Ohmic iR drop

Significance of the Topic

Ohmic iR drop represents the voltage loss caused by the uncompensated resistance in an electrochemical cell. Precise iR compensation is essential for accurate voltammetric and impedance measurements, as uncorrected resistance leads to distorted peak shapes, shifted potentials, and misinterpretation of reaction kinetics. Employing robust measurement and compensation strategies enhances data reliability across applications in corrosion studies, sensor development, and fundamental redox investigations.

Objectives and Study Overview

This application note (AN-EC-036) concludes a three-part series on managing ohmic drop by introducing electrochemical impedance spectroscopy (EIS) as a tool to measure and compensate for uncompensated resistance (Ru). The note compares EIS with current interrupt and positive feedback, outlines manual and automatic compensation workflows using Metrohm Autolab instruments with FRA32M and VIONIC modules, and illustrates the impact of iR correction on cyclic voltammetry of a model redox system.

Instrumentation

• Metrohm Autolab potentiostat/galvanostat equipped with an FRA32M impedance module
• VIONIC potentiostat/galvanostat powered by INTELLO software
• Three-electrode cell: Pt disc working electrode (3 mm), Pt sheet counter electrode, Ag/AgCl reference electrode
• Electrolyte: 0.05 M K₄[Fe(CN)₆] in ultrapure water (no supporting electrolyte added to accentuate iR drop)

Methodology

EIS measurements were performed at open-circuit potential with a 5 mV sine amplitude over 100 kHz to 100 Hz. Ru was extracted from the high-frequency intercept on Nyquist plots or the plateau of Bode |Z| vs. log f. Data fitting in NOVA software provided an alternative Ru value. Manual compensation involves entering 80–90% of Ru into NOVA or INTELLO “Apply Settings.” The automatic “Measure iR Drop” command in INTELLO performs a single-frequency EIS (default 30 kHz) to determine and apply Ru within user-defined safety limits, preventing overcompensation and oscillations.

Main Results and Discussion

Manual EIS analysis yielded an uncompensated resistance of approximately 87 Ω. Automatic routines successfully measured Ru and applied a calculated compensation. Cyclic voltammetry of ferrocyanide in water demonstrated a dramatic improvement after iR correction: peak-to-peak separation decreased to values consistent with a reversible one-electron process, and peak currents increased, avoiding erroneous conclusions about electrode kinetics.

Benefits and Practical Applications

• Improved accuracy in voltammetric peak potentials and currents
• Enhanced reliability in kinetic parameter determination
• Prevention of misinterpretation in reversibility studies
• Streamlined workflow via automated compensation in INTELLO

Future Trends and Opportunities

As impedance technology advances, integrated hardware-software solutions will further simplify iR compensation, enabling real-time adaptive correction. Higher-frequency EIS and machine-learning algorithms for equivalent-circuit fitting may deliver even faster and more precise resistance measurements. Expanded use in microelectrode arrays and nonaqueous systems presents new application domains.

Conclusion

EIS provides the most accurate approach to quantify and correct ohmic iR drop. Metrohm’s AUTOLAB/FRA32M and VIONIC/INTELLO platforms support both manual and fully automated workflows, ensuring high fidelity in electrochemical data. Proper iR compensation prevents misinterpretation of electrode processes and is indispensable for high-precision research and quality control.

Reference

• Metrohm Application Note AN-EC-036: Ohmic iR Drop Part 3 – Measurement with EIS
• Metrohm Application Note AN-EC-003: Ohmic Drop Concepts
• Metrohm Application Note AN-EC-004: Current Interrupt and Positive Feedback Methods