Adsorption of the analyte on the electrode can disrupt SWV calibration by altering the electrochemical response, making it challenging to establish a linear relationship between peak current and concentration. Irreversible adsorption can lead to non-reproducible signals, complicating calibration efforts even in diffusion-controlled processes.

This is a common issue when dealing with adsorption alongside diffusion-controlled processes, especially when transitioning from Cyclic Voltammetry (CV) to Square Wave Voltammetry (SWV). Here's a breakdown of why this might be happening and how to address it:

Understanding the Discrepancy:

• CV vs. SWV:
  • CV is a dynamic technique that probes the electrochemical behavior over a range of potentials, revealing information about the reversibility and kinetics of the reaction. It can be less sensitive to adsorption effects at higher concentrations where diffusion dominates.
  • SWV is a sensitive technique that minimizes capacitive current and enhances faradaic current, making it ideal for quantitative analysis. However, it's also more susceptible to adsorption effects, which can significantly distort peak currents and linearity.
• Adsorption Interference:
  • Adsorption of the analyte or its reaction products onto the electrode surface can:
    • Saturate the electrode surface, limiting the amount of analyte that can participate in the electrochemical reaction.
    • Alter the kinetics of the electron transfer, leading to non-linear relationships between peak current and concentration.
    • Cause peak broadening or shifts in peak potential.
  • Even though the overall process is diffusion-controlled, the adsorption process occurs first and can limit the amount of analyte that reaches the electrode surface.
• High vs. Low Concentration:
  • At high concentrations, diffusion can overwhelm adsorption effects in CV, leading to a reasonable calibration.
  • At lower concentrations, or in SWV, adsorption becomes more significant, disrupting the linear relationship between peak current and concentration.

Troubleshooting and Solutions:

By carefully considering these factors and implementing the appropriate solutions, you should be able to improve the linearity of your SWV calibration and obtain reliable quantitative results.

Monika Antil made and exhaustive explanation for you question. I am going to add some points based on the description made for your question.

• Cyclic Voltammetry is not an analytical tool: The peak current is due to a compention between mass transport and the kinetics of interfacial charge transfer. In addition, capacitive currents affect the faradaic current, specially at high scan rates.
• Square Wave Voltametry: The main difference with CV is the profile of the pulse. Hence, SWV is not the cause of the adsorption. The advantage of SWV is that the capacitive currents are eliminated, so the peaks are truly correlated to the concentration. It is a propper analytical tool.
• Adsoprtion: It may happen because your electrode interacts with the analyte or because at a certain potential either the reduced or oxydized form interacts with the electrode's surface. Hence, you may have charge transfer via adsorption (not only by mass transport) or blocking effect of your electrode, if the the specie is not electroactive.
• Cleaning the electrode: This is a critical part. Clean your electrode every time you perform an experiment.

More information can be found in the following links in which there are some interactive apps that illustrates the phenomena:

• Adsorption: https://electrochemeisbasics.blogspot.com/2025/04/understanding-surface-adsorption-in.html
• Cyclic Voltammetry: https://electrochemeisbasics.blogspot.com/2024/07/understanding-cyclic-voltammetry-ec.html
• Ohmic Drop: https://electrochemeisbasics.blogspot.com/2025/05/how-ohmic-resistance-shapes-your.html