6.1 Principles of Voltammetry and Polarography

Voltammetry and polarography measure current responses to applied potentials, revealing crucial info about analytes in solution. These techniques use a three-electrode setup: working, reference, and counter electrodes, each playing a unique role in the electrochemical cell.

Interpreting voltammograms and polarograms is key to understanding electrochemical processes. The shape, peak currents, and potentials provide valuable data for qualitative and quantitative analysis, helping identify analytes and determine their concentrations in solution.

Fundamentals of Voltammetry and Polarography

Principles of voltammetry and polarography

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• Voltammetry and polarography are electroanalytical techniques that study the electrochemical behavior of analytes in solution by measuring the current response as a function of applied potential
• Voltammetric cell consists of three electrodes immersed in an electrolyte solution containing the analyte
  • Working electrode: site of the electrochemical reaction of interest (glassy carbon, platinum, gold, or mercury)
  • Reference electrode: maintains a constant potential and serves as a reference point (Ag/AgCl or SCE)
  • Counter electrode: completes the electrical circuit and allows current to flow (platinum or graphite)
• Potential is varied in a controlled manner, either linearly (linear sweep voltammetry) or in a stepwise fashion (differential pulse voltammetry)
• Resulting current is measured and plotted against the applied potential to generate a voltammogram or polarogram, which provides information about the electrochemical processes occurring at the electrode surface

Electrodes in voltammetric cells

• Working electrode: electrode at which the electrochemical reaction of interest takes place
  • Material choice depends on the analyte and the potential range of interest
• Reference electrode: provides a stable and reproducible potential against which the potential of the working electrode is measured
  • Maintains a constant composition and potential throughout the experiment
• Counter electrode: facilitates the flow of current through the cell
  • Made of an inert material to balance the charge transfer at the working electrode and maintain electroneutrality in the solution

Interpreting Voltammetric and Polarographic Data

Potential-current relationship in voltammetry

• As the potential is scanned, the current response is recorded, generating a voltammogram
• Voltammogram shape depends on the electrochemical processes occurring at the electrode surface
  • Faradaic current: results from the transfer of electrons during the oxidation or reduction of the analyte
  • Capacitive current: arises from the charging of the electrical double layer at the electrode-solution interface
• Peak current $(i_p)$ in a voltammogram is proportional to the concentration of the analyte in solution, as described by the Randles-Sevcik equation: $i_p = (2.69 \times 10^5) n^{3/2} A D^{1/2} v^{1/2} C$
  • $n$: number of electrons transferred per molecule
  • $A$: electrode surface area
  • $D$: diffusion coefficient of the analyte
  • $v$: scan rate
  • $C$: analyte concentration
• Potential at which the peak current occurs (peak potential, $E_p$) is characteristic of the analyte and provides information about its redox properties

Interpretation of voltammograms and polarograms

• Voltammograms and polarograms can be used to identify and quantify analytes based on their characteristic peak potentials and currents
• Qualitative analysis: comparing the peak potentials of unknown analytes to those of known standards to identify the species present
• Quantitative analysis: using the peak current to determine the concentration of the analyte in solution
  1. Construct a calibration curve by plotting peak current vs. concentration for a series of standard solutions
  2. Measure the peak current of the unknown solution
  3. Determine the unknown concentration by comparing its peak current to the calibration curve
• Reversibility of the electrochemical reaction can be assessed by examining the separation between the anodic and cathodic peak potentials
  • Reversible systems: $\Delta E_p = E_{pa} - E_{pc} \approx 59/n$ mV at 25℃
  • Irreversible systems: larger peak separation and broader peaks
• Coupled chemical reactions, adsorption processes, or other complex mechanisms can be inferred from the shape and features of the voltammogram