Ohmic Drop (iR) Compensation Tutorial
A Practical Guide to iR Compensation
Testing Your System Instead of Trusting a Rule of Thumb
iR compensation is one of those topics where a simple guideline (“80–90% is safe”) can quietly harden into a rigid rule. That rule is often helpful—but it can also hide what your specific system is actually capable of.
This short tutorial is not about pushing everyone to 100% iR compensation. It’s about learning how to test your system, recognize artifacts, and make informed decisions grounded in measurement rather than inheritance.
If nothing else, I hope this helps you develop a more curious relationship with your potentiostat and cell.
1. Start by Clarifying What You Mean by “100% iR Compensation”
Before touching a knob or a setting, it helps to pause and ask:
What do I actually want “100%” to mean in this experiment?
There are at least two common (and often conflated) meanings:
- Low‑frequency / steady‑state correctness:
The potential at the electrode interface is correct once things have settled. - Perfect correction at all frequencies and time scales:
The electrode sees the exact applied waveform at every instant.
The second definition quietly assumes infinite bandwidth, which no real potentiostat–cell system has. Under that definition, 100% iR compensation is not physically or practically achievable.
The first definition is achievable in many systems—if bandwidth and stability are treated intentionally.
Much of the debate around iR compensation disappears once this distinction is made explicit.
Here is the question that you need to ask yourself:
What are the shortest sampling times that I need?
Can the measured signal well before that time while applying active iR compensation from my potentiostat?
2. Use a Dummy Cell Before Real Chemistry
A dummy cell is your safest sandbox.
Why this matters:
- It separates instrument behavior from electrochemical complexity
- It lets you explore stability limits without sacrificing samples or sanity
How to do it:
- Build a simple series R–C dummy cell with values comparable to your real system (Dummy cells are often provided with a potentiostat).
- Apply the same waveform (CV, step, pulse) you plan to use experimentally
- Gradually increase iR compensation while watching the response
What to watch for:
- Clean, well‑damped responses → good sign
- Overshoot, ringing, or sustained oscillation → you’re past a stability boundary
This exercise builds intuition fast and makes later artifacts much easier to recognize.
3. Measure Ru (Don’t Guess It)
iR compensation is only as good as your estimate of Ru.
Practical approaches:
- Electrochemical impedance spectroscopy (EIS)
Measure impedance in your solution with and/or without your analyte present.
Check to make sure your current ranges are set appropriately on the potentiostat during the measurement.
Fit a simple equivalent circuit and extract Ru and Cdl.
- High‑frequency impedance comparison
At sufficiently high frequency, the double‑layer impedance collapses and the measured impedance approaches Ru.
Re‑measure Ru if you change:
- Electrolyte composition or concentration
- Temperature
- Cell geometry or electrode placement
- Reference electrode configuration
Ru is not a universal constant—it’s a property of this experiment.
4. Diagnose Overcompensation in Real Voltammograms
Once you move to real electrochemistry, artifacts become more subtle—and more interesting.
Identify the highest scan rate or shortest sampling time that you plan to run. Then apply iR compensation while running at that scan rate or sampling time. Start with low iR compensation (70-80% of Ru) and gradually work your way up. You want to go just beyond stability without cooking your solution. Use your measurement of your estimated Ru to guide you. As you step up iR compensation watch for these signs:
Dramatic signs (easy to spot):
- Ringing or oscillations
- Back‑bending peaks
- Noise that grows rather than damps
Subtle signs (easy to miss):
- Peak separations smaller than theoretically expected for reversible systems
- Peaks that become unnaturally narrow
- Voltammograms that change shape as compensation is increased, even when chemistry shouldn’t
If your data look “too perfect,” that’s often a reason to pause, not celebrate.
5. Sweep Time Scale or Scan Rate Intentionally
A powerful diagnostic is to vary scan rate or time scale at a fixed compensation setting.
Ask:
- Does the voltammogram behave sensibly across decades of scan rate?
- Do artifacts emerge only at faster scans?
- Does stability depend on sampling speed?
If behavior degrades as you speed up, you’re likely running beyond the compensated bandwidth of your system.
6. Document the “Sweet Spot” for Your Setup
Once you’ve done this work, write it down.
For each important configuration, record:
- Ru and Cdl
- Compensation fraction
- Damping / filter settings
- Maximum scan rate or frequency with clean behavior
This turns iR compensation from a recurring mystery into institutional memory—for you, your students, and your future self.
A Final Perspective
iR compensation is a dynamic conversation between:
- Your potentiostat
- Your electrochemical cell
- Your experimental goals
- And your willingness to test assumptions rather than inherit them
Suggested References
- Britz, D. (1978). iR elimination in electrochemical cells. Journal of Electroanalytical Chemistry, 88, 309–352.
- Britz, D. (1980). 100% iR compensation by damped positive feedback. Electrochimica Acta, 25, 1449–1452.
- Wipf, D. O. (1996). Ohmic drop compensation in voltammetry: Iterative correction of the applied potential. Analytical Chemistry, 68, 1871–1876.