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This article explains how to diagnose and fix iR compensation errors in electrochemical measurements by quantifying uncompensated resistance, selecting a robust compensation method, and validating results with simple checks.
1. Understand iR Drop and Why It Skews Potentials
The measured potential includes a voltage loss equal to current times uncompensated resistance between the reference probe and the reaction interface.
E_true = E_measured - i · R_u.Large i or large R_u produces distorted peak positions, shifted Tafel slopes, and false overpotentials.
Caution: Overcompensation creates positive feedback that can drive oscillation and artifact peaks. Do not exceed stability limits of your cell and potentiostat.
2. Map the Resistance Path
Separate solution resistance from hardware resistance.
- R_solution: electrolyte conductivity, temperature, geometry, and reference tip distance to the working surface.
- R_hardware: reference frit, leads, connectors, and current interrupt relays.
- R_contact: clip and solder joints on electrodes.
3. Choose the Right Compensation Strategy
| Method | How It Works | Best Use | Limitations |
|---|---|---|---|
| Electronic positive feedback | Continuously subtracts i·R_u during the scan. | Fast scans and kinetic studies. | Risk of oscillation when % compensation is too high. |
| Current interrupt (CI) | Momentarily breaks current and measures immediate voltage step. | Steady currents in chrono methods. | Undershoots if interruption is slow or electrode is capacitive. |
| EIS high-frequency intercept | Extracts R_u from real-axis intercept of Nyquist at high frequency. | General purpose and benchmarking. | Needs a clean fit and adequate frequency range. |
| Manual post-correction | Applies E_true = E_measured - i·R_u after the experiment. | Publishing clarity and sensitivity analysis. | Not helpful for feedback-driven instability during acquisition. |
4. Measure Ru Reliably
4.1 EIS High-Frequency Intercept
Run small-signal EIS around the operating potential with an amplitude that keeps the system linear. Fit the spectrum and read R_u from the real-axis intercept at the highest frequency branch.
Caution: If the reference frit adds significant impedance, soak or replace the frit and repeat EIS. A clogged frit inflates R_u.
4.2 Current Interrupt Procedure
- Hold at the target current or potential until stable.
- Trigger the instrument’s current interrupt and record the instantaneous voltage step ΔE.
- Compute R_u = ΔE / i using the pre-interrupt current.
- Repeat at several currents to verify linearity.
4.3 Step-Current Method
Apply small current steps ±Δi around the operating point. Measure steady-state ΔE for each step. Fit slope dE/di. Use the highest-frequency response window to minimize double-layer effects.
5. Set Compensation Without Instability
- Start with 60–80% electronic compensation based on measured R_u.
- Increase in 5% steps while monitoring baseline noise and phase lag.
- Stop increasing when noise rises, peaks sharpen unrealistically, or control becomes erratic.
- Record the final % and the R_u source in your method file.
Caution: If oscillation occurs, halve the compensation, reduce scan rate, or increase cell capacitance with a small series resistor on the counter lead if your instrument supports it.
6. Lower Ru at the Source
- Shorten the reference tip–to–working distance using a Luggin capillary aligned to the electrode surface.
- Increase supporting electrolyte concentration within compatibility limits.
- Raise temperature moderately if chemistry allows since conductivity increases with temperature.
- Use high-conductivity separators and fresh membranes in cells that require them.
- Select low-resistance frits and maintain them clean and bubble-free.
- Use four-terminal lead configurations when available to reduce lead resistance effects.
7. Validate After Compensation
- Cyclic voltammetry: Peak separation for a reversible redox should approach theory when iR error is removed.
- Tafel plots: Slopes should be insensitive to small changes in % compensation if R_u is correct.
- Duplicate with manual post-correction to confirm that on-line compensation did not overcorrect.
8. Quick Diagnostic Checklist
| Symptom | Likely Cause | Action |
|---|---|---|
| Unrealistically sharp CV peaks after enabling compensation. | Overcompensation and positive feedback. | Lower % compensation by 10–20 points and retest. |
| Peak positions still shift with current. | Undercompensation or drifting R_u. | Re-measure R_u via EIS or CI and update. |
| Noisy baseline or oscillation. | Feedback loop unstable. | Reduce compensation, reduce scan rate, shorten reference distance. |
| Tafel slope changes when % compensation changes. | Incorrect R_u or capacitive distortion. | Use EIS-derived R_u and minimize current ripple. |
| Different R_u by method. | Frit impedance or nonlinearity. | Service reference, repeat at multiple currents, prefer EIS intercept. |
9. Worked Examples
9.1 Manual iR Post-Correction for CV
# Inputs R_u = 18.5 # ohm, from EIS intercept i = [ -0.003, -0.001, 0.000, 0.002, 0.004 ] # A E_m = [ 0.412, 0.423, 0.435, 0.449, 0.463 ] # V vs Ag/AgCl
Correction
E_true = [ Em - ii*R_u for Em, ii in zip(E_m, i) ]
Report
Use E_true for peak analysis and Tafel extraction.
9.2 Setting Safe Positive-Feedback Compensation
# Measured R_u = 22 ohm. target_pct = 0.75 R_comp = target_pct * 22 # 16.5 ohm
Instrument: set 'iR comp (ohm)' to 16.5 and enable dynamic tracking if supported.
Validate with a 10 mV test pulse. If ringing appears, reduce to 0.65 · R_u.
9.3 Current Interrupt Calculation
# During chronoamperometry: i = 12.0 mA. Interrupt yields ΔE = 145 mV. R_u = 0.145 / 0.012 # 12.08 ohm # Apply 70% compensation first. Increase carefully while monitoring stability. 10. Cell and Hardware Setup That Prevents iR Problems
- Use a Luggin capillary with a 0.5–1.0 mm tip distance to the electrode center for lab benchmarks.
- Keep the reference junction below the bubble line. Remove trapped gas near the tip.
- Use short, twisted pairs for working and reference leads to minimize inductive artifacts at high scan rates.
- Replace aged reference frits and salt bridges that show slow potential recovery after current interruptions.
11. Reporting Template for Methods Sections
Cell: three-electrode glass cell, 298 K. Electrolyte: 0.1 M supporting salt, resistance measured by EIS. Reference: Ag/AgCl (sat'd), Luggin tip at 0.8 mm. R_u: 16.2 Ω from HF intercept; verified by CI at 15.9 Ω. Compensation: 75% electronic positive feedback, validated by stable CV baseline. Post-processing: residual iR corrected offline with E_true = E_measured - i · 16.2 Ω. Uncertainty: ±0.8 Ω across three repeats. FAQ
How close can I place the reference tip to the working electrode?
Keep 0.5–1.0 mm for flat electrodes. Avoid physical contact and vortex zones. Recheck R_u after any geometry change.
What % iR compensation is safe?
Begin at 60–80% of measured R_u. Increase only if the baseline remains stable and peaks remain physically plausible.
When should I prefer EIS over current interrupt?
Use EIS when the system has significant capacitance or when you need small-signal linear values across potentials. Use CI for steady-state currents during chrono methods.
Do I still need manual correction after enabling electronic compensation?
Yes. Apply residual correction using the uncompensated remainder to ensure transparent reporting and reproducibility.