Voltammetry: A Look at Theory and Application
Bobby Diltz
14 March 2005
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
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)
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
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)
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
Working Electrode (cont)• Rotating Disk Electrode (RDE)
– Ultra Trace Graphite
– Gold
– Glassy Carbon*
• Many other types of WE
Auxiliary Electrode
• Completes the circuit between the potentiostat and the WE
• Two different types available– Platinum– Glassy Carbon
Reference Electrode
• Provides a reference potential to the WE/AE circuit
• Two types of RE– Ag/AgCl in KCl– Hg/HgCl in saturated KCl
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
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
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
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
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
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