7.3 Voltammetry and amperometry

Voltammetry and amperometry are powerful electroanalytical techniques that measure current in response to applied potential. These methods allow us to study redox reactions, determine analyte concentrations, and gain insights into reaction kinetics and mechanisms.

These techniques offer versatility and selectivity, making them valuable for analyzing various substances in different matrices. By interpreting voltammograms and amperograms, we can extract both qualitative and quantitative information about electroactive species in our samples.

Principles of Voltammetry and Amperometry

Electroanalytical Techniques and Redox Reactions

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• Voltammetry and amperometry measure the current response to an applied potential in an electrochemical cell
• The applied potential causes redox reactions at the electrode surface, transferring electrons between the electrode and analyte species in solution
• The current measured is proportional to the rate of the redox reaction and the concentration of the electroactive species
• The potential at which the redox reaction occurs is characteristic of the analyte, providing qualitative information about the species present

Quantitative Analysis and Governing Equations

• The magnitude of the current relates to the concentration of the analyte, allowing for quantitative analysis
• The Nernst equation and the Butler-Volmer equation describe the relationship between the applied potential and the resulting current
  • These equations consider factors such as the standard reduction potential, the number of electrons transferred, and the rate constants of the redox reaction
  • They help to understand the thermodynamics and kinetics of the redox processes occurring at the electrode surface

Voltammetric Techniques: A Comparison

Cyclic Voltammetry (CV)

• CV sweeps the potential linearly between two values at a constant rate and measures the resulting current, then reverses the potential and repeats the sweep in the opposite direction
• Provides information about the reversibility of the redox reaction, the presence of intermediates, and the stability of the analyte species
• The shape of the voltammogram, including peak potentials and currents, can be used to study the kinetics and mechanism of the redox reaction
• Useful for investigating the electrochemical behavior of new compounds or modified electrodes

Linear Sweep Voltammetry (LSV) and Square Wave Voltammetry (SWV)

• LSV is similar to CV but only sweeps the potential in one direction without reversing, making it useful for studying irreversible redox reactions or determining analyte concentration in a single sweep
• SWV applies a square wave potential superimposed on a staircase potential ramp, offering high sensitivity and fast scan rates
  • Suitable for trace analysis and studying rapid electrode processes
  • Measures current at the end of each square wave cycle, reducing the contribution of capacitive current and enhancing the faradaic current signal
  • Can be used for the detection of low concentrations of analytes in complex matrices (environmental samples, biological fluids)

Applications of Voltammetry and Amperometry

Studying Redox Reactions and Determining Analyte Concentrations

• Voltammetry and amperometry can study the thermodynamics and kinetics of redox reactions
  • Determine standard reduction potentials, electron transfer coefficients, and rate constants
  • Investigate the mechanism and rate-determining steps of redox processes
• The concentration of an electroactive analyte can be determined by measuring the peak current or limiting current in a voltammogram or amperogram, respectively
• Calibration curves can be constructed by plotting the peak current or limiting current against known concentrations of the analyte, allowing for the quantification of unknown samples

Versatility and Selectivity

• Voltammetric and amperometric techniques can be applied to a wide range of analytes (inorganic ions, organic compounds, biological molecules) in various sample matrices (environmental, industrial, clinical samples)
  • Examples: heavy metal ions (lead, cadmium) in water samples, ascorbic acid in fruit juices, neurotransmitters (dopamine, serotonin) in brain tissue
• The selectivity of the method can be enhanced by using modified electrodes or applying potential waveforms that exploit the unique redox properties of the analyte
  • Chemically modified electrodes (polymer coatings, nanoparticles) can improve selectivity and sensitivity
  • Pulsed voltammetric techniques (differential pulse voltammetry) can discriminate against interfering species and enhance the signal-to-noise ratio

Interpreting Voltammograms and Amperograms

Qualitative and Quantitative Information

• The peak potential (Ep) in a voltammogram provides qualitative information about the identity of the analyte, as it is characteristic of the specific redox reaction
• The peak current (Ip) in a voltammogram is proportional to the concentration of the analyte, allowing for quantitative analysis using calibration curves or standard addition methods
• In amperometry, the limiting current (Ilim) is proportional to the concentration of the analyte and is used for quantitative analysis

Additional Insights from Voltammograms and Amperograms

• The shape of the voltammogram, including the presence of additional peaks or waves, can indicate the occurrence of coupled chemical reactions, adsorption processes, or the formation of insoluble products
  • Example: the appearance of a pre-peak in the voltammogram of an organic compound may suggest the formation of a surface-adsorbed species prior to the main redox reaction
• The time dependence of the current in amperometry can provide information about the kinetics of the redox reaction and the mass transport processes occurring at the electrode surface
  • Example: a steady-state current in an amperogram indicates that the redox reaction is controlled by the diffusion of the analyte to the electrode surface
• The half-wave potential (E1/2) in a polarographic experiment relates to the standard reduction potential of the analyte and can be used for qualitative identification