Non-purged voltammetry explored with AGNES.

Area of Science:

• Electrochemistry
• Analytical Chemistry
• Physical Chemistry

Background:

• Electrode reactions, specifically oxygen reduction, elevate the pH near the electrode surface.
• This localized pH increase can alter chemical speciation and complicate electrochemical analysis.
• Existing models often overlook this significant surface pH phenomenon.

Purpose of the Study:

• To develop a theoretical model for predicting the steady-state pH profile around an electrode.
• To experimentally validate the model using the Absence of Gradients and Nernstian Equilibrium Stripping (AGNES) technique.
• To assess the impact of surface pH changes on the quantification of metal ions like Zn(2+) and Cd(2+).

Main Methods:

• Development of a theoretical model to estimate the pH profile from the electrode surface to the bulk solution.
• Implementation of the electroanalytical technique Absence of Gradients and Nernstian Equilibrium Stripping (AGNES) with Screen Printed Electrodes (SPE).
• Experimental estimation of surface pH by probing free metal ion concentrations at the electrode surface.

Main Results:

• The theoretical model predicts significant pH increases (up to 10.3) in non-deaerated solutions within a specific bulk pH range (4.0-10.0).
• The AGNES technique experimentally confirmed the theoretical model's predictions for surface pH changes.
• AGNES, with modifications, allows for more accurate quantification of bulk free metal concentrations (Zn(2+), Cd(2+)) in non-purged solutions.

Conclusions:

• Localized pH increases at electrode surfaces are a critical factor in electrochemical analyses.
• The developed theoretical model and AGNES technique provide reliable methods for assessing and accounting for surface pH effects.
• Improved accuracy in determining free metal ion concentrations in real-world (non-purged) solutions is achievable by considering these pH variations.