Transcript Voltammetry: A Look at Theory and Use

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
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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