1. Voltammetry
Nov 16, 2004
Lecture Date: April 10th, 2013

2. 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: a current proportional to analyte concentration is monitored at a fixed potential
In other words, voltammetry at a constant potential

3. 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
Currents in potentiometry are negligible. In voltammetry (and coulometry), currents must be taken into account.

4. Current in Electrochemical Cells
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
Currents in potentiometry are negligible. In voltammetry (and coulometry), currents must be taken into account.
Current in a voltammetric cell flows from the counter electrode to the working electrode. The reference electrode essentially draws no current.

5. Electrodes in cells are polarized over certain current/voltage ranges
Polarization
Electrodes in cells are polarized over certain current/voltage ranges
Electrodes are purposely kept small (mm2 to um2) in voltammetry to promote polarization
“Ideal” polarized electrode: current does not vary with potential
Currents in potentiometry are negligible. In voltammetry (and coulometry), currents must be taken into account.

6. 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:
Currents in potentiometry are negligible. In voltammetry (and coulometry), currents must be taken into account.
IR Drop
(needed when current
is significant)
Electrodes

7. Overvoltage and Polarization Sources
Overvoltage (overpotential) the difference between the equilibrium potential and the actual potential; it develops because of polarization
Net result is you must means must apply greater potential before redox chemistry occurs
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)
Currents in potentiometry are negligible. In voltammetry (and coulometry), currents must be taken into account.
Overvoltage reduces theoretical cell potential when current is flowing.

8. 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)

9. Instrumentation for Voltammetry
Block diagram of a typical 3-electrode voltammeter:
Counter electrode
← e-
Waveform generator
Potentiostat
e- →
Working electrode
Eapplied
Reference electrode
(i = 0)
Computer
Current-to-voltage converter
Note the counter electrode is sometimes called an auxiliary electrode.
Cell
See Fig in Stroebel and Heineman, Chemical Instrumentation, A Systematic Approach 3rd Ed. Wiley 1989.

10. 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
Basic voltammetry system suitable for undergraduate laboratory work
From

11. 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

12. 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 E1/2 is often used for qualitative analysis
n can also be fitted
The limiting current is proportional to analyte concentration and is used for quantitative analysis
Nernst Plot
A + n e-  P
Half-wave potential
E1/2
Limiting current
Remember, E is scanned linearly to higher values as a function of time in linear sweep voltammetry

13. 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 processes.
Most importantly, migration rate becomes independent of applied potential

14. 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
The E1/2 is pH sensitive in this case
Anodic wave
Diagram from Stroebel and Heineman, Chemical Instrumentation, A Systematic Approach 3rd Ed. Wiley 1989.

15. 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.

16. 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

17. The Clark Voltammetric Oxygen Sensor
At the platinum cathode:
O2 + 2H2O + 4e OH-
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
A drawback to the Clark sensor is that oxygen is consumed during the measurement with a rate equal to the diffusion in the sensor. To avoid this, the system must be stirred to get an accurate measurement and avoid stagnant pools of electrolyte. The oxygen consumption increases and so does the stirring sensitivity with increasing sensor size. Clark oxygen sensors can be made very small (e.g. a few µm). The oxygen consumption of a microsensor is small, and its measurements are not affected by stagnant media (so stirring is not needed).
analyte solution
electrolyte
O2 permeable membrane
(O2 crosses via diffusion)
platinum electrode
(-0.6 volts)

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

19. 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

20. 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

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

22. CV for a simple system: hexacyanoferrate(III) and (II) ions
Cyclic Voltammetry
CV for a simple system: hexacyanoferrate(III) and (II) ions
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
At point A, the potential is +0.8 V and a tiny anodic current is briefly observed (from the oxidation of water to give oxygen). From then onto point B (from 0.7 V to 0.4 V), no current is observed as there is no reduction or oxidation occurring.
At point B (about 0.4 V), a cathodic current begins to grow as hexacyanoferrate(III) ion is reduced to hexacyanoferrate(II) ion. At the cathode we have: Fe(CN)63- + e- <-> Fe(CN)64-
Between points B and D the surface concentration of Fe(CN)63- becomes smaller. At the peak cathodic current ipc, the current is made up of the initial surge required to adjust the surface concentration of the reactant to its equilibrium concentration (obtained from the Nernst equation) plus the diffusion-controlled component. The current reaches the peak cathodic potential Epc at the peak cathodic current ipc, then (point E) decays rapidly to point F as the diffusion layer extends further from the electrode.
At point F, the scan direction is changed (see the CV waveform). As the potential sweeps in the positive direction, the current is still cathodic for a while but then eventually reduction of Fe(CN)63- ceases. The current goes to zero and then becomes anodic between H and I. The anodic current occurs because Fe(CN)64- is reoxidizing near to the electrode, where it accumulated during the forward scan.
The anodic current peaks at pointJ at current ipa and potential Epa, and then decreases (point K) as the accumulated Fe(CN)64- is used up by the anodic reaction.
P. T. Kissinger and W. H. Heineman, J. Chem. Ed. 1983, 60, 702.

23. Electrochemical Stripping Voltammetry
A two step process:
(1) The analyte is deposited (accumulated) on the working electrode from solution.
(2) The analyte is then stripped off of the electrode with observation of current by a voltammetric method.
The aim is to concentrate the analyte to obtain lower LOD and LOQ.
Anodic stripping: the working electrode behaves as a cathode during the deposition step, then behaves as an anode during the stripping step.
Cathodic stripping (less common) is the opposite process.
Although a voltammetric method is usually used, other electrochemical methods can be combined with stripping.
See pages 748 of the text for more about electrochemical stripping techniques.

24. Electrochemical Stripping Voltammetry
Cd => Cd2+ + 2e-
The currents observed for Cd and Cu are proportional to the concentration of each metal in solution.
Cu => Cu2+ + 2e-
The currents observed for Cd and Cu are proportional to the concentration of each metal in solution.
See pages 748 of the text for a similar figure.

25. Electrochemical Stripping Voltammetry: Elemental Analysis
Elemental detection using a bismuth-modified carbon paste electrode
Three toxic elements (Cd, Pb, Tl) are easily detected at 200 ppb in this example.
Svancara, et al., Electroanalysis 18, 2006,

26. Electrochemical Stripping Voltammetry: Molecular Analysis
An early example of stripping voltametry (polarography) using a hanging mercury drop electrode on the drug diazepam:
R. Kaldova, Analytica Chimica Acta, 162 (1984) 197—205.

27. Electrochemical Stripping Voltammetry: Molecular Analysis
Detection of the insecticide methyl parathion using stripping square-wave voltammetry with an electrode made from tetrasulfonated phtalocyanine (p-NiTSPc) electrodeposited on a carbon surface with a Nafion® sulfonated tetrafluoroethylene copolymer coating
irreversible reduction (a, Epa V)
reversible reduction-oxidation (b, Epa =
-0.08 V, c, Epc = 0.0 V)
(c)
Note the y-axis of the square-wave stripping voltammogram from the actual method (right), which is 10x that of the “exploratory” CV (left).
Note that the CV shows two redox peaks (Epa = 0.08V and Epc 0.0 V) and on irreversible reduction peak (Epc =−0.61 V) in the potential range from −1.0 to 1.0V.
(b)
(a)
M. Sbai, et al. Sensors and Actuators B 124 (2007) 368–375.

28. CV and 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,

29. More Spectroelectrochemistry
A typical system (Gamry Interface 1000 and Agilent/Varian Cary 50 UV-Vis)

30. SECM and SECM-AFM
Scanning electrochemical microscopy (SECM) uses nanometer sized tips (electrodes) to probe surface phenomena
Analyses are run in constant height mode or constant current mode
Can be combined with AFM
The figures compare steady-state voltammograms of 1 mM ferrocenemethanol and 0.2 M NaCl obtained using a bulk system and using a SECM with a 36 nm polished Pt tip
Bulk
SECM
SECM systems are commercially available – see Ametek, PAR, etc…
In SECM, the tip is the electrode. Note that SECM-AFM is a distinct technique from SECM that combines the two methods.
Sun and Mirkin, Anal. Chem. 2006, 78,

31. SECM: Applications to Metal Corrosion
SECM can be used to identify precursor sites for corrosion in passive oxide films that protect metals
The metal substrate is biased with a voltage and the SECM tip detects the product of a reaction, providing an image of the reactive site.
Allows imaging of surface reactivity
Basame and White, Langmuir 1999, 15,

32. SECM Instrumentation
Princeton Applied Research/Ametek VersaSCAN:

33. Reading Material Skoog, Holler and Crouch: Ch. 25 Cazes: Chapter 17
Optional 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,
A. J. Bard and L. R. Faulkner, “Electrochemical Methods”, 2nd Ed., Wiley, 2001.