Understanding Voltammetry: Techniques and Polarization Effects

1. Voltammetry Nov 16, 2004 Lecture Date: April 28th, 2008

2. Reading Material • Skoog, Holler and Crouch: Ch. 25 • Cazes: Chapter 17 • For those using electroanalytical chemistry in their work, see: A. J. Bard and L. R. Faulkner, “Electrochemical Methods”, 2nd Ed., Wiley, 2001.

3. Voltammetry • Voltammetry techniques measure current as a function of applied potential under conditions that promote polarization of a working electrode • Polarography: Invented by J. Heyrovsky (Nobel Prize 1959). Differs from voltammetry in that it employs a dropping mercury electrode (DME) to continuously renew the electrode surface. • Amperometry: current proportional to analyte concentration is monitored at a fixed potential

4. Polarization • Some electrochemical cells have significant currents. • Electricity within a cell is carried by ion motion • When small currents are involved, E = IR holds • R depends on the nature of the solution (next slide) • When current in a cell is large, the actual potential usually differs from that calculated at equilibrium using the Nernst equation • This difference arises from polarization effects • The difference usually reduces the voltage of a galvanic cell or increases the voltage consumed by an electrolytic cell

5. Ohmic Potential and the IR Drop • To create current in a cell, a driving voltage is needed to overcome the resistance of ions to move towards the anode and cathode • This force follows Ohm’s law, and is governed by the resistance of the cell: IR Drop Electrodes

6. More on Polarization • Electrodes in cells are polarized over certain current/voltage ranges • “Ideal” polarized electrode: current does not vary with potential

7. Overvoltage and Polarization Sources • Overvoltage: the difference between the equilibrium potential and the actual potential • Sources of polarization in cells: • Concentration polarization: rate of transport to electrode is insufficient to maintain current • Charge-transfer (kinetic) polarization: magnitude of current is limited by the rate of the electrode reaction(s) (the rate of electron transfer between the reactants and the electrodes) • Other effects (e.g. adsorption/desorption)

8. DC Polarography • The first voltammetric technique (first instrument built in 1925) • DCP measures current flowing through the dropping mercury electrode (DME) as a function of applied potential • Under the influence of gravity (or other forces), mercury drops grow from the end of a fine glass capillary until they detach • If an electroactive species is capable of undergoing a redox process at the DME, then an S-shaped current-potential trace (a polarographic wave) is usually observed www.drhuang.com/.../polar.doc_files/image008.gif

9. Voltage-Time Signals in Voltammetry • A variable potential excitation signal is applied to the working electrode • Different voltammetric techniques use different waveforms • Many other waveforms are available (even FT techniques are in use)

10. Linear Sweep Voltammetry • Linear sweep voltammetry (LSV) is performed by applying a linear potential ramp in the same manner as DCP. • However, with LSV the potential scan rate is usually much faster than with DCP. • When the reduction potential of the analyte is approached, the current begins to flow. • The current increases in response to the increasing potential. • However, as the reduction proceeds, a diffusion layer is formed and the rate of the electrode reduction becomes diffusion limited. At this point the current slowly declines. • The result is the asymmetric peak-shaped I-E curve

11. The Linear Sweep Voltammogram • A linear sweep voltammogram for the following reduction of “A” into a product “P” is shown A + n e-  P • The half-wave potential E1/2is often used for qualitative analysis • The limiting current is proportional to analyte concentration and is used for quantitative analysis A + n e- P Half-wave potential Limiting current Remember, E is scanned linearly to higher values as a function of time in linear sweep voltammetry

12. Hydrodynamic Voltammetry • Hydrodynamic voltammetry is performed with rapid stirring in a cell • Electrogenerated species are rapidly swept away by the flow • Reactants are carried to electrodes by migration in a field, convection, and diffusion. Mixing takes over and dominates all of these • Most importantly, migration rate becomes independent of applied potential

13. Hydrodynamic Voltammograms • Example: the hydrodynamic voltammogram of quinone-hydroquinone • Different waves are obtained depending on the starting sample • Both reduction and oxidation waves are seen in a mixture Cathodic wave Anodic wave Diagram from Stroebel and Heineman, Chemical Instrumentation, A Systematic Approach 3rd Ed. Wiley 1989.

14. Oxygen Waves in Hydrodynamic Voltammetry • Oxygen waves occur in many voltammetric experiments • Here, waves from two electrolytes (no sample!) are shown before and after sparging/degassing • Heavily used for analysis of O2 in many types of sample • In some cases, the electrode can be dipped in the sample • In others, a membrane is needed to protect the electrode (Clark sensor) Diagram from Stroebel and Heineman, Chemical Instrumentation, A Systematic Approach 3rd Ed. Wiley 1989.

15. The Clark Voltammetric Oxygen Sensor • Named after its generally recognized inventor (Leyland Clark, 1956), originally known as the "Oxygen Membrane Polarographic Detector“ • It remains one of the most commonly used devices for measuring oxygen in the gas phase or, more commonly, dissolved in solution • The Clark oxygen sensor finds applications in wide areas: • Environmental Studies • Sewage Treatment • Fermentation Process • Medicine

16. The Clark Voltammetric Oxygen Sensor At the platinum cathode: O2 + 2H2O + 4e- 4OH- At the Ag/AgCl anode: O2 Ag + Cl- AgCl + e- O2 dissolved O2 id = 4 F Pm A P(O2)/b id - measured current F - Faraday's constant Pm - permeability of O2 A - electrode area P(O2) - oxygen concentration b - thickness of the membrane O2 analyte solution electrolyte platinum electrode O2 permeable membrane (O2 crosses via diffusion) (-0.6 volts)

17. The Clark Voltammetric Oxygen Sensor • General design and modern miniaturized versions:

18. Hydrodynamic Voltammetry as an LC Detector • One form of electrochemical LC detector: Classes of Chemicals Suitable for Electrochemical Detection: Phenols, Aromatic Amines, Biogenic Amines, Polyamines, Sulfhydryls, Disulfides, Peroxides, Aromatic Nitro Compounds, Aliphatic Nitro Compounds, Thioureas, Amino Acids, Sugars, Carbohydrates, Polyalcohols, Phenothiazines, Oxidase Enzyme Substrates, Sulfites

19. Cyclic Voltammetry • Cyclic voltammetry (CV) is similar to linear sweep voltammetry except that the potential scans run from the starting potential to the end potential, then reverse from the end potential back to the starting potential • CV is one of the most widely used electroanalytical methods because of its ability to study and characterize redox systems from macroscopic scales down to nanoelectrodes

20. Cyclic Voltammetry • The waveform, and the resulting I-E curve: • The I-E curve encodes a large amount of information (see next slide)

21. Cyclic Voltammetry • A typical CV for a simple electrochemical system • CV can rapidly generate a new oxidation state on a forward scan and determine its fate on the reverse scan • Advantages of CV • Controlled rates • Can determine mechanisms and kinetics of redox reactions P. T. Kissinger and W. H. Heineman, J. Chem. Ed. 1983, 60, 702.

22. Spectroelectrochemistry (SEC) • CV and spectroscopy can be combined by using optically-transparent electrodes • This allows for analysis of the mechanisms involved in complex electrochemical reactions • Example: ferrocene oxidized to ferricinium on a forward CV sweep (ferricincium shows UV peaks at 252 and 285 nm), reduced back to ferrocene (fully reversible) Y. Dai, G. M. Swain, M. D. Porter, J. Zak, “New horizons in spectroelectrochemical measurements: Optically transparent carbon electrodes,” Anal. Chem., 2008, 80, 14-27.

23. Instrumentation for Voltammetry • Sweep generators, potentiostats, cells, and data acquistion/computers make up most systems Cyclic voltammetry cell with a hanging mercury drop electrode From www.indiana.edu/~echem/cells.html Basic voltammetry system suitable for undergraduate laboratory work From www.edaq.com/er461.html

24. Homework Problems and Further Reading • Optional Homework Problems: • 25-1, 25-2, 25-5 • Further Reading: • C. Amatore and E. Maisonhaute, “When voltammetry reaches nanoseconds”, Anal. Chem., 2005, 303A-311A. • Y. Dai, G. M. Swain, M. D. Porter, J. Zak, “New horizons in spectroelectrochemical measurements: Optically transparent carbon electrodes,” Anal. Chem., 2008, 80, 14-27.