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Like most of the other electrochemical techniques offered by the AfterMath software, this experiment begins with an induction period, also called a cleaning period for this technique. During this time, a set of initial conditions is applied to the electrochemical cell and the cell is allowed to equilibrate to these conditions. The default initial condition involves holding the working electrode potential at a cleaning potential for a specified period of time.

After the induction period, the potential of the working electrode is stepped to a deposition potential for another specified period of time. Solutions are typically stirred for an HMDE, or a rotating disk electrode (glassy carbon or wax-impregnated graphite) is employed for MFE in order to maintain constant conditions for the deposition process. After the deposition period, stirring is stopped in the case of a HMDE. Next, the potential of the working electrode is linearly swept, or stepped through a series of pulses, from the Initial potential to the Final potential.

After the sequence has finished, the experiment concludes with a relaxation period. The default condition during the relaxation period involves holding the working electrode potential at the final potential for an additional brief period of time (i.e., 1 seconds).

At the end of the relaxation period, the post experiment conditions are applied to the cell and the instrument returns to the idle state.

Current is plotted as a function of the potential applied to the working electrode, resulting in a voltammogram.

The three variants mentioned above, Anodic Stripping Voltammetry (ASV), Differential Pulse Stripping Voltammetry (DPSV), and Square Wave Stripping Voltammetry (SWSV), are three separate experiment types in AfterMath but will all be covered in this wiki. Another variant, called Cathodic Stripping Voltmmetry (CSV) can be employed by using an anodic deposition potential, and is useful in examination of halides. CSV will not be covered in the parameter setup area.

The parameters for these methods are arranged on various tabs on the setup panel. The most commonly used parameters are on the Basic tab, and less commonly used parameters are on the Advanced tab. Each of these two tabs will be discussed separately for ASV, DPSV, and SWSV. Additional tabs for Ranges and Post experiment conditions are common to all of the electrochemical techniques supported by the AfterMath software and will be discussed after the Basic and Advanced tabs sections.

Below are the typical results for the determination of in solution ( in with GC WE, deposition = @ ). Note for quantitation purposes, it is best to build a calibration curve.

Figure 7: ASV of a Lead(II) solution

Figure 8 shows the typical results for the detection of several metal species, with concentrations ranging from to , in solution (experimental concentrations and conditions are as listed: , , and in with , GC WE, deposition = @ .

Figure 8: ASV of a Solution of Mixed Metal Species

The following is a brief introduction to to the theory Stripping Voltammetry using a Mercury Film Electrode (MFE). Stripping Voltammetry was traditionally done using an HMDE, which will not be discussed in detail here. For more information, please consult the literature.² ASV will be discussed first, followed by DPSV, SWSV, and finally CSV. All of the techniques discussed here consist of two steps – 1. Electrodeposition and 2. Stripping. Electrodeposition for the three anodic techniques (ASV, DPSV, and SWSV) is the same and will be discussed first, followed by the stripping analysis for each one individually. CSV will be discussed lastly and independently of the other techniques.

The first application is chosen to show the effect of the substrate on the determination of ions in solution. Florence⁹ compared the stripping curves for an HMDE, pyrolytic graphite, unpolished glassy carbon and polish glassy carbon electrodes. The graphite and glassy carbon electrodes were rotated in the solutions that also contained Hg²⁺ in order to produce a thin film. Depositions times were 30 minutes for the HMDE and 5 minutes for the other electrodes. In the final side-by-side comparison, the three carbon-based electrodes showed superior selectivity with the polished glassy carbon electrode also showing the best signal-to-noise ratio of the three.

The second application utilizes DPSV for determination of Hg in seawater. Gustavvson¹⁰ deposited Hg on to a rotating Au disc electrode. By removing interfering organic material through decomposition with UV light prior to analysis, the researcher was able to detect Hg at concentrations as low as 10 pM.

In the third example, Wang et al.¹¹ used bismuth-coated carbon electrodes for the detection of cadmium, lead, thallium, and zinc. Bi is an attractive alternative to Hg from an environmental standpoint but also from a selectivity standpoint since the formal potentials in Bi are slightly different than in Hg. The researchers used short depositions times (2-10 minutes) to create the thin films and square wave voltammetry for the determination down to the ppb level.

The fourth application is an example of CSV. Piche and Kubiak¹² used CSV to detect arsenic at concentrations as low as 0.06 nM. The researchers speculate that copper and sodium diethyldithiocarbamate aid to form an amalgam with Ar(III) on the surface of the electrode in as little as 50 s. Upon sweeping cathodically, Ar(III) is reduced to ArH₃. One of the advantages with this method is the ability to discern Ar(III) from Ar(V) making it superior to methods such as ICP-MS and Atomic Absortpion Spectroscopy.

1. He, P. Anal. Chem., 1995, 67, 986-992.
2. Faulkner, L. R.; Bard, A. J. Bulk Electrolysis Methods. Electrochemical Methods: Fundamentals and Applications, 2^nd ed.; Wiley: New Jersey, 2000; 417-470.
3. Copeland, T. R.; Christie, J. H.; Osteryoung, R. A.; Skogerboe, R. K. Anal. Chem. 1973, 45, 2171-2174.
4. Osteryoung, R. A.; Christie, J. H. Anal. Chem. 1974, 46, 351-355.
5. Knouvaes, S. P.; O'Dea, J. J.; Chandresekhar, P.; Osteryoung, J. Anal. Chem. 1986, 58, 3199-3202.
6. Brainina, K. Z. Talanta 1971, 18, 513.
7. Brainina, K. Z., translated to English by P. Sheinitz, Stripping Voitammetry in Chemical Analysis, Wiley: New York, 1974.
8. Vypra, F.; Stulyk, K.; Juakova, E., translated to English by J. Tyson, Electrochemical Stripping Analysis, Wiley: New York, 1976.
9. Florence, T. M. J. Electroanal. Chem. 1970, 27, 273-281.
10. Gustavvson, I. J. Electroanal. Chem. 1986, 214, 31-36.
11. Wang, J.; Lu, J.; Hocevar, S. B.; Farias, P. A. M.; Ogorevc, B. Anal. Chem. 2000, 72, 3218-3222.
12. Piche, R.; Kubiak, W. W. J. Electroanal. Chem. 2007, 599, 59-64.

Links: Electrochemist's Guide, AfterMath User's Guide, AfterMath Main Support Page

Related Techniques: Differential Pulse Voltammetry (DPV), Square Wave Voltammetry (SWV)

Like most of the other electrochemical techniques offered by the AfterMath software, this experiment begins with an induction period. During the induction period, a set of initial conditions is applied to the electrochemical cell and the cell is allowed to equilibrate to these conditions. The default initial condition involves holding the working electrode potential at the Initial Potential for a brief period of time (i.e., 3 seconds). If the potentiostat is being used to control the rotator speed, the rotator is also spun at the desired Speed during this time. The WaveNow and WaveNano are capable of outputting a potential proportional to the desired rotator speed. This output can then be coupled to the input on an MSR rotator for fully automated control of the rotation speed.

For a typical one segment RDE experiment, after the induction period, the potential of the working electrode is swept linearly at the specified Sweep Rate from the Initial Potential to the Final Potential. Typical sweep rates are . The Typical Results section below shows the effect of sweep rates that are too large.

After the sweep has finished, the experiment concludes with a relaxation period. The default condition during the relaxation period involves holding the working electrode potential at the final potential for an additional brief period of time (i.e., 1 seconds).

At the end of the relaxation period, the post experiment idle conditions are applied to the cell and the instrument returns to the idle state.

Current is plotted as a function of the potential applied to the working electrode, resulting in a voltammogram.

The parameters for this method are arranged on various tabs on the setup panel. The most commonly used parameters are on the Basic tab, and less commonly used parameters are on the Advanced tab. Additional tabs for Ranges and post experiment idle conditions are common to all of the electrochemical techniques supported by the AfterMath software.

The typical results for a one electrode reversible redox process is a sigmoidal shaped voltammogram (see Figure 7, specific parameters were: Ferrocene in , Pt disk, , ).

Figure 7: Rotating Disk Voltammogram of a Ferrocene Solution

The limiting current can be obtained from the voltammogram by right clicking on the trace and selecting “Add Tool » Peak Height” (see Figure 8, specific parameters were: Ferrocene in , Pt disk, , ). The peak height that is initially drawn (see Figure 9, specific parameters were identical to Figure 8) may have to be changed to properly measure the limiting current.

Figure 8: Addition of Peak Height Tool to Measure Limiting Current

Figure 9: Peak Height Tool Added

By dragging the control points on the tool around you can draw a proper baseline (see Figure 10).

Figure 10: A Proper Baseline for Measurement of the Limiting Current

The baseline type that is initially chosen typically does a good job for a one component reversible system such as that shown above. However, you can change the baseline type by right clicking on the tool and selecting “Properties” (see Figure 11).

Figure 11: Selection of Baseline Properties

This will bring up a dialog box where you can select the type of baseline from the drop-down menu (see Figure 12).

Figure 12: Dialog Box Showing Baseline Types

A series of rotation speeds was used to examine the Ferrocene solution mentioned above (see Figure 13). The limiting current for a freely diffusing species in solution is proportional to the square root of the rotation speed (see Figure 13, inset).

Figure 13: Effect of varying the rotation speed for a Ferrocene Solution from 300 – 2500 rpm. Inset: Plot of Limiting Current versus Square Root of Rotation Speed.

Typical sweep rates are . As long as the system being studied remains in steady-state conditions, sweep rate will have no effect on the limting current. When sweep rates get fast, voltammograms obtained begin to look like typical cyclic voltammograms where the electrode is not rotated. This is due to depletion of the Levich layer of the species being oxidized or reduced. As an example, there is no difference between and sweep rates for a Ferrocene solution when the rotator speed is (see Figure 14, Black trace = (underneath Red trace), Red trace = , Blue trace = , Green trace = ). However, when the sweep rate is or , the voltammograms show signs of depletion of the Levich layer and begin to resemble voltammograms seen in traditional CV. Please see the Theory section for more details if necessary.

Figure 14: Effect of varying the Sweep Rate for a Ferrocene Solution.

The following theoretical introduction to RDE is intended to give the reader a general understanding so that they may better understand what parameters affect the outcome in a typical experiment. A more detailed description can be found in Bard and Faulkner.² Rotating the electrode is a method of forced convection with the purpose of continually delivering material to the electrode in a controlled manner. The rotating rod creates a vortex flow underneath the electrode which pulls material upwards.

The purpose of rotating the electrode is to keep the solution homogeneous. However, next to the electrode is a stagnant layer, called the Levich layer which actually “clings” to the electrode and rotates with it. Inside this layer, the primary mode of mass transport is diffusion. Even though the primary mode of mass transport is diffusion like in cyclic voltammetry, linear sweep voltammetry, or chronoamperometry the concentration gradient at the electrode remains constant with respect to time. Since the concentration gradient remains constant with respect to time, the current is a steady-state current.

The thickness of the Levich layer will depend upon the experimental conditions and is governed by the equation

where is the diffusion coefficient (in ), is the rotation speed (), and is the kinematic viscosity (in , see Table 1).

Table 1: Kinematic Viscosities for Solutions at 1
Solution
(acetonitrile)
(dimethylsulfoxide)
Pyridine
(dimethylformamide)
N,N-Dimethylacetamide
(hexamethylphosphoramide)

The limiting current at electrode is proportional to the thickness of the Levich layer and is defined by

where is the number of electrons in the electrochemical reaction, is Faraday's constant (), is the electrode area (in ), and is the concentration (in ). Finally, the fully expanded limiting current is defined by the Levich equation,

.

In the first example, Gallaway and Barton³ coated an electrode with an oxygen-reducing enzyme and a series of redox-polymers. The purpose of this was to examine mediated electron transfer in a biocatalyzed fuel cell. Varying the type of redox-polymer used, the researchers were able to examine rate constants as a function of overpotential between the enzyme and mediator. These data were then used to determine the optimum polymer redox potential in order to get maximum power output for a hypothetical biofuel cell.

The next example uses RDE to examine the reduction of ferricyanide, in the prescence of methylene blue (MB), at a DNA-modified electrode. Boon et al.⁴ used a gold-disk electrode that was modified with self-assembled monolayers of a double-stranded oligonucleotide selective for the adsorption of MB. The rate limiting step was determined, from RDE, to be the adsorption of MB onto the electrode. Despite slow sweep rates, the resulting voltammograms showed interesting “peak-shaped” i-V traces that took several seconds to reach their steady-state values. These “peak-shaped” traces were attributed to the slow transfer of MB into and out of the film.

The third example, Orilall et al.⁵ synthesized Pt and Pt-Pb nanoparticles and incorporated them into mesoporous niobium-oxide/carbon composites. Heat treating this composite made a graphitic-like material that was assembled onto a glassy carbon disk electrode. The researchers investigated this material for formic acid oxidation and found that it had a four times higher mass activity and lower onset potential than any previously reported Pt-Pb nanoparticles.

In the fourth example, Zhang et al.⁶ used RDE to examine the reduction of oxygen. Multiwalled carbon nanotubes (MWCNTs) were functionalized with a Co-porphyrin complex and then mixed with Nafion. This material was then dropcast onto a glassy carbon disk electrode. RDE was then used to show that the mechanism for oxygen reduction was a direct four-proton and four-electron reduction of oxygen to water. These results showed that the materials would be a suitable substitute for platinum or other metal-based cathode materials in proton conducting membrane fuel cells.

The fifth example uses RDE to examine the mechanism of electrochemical reduction of H2O2 on Cu in acidic sulfate solutions. Stewart and Gerwith⁷ studied the interaction of H2O2 with Cu because H2O2 is commonly used as an oxidizer component in chemical mechanical planarization (CMP) slurries. CMP is used in microelectronics applications. By using RDE, the researchers were able formulate a mechanism that involved a never observed before Cu(I) intermediate following peroxide exposure. The researchers went on to use other techniques to look for and find this Cu(I) intermediate.

The final example shows how RDE can be used in a teaching lab for advanced undergraduate or first-year graduate chemistry or chemical engineering labs. Kear et al.^8,9 use Cu disk electrodes in aerated sea water to examine the reduction of oxygen at a cathode surface. This is a way to introduce electrochemical engineering and corrosion to students and is informative in explaining cathodic kinetics in corrosion reactions. The students also used this experiment as a hand-on approach to learning about mass transfer since the examination of the limiting current at various rotation speeds allows for the calculation of the diffusion coefficient for oxygen.

1. He, P. Anal. Chem., 1995, 67, 986-992.

2. Faulkner, L. R.; Bard, A. J. Methods Involving Forced Convection – Hydrodynamic Methods, Electrochemical Methods: Fundamentals and Applications, 2^nd ed.; Wiley: New Jersey, 2000; 331-367.

3. Gallaway, J. W.; Barton, S. A. C. J. Am. Chem. Soc., 2008, 130, 8527–8536.

4. Boon, E. M.; Barton, J. K.; Bhagat, V.; Nersissian, M.; Wang, W.; Hill, M. G. Langmuir, 2003, 19, 9255–9259.

5. Orilall, M. C.; Matsumoto, F.; Zhou, Q.; Sai, H.; Abruna, H. D.; DiSalvo F. J.; Wiesner, U. J. Am. Chem. Soc., 2009, 131, 9389–9395.

6. Zhang, W.; Shaikh, A. U.; Tsui, E. Y.; Swager, T. M. Chem. Mater., 2009, 21, 3234–3241.

7. Stewart, K. L.; Gewirth, A. A. Langmuir, 2007, 23, 9911–9918.

8. Chemical Engineering Education

9. Kear, G.; A. P.-de-L., C.; Walsh, F. C. Chem. Eng. Ed., 2005, 39, 14-21.

Links: Electrochemist's Guide, AfterMath User's Guide, AfterMath Main Support Page

Related Techniques: Rotating Ring Disk Electrochemistry (RRDE), RDE – Levich Variant (KL-RDE)