Ohmic Drop Part 2 – Measurement

Importance of the Topic

Electrochemical measurements are highly sensitive to the potential drop caused by solution resistance between the working and reference electrodes. This uncompensated resistance, or ohmic drop, can lead to inaccurate control of electrode potential, compromising data quality in kinetic studies, mechanistic investigations and quality control applications. Accurate determination and compensation of the ohmic drop is essential to ensure reliable electrochemical results.

Objectives and Overview of the Application Note

This application note evaluates three experimental approaches to measure and compensate the ohmic drop in a three-electrode cell: current interrupt, positive feedback and electrochemical impedance spectroscopy (EIS). It compares their precision, speed and practical considerations when applied to a dummy cell representing typical electrochemical circuits.

Methodology and Used Instrumentation

The experiments employ a rotating disc electrode or planar and spherical model electrodes in a three-electrode configuration. The equivalent circuit for the dummy cell is defined by solution resistance (R), double-layer capacitance (C) and additional resistive elements. Key instrumentation includes:

• Autolab PGSTAT302N potentiostat/galvanostat
• ADC10M high-speed sampling module for current interrupt measurements
• FRA32M frequency response analyzer for EIS
• Nova software for setup, control and data analysis

Main Results and Discussion

1. Current Interrupt: Interrupting the current and comparing potential just before and after the break yields the ohmic drop (ΔE). High-speed sampling via ADC10M improves accuracy by capturing rapid potential changes. Without ADC10M, fewer data points reduce precision.

2. Positive Feedback: A trial-and-error approach applies a feedback voltage proportional to measured current. Adjusting the feedback resistance to 80–90% of the true solution resistance yields damped oscillations without destabilizing the control loop. Overcompensation risks oscillation and may trigger side reactions.

3. Electrochemical Impedance Spectroscopy: Nyquist plot analysis at high frequency intercept provides an accurate measurement of solution resistance. Fitting to the equivalent circuit delivers low fitting errors and precise resistance values. EIS is reliable and non-perturbing but requires the FRA32M module.

Benefits and Practical Applications of the Method

• Real-time compensation during experiments: The measured resistance can be entered in the potentiostat control settings to apply iR compensation on the fly.
• Post-processing correction: Measured current can be multiplied by resistance and subtracted from recorded potential to correct existing data.
• Improved data fidelity: Reducing potential errors enhances the accuracy of kinetic constants, redox potential determinations and analytical quantification.

Future Trends and Possibilities for Application

Advances in high-speed electronics and software algorithms will enable automated, real-time measurement and compensation of ohmic drop in more complex geometries and flow systems. Integration of EIS with rapid scanning techniques and adaptation to microelectrode arrays and battery cells will further broaden the applicability of precise ohmic drop management in academic research and industrial quality control.

Conclusion

This work highlights three complementary approaches for measuring and compensating solution resistance in electrochemical cells. Current interrupt and positive feedback methods offer rapid results with appropriate care, while EIS provides the most reliable resistance values. Both real-time and post-processing corrections improve the accuracy of electrochemical studies.

Reference

• Metrohm Application Note AN-EC-004, March 2019