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 mol/L –Low separation ability –Wave form of polarogram Differential Pulse –Sensitivity of 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: Cr 6+  Cr 3+ 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