the counter reaction needs isolation. Counter electrode products can reach the WE and interfere.

Sensible defaults before pressing start:

Confirm the reference is a known working one. See Reference Electrodes. Check that it has not been sitting dry.
Check that WE and CE leads are not swapped. Most instruments misbehave obviously if they are. Runaway current or a nonsensical OCP are signs.
Purge or sparge if the technique requires it.
Let OCP stabilize before starting any perturbation. See OCP. Do not assume the first reading is representative.