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