Voltammetry: A Look at Theory and Application

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Voltammetry A Look at Theory and Application

• Bobby Diltz
• 14 March 2005

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Analytical Methods

• Chromatography
• Large amounts of materials used
• Not very sensitive
• Spectroscopy
• Very sensitive
• Able to separate species
• Voltammetry
• Extremely sensitive
• Few components used
• Wide range of concentrations

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What is Voltammetry?

• As an applied potential is changed over time a
  current is measured
• Reduces ions in the electrode
• Commonly uses three electrodes
• Working Electrode (WE)
• Auxiliary Electrode (AE)
• Reference Electrode (RE)

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Applications

• Ultra Trace range metals (sub ppb)
• Wastewater Analysis
• Industrial Water/Liquor Analysis
• Polarographic Determination of Sulfur Compounds
  in Pulping Liquors Dr. J.J. Renard
• Sulfur compounds in weapons
• Pharmaceutics
• Environmental Studies
• Biological/Biochemical Analysis
• Plating Analysis

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Techniques

• Polarography
• Uses direct reduction at the electrode
• This technique has limited sensitivity
• Uses the mercury drop electrode (SDME, DME)
• Voltammetry (Anodic Stripping/Adsorptive
  Cathodic Stripping)
• Electrochemical deposition
• Determination by stripping
• Very high sensitivity, down to the parts per
  trillion range
• Uses a stationary electrode (HDME, RDE)

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Working Electrode

• The working electrode is used to show the
  response of the analyte to the potential
• Mercury Electrode
• Hanging Drop Mercury Electrode (HDME)
• Used in the ppb to low ppm range
• Static Drop Mercury Electrode (SDME)
• Used in the low ppm range
• Dropping Mercury Electrode (DME)
• Used in the ppm range

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Working Electrode (cont)

• Rotating Disk Electrode (RDE)
• Ultra Trace Graphite
• Gold
• Glassy Carbon
• Many other types of WE

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Auxiliary Electrode

• Completes the circuit between the potentiostat
  and the WE
• Two different types available
• Platinum
• Glassy Carbon

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Reference Electrode

• Provides a reference potential to the WE/AE
  circuit
• Two types of RE
• Ag/AgCl in KCl
• Hg/HgCl in saturated KCl

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Electrolyte

• One of the most difficult parts of method
  development with voltammetry!!
• Electrolyte selection is greatly based on type of
  matrix
• Electrolytes reduce voltage drop and promote a
  better flow of electrons through the solution,
  while also stabilizing half-wave potentials of
  analytes
• Adjusts the pH
• Increases selectivity

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Measurement Modes

• Direct Current
• Sensitivity of 10-5 mol/L
• Low separation ability
• Wave form of polarogram
• Differential Pulse
• Sensitivity of 10-8 mol/L
• High separation
• Peak form of polarogram

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More Measurement Modes

• Alternating Current
• Only for reversible reactions
• Square Wave
• Better for reversible reactions
• Fast determinations
• Cyclic Voltammetry
• Useful in kinetic studies/qualitative analysis
• Uses both oxidation and reduction steps

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Why Use Voltammetry?

• Handles high salt concentrations better than
  chromatographic instrumentation
• Can differentiate between ionic species
• Example Cr6 ? Cr3
• Extremely low detection limits
• Can detect a wide range of species

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Summary

• Voltammetry is a very useful form of instrumental
  analysis
• Wide range of applications
• Assortment of electrodes allows for a very
  specific analysis of different chemicals
• Capable of very low to moderate detection limits-
  parts per trillion to low ppm range

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References

• Chloride by Cathodic Stripping Voltammetry
  Princeton Applied Research- Analytical
  Instrument Division
• Voltammetric Determination of Zinc, Cadmium,
  Lead, Copper, Thallium, Nickel, and Cobalt in
  Water Samples Application note No. 231/2 E