Voltammetry: An Overview of Electrochemical Techniques
Voltammetry
Voltammetry encompasses a group of electroanalytical methods that provide information about an analyte by measuring current as a function of applied potential. This process promotes polarization of a working electrode.
- Amperometry measures current at a fixed potential, which is proportional to analyte concentration.
- Polarography, a foundational field in voltammetry, was historically used to determine inorganic ions and certain organic species, though many of these applications have been superseded by spectroscopic methods. However, it still finds use in specific applications like determining molecular oxygen in solutions.
Various Voltammetric Techniques
- Voltammetry: Measuring current as potential varies.
- Linear Scan:
- Hydrodynamic Voltammetry
- Polarography
- Differential Pulse:
- Differential-Pulse Voltammetry
- Square Wave:
- Square-Wave Voltammetry
- Triangular:
- Cyclic Voltammetry
Electrochemical Setup
- Solvent
- Supporting Electrolyte: Increases solution conductivity and limits analyte migration.
- Three-Electrode System
Voltammograms
- Cathodic currents are positive, while anodic currents are negative.
- Linear-sweep voltammograms typically have a sigmoid shape, known as voltammetric waves.
- The constant current after the steep rise is the limiting current (i1), limited by the rate at which reactants reach the electrode surface via mass transport.
- Limiting currents are generally directly proportional to reactant concentration.
- The half-wave potential (E1/2) is the potential at which the current is half the limiting current.
Cyclic Voltammetry (CV)
In cyclic voltammetry (CV), the current response of a stationary electrode in an unstirred solution is measured using a triangular voltage waveform.
- Switching potentials are the voltage extrema where the scan direction reverses.
- A forward scan moves towards more negative potentials, while a reverse scan moves towards more positive potentials.
- The Randles-Sevcik equation provides quantitative information (at 25°C):
- ip = peak current (A), A = electrode area (cm2), D = diffusion coefficient (cm2/s), c = concentration (mol/cm3), and ν = scan rate (V/s).
- CV can determine diffusion coefficients if concentration, electrode area, and scan rate are known.
- CV is primarily used for fundamental and diagnostic studies, providing qualitative information about electrochemical processes.
- CV is widely used in organic and inorganic chemistry to study modified electrodes. Platinum, mercury film (for negative potentials), glassy carbon, carbon paste, graphite, gold, diamond, and carbon nanotubes are common electrode materials.
Three-Electrode System
- The system consists of three electrodes immersed in a solution containing the analyte and a supporting electrolyte.
- Working Electrode (WE): Small dimensions to enhance polarization. Materials include platinum, gold, carbon, and mercury. Sizes range from conventional (>mm) to microelectrodes (>25 μm), ultramicroelectrodes (0.1-25 mm), and nanoelectrodes (nm). Microelectrodes minimize solution resistance and offer fast response times.
- Reference Electrode (RE): Maintains a constant potential (e.g., saturated calomel or silver-silver chloride electrode).
- Counter Electrode (CE): Conducts electricity from the source to the WE (e.g., platinum coil with a large contact area).
- The three-electrode system is necessary because a reference electrode cannot maintain a constant potential under high current flow.
Data Acquisition Steps
- Record a background scan.
- Maintain an inert atmosphere.
- Measure the open circuit potential.
- Minimize ohmic drop by:
- Reducing WE size
- Increasing conductivity
- Decreasing the distance between WE and RE
Factors Influencing Electrode Reaction Rate
- Electrode reaction rates are influenced by:
- Mass transfer (e.g., from bulk solution to electrode surface)
- Electron transfer at the electrode surface
- Chemical reactions before or after electron transfer
- Other surface reactions
Multi-Electron Transfer CV
- CV can analyze two reversible electron transfer reactions with high heterogeneous electron transfer rates.
- As the separation between the two reduction potentials (ΔE1/2) decreases, the peaks merge into a single two-electron peak.
Electron Transfer Kinetics
- In reversible systems, the peak potential is constant and independent of the sweep rate.
- In quasi-reversible or irreversible electron transfer reactions (slower rate constants), the peak potential (Ep) shifts depending on the reduction rate constant and the voltage scan rate.
- Slow electrode kinetics require more negative applied potentials for appreciable current flow.
Electrode Reactions with Coupled Homogeneous Chemical Reactions (EC)
- E: Electron transfer at the electrode surface
- C: Homogeneous chemical reaction
- r: Reversible
- i: Irreversible
- Examples:
- ErCi mechanism
- ErCi‘ mechanism
ErCi Mechanism
- Reversible electron transfer followed by an irreversible chemical reaction.
ErCi‘ Mechanism (Catalytic Regeneration)
- Reversible electron transfer followed by an irreversible chemical reaction that regenerates the starting material.
- Assumes species Z is present in large concentration.
- Ox regeneration is controlled by the rate constant k.
- If k is small relative to the scan rate (ν), regeneration is not obvious (diffusion control).
- As k increases, more Ox is catalytically regenerated, transitioning the voltammogram shape between diffusion control and steady-state behavior.