Ohmic Drop Part 1 – Basic Principles

Significance of the topic

The accurate control and measurement of potential in electrochemical experiments is essential to obtain reliable data. The potential error from uncompensated resistance or ohmic drop can distort voltammetric curves, complicate mechanistic interpretations, and compromise electroanalysis in low-conductivity media, fast scans, or high-current processes.

Study objectives and overview

In this application note, fundamentals of ohmic drop in a typical three-electrode setup are discussed. A sense lead monitors the working electrode (WE) potential relative to a reference electrode (RE). The note aims to quantify how solution resistance, electrode placement, and current affect measured potentials and describe strategies to mitigate errors.

Methodology and instrumentation

The analysis is based on Ohm’s law: ΔE = iR, and the measured electrode potential Eapplied = Ewe - iR. It explores the electrochemical cell geometry and equipotential distribution using different RE positions (RE1, RE2, RE3). The transient response of the double layer in fast scans is modeled by E = Eapplied(1 - exp(-t/RC)).

• Electrodes: Working electrode (WE), counter electrode (CE), reference electrode (RE) with a Luggin capillary.
• Instrumentation: Potentiostat with sense lead, low-polarizable RE, optional Luggin-Haber capillary setup.
• Software: NOVA for ohmic drop measurement and compensation.

Main results and discussion

• Ohmic drop increases with solution resistivity, current magnitude, and RE distance from WE.
• Small WE areas reduce capacitance C, minimizing transient errors in fast scans.
• Placing RE close via Luggin capillary reduces R but may cause shielding errors if too close (<2 x capillary diameter).
• Industrial electrosynthesis often uses galvanostatic control to avoid iR compensation issues.
• In low-conductivity media (organic electrolytes, concrete), even microamp currents produce volt-level errors.

Benefits and practical applications

Accurate quantification of ohmic drop enables:

• Improved potential control in high-rate techniques (fast CV).
• Enhanced electrolysis and electrosynthesis reproducibility.
• Reliable data in low-conductivity matrices and nonaqueous media.
• Better sensor calibration and mechanistic studies.

Future trends and opportunities

• Development of non-invasive micro-reference electrodes to further reduce R without shielding.
• Advanced software algorithms for real-time iR compensation.
• Integration of impedance measurement to dynamically adjust potentiostat settings.
• Miniaturized cell designs for high-throughput screening with reduced RC time constants.
• Application to emerging fields: concrete corrosion monitoring, biological electrochemistry.

Conclusion

Managing ohmic drop is critical for accurate electrochemical measurements. By optimizing electrolyte conductivity, electrode geometry, and reference placement, errors can be minimized. Combined hardware and software strategies ensure that applied potentials at the electrode surface match nominal values, enhancing data quality across research and industrial applications.

Reference

• Metrohm Application Note AN-EC-003, Ohmic Drop Part 1 – Basic Principles, March 2019.