Knowledgebase

Though there are similarities between SCV and CV and LSV, SCV allows more control of the waveform that is applied to the working electrode. CV and LSV waveforms are optimized to take full advantage of the resolution of the potentiostat’s digital-to-analog converter, while the waveform in SCV is designed for much higher sweep rates than typically encountered in CV or LSV.

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).

In Linear Staircase Voltammetry (LSCV), after the induction period, the potential of the working electrode is stepped from the Initial Potential to the Final Potential in Step amplitude increments. The period of each step is controlled by the Step period increment. In Cyclic Staircase Voltammetry (CSCV), using a two segment experiment as an example, the electrode potential is swept from the Initial potential to the Upper potential or Lower potential, then reversed and swept to the Final potential.

After the sweeping 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, the instrument returns to the idle state and, if necessary, post experiment processing is done.

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. There is also a tab for Post experiment processing.

Typical results for a solution of Ferrocene in are shown below. Two different sweep rates have been shown along with different Sample windows.

The first typical result is at with Sample windows of (see Figure 5A) and (see Figure 5B). Note that the Electrode current range was set to “Auto.”

Figure 5: Staircase Voltammograms of a Ferrocene Solution with a) 1 ms and b) 5 ms Sample Windows

The following section will discuss differences between SCV and CV. Please see Bard and Faulkner¹ for a more detailed description of the technique. Under ideal conditions or conditions where uncompensated resistance is negligible SCV should produce results identical to CV. The voltammogram in Figure 6 is an example where uncompensated resistance is not negligible, as evidenced by the large peak separation. At very high sweep rates, it is possible that kinetic effects can be masked as uncompensated resistance. A plot of should extrapolate to when a large is caused only by uncompensated resistance.

Both CV and SCV utilize staircase waveforms (as opposed to truly linear waveforms generated in analog instruments), however, CV is optimized to take full advantage of the resolution of the potentiostat’s digital-to-analog converter. SCV gives you the ability to control the step height and duration giving you access to sweep rates greater than . Both techniques give you the ability to choose when current is measured and thus give the ability to minimize background charging currents. In CV, the parameter Alpha determines when current is sampled. A value of “0” means that the current is sampled immediately after the potential step, and a value of “100” means that the current is sampled immediately prior to the potential step. In SCV, the parameter Sample window determines the point at which current is sampled before the next potential step. Therefore, in SCV, a Sample window of and Step period increment of corresponds to an Alpha of 50 in CV. Theoretically, it is best to sample at of the sample window, meaning an Alpha of 75 in CV or a Sample window of 1/4*(Step period increment) in SCV.² Today’s high resolution digital-to-analog converters make it unlikely that changing the point at which the current is sampled will noticeably alter the voltammogram of a species in solution. It is possible that changing the point at which current is sampled for a surface-bound species will introduce artifacts or alter the voltammogram significantly. Pexing He has explored this point in the literature.³ Finally, several discussions comparing SCV and CV have been presented in the literature.^4-8

The following two examples employ staircase voltammetry to generate high sweep rates. Please see the Application section of CV for examples using staircase voltammetry to generate waveforms with lower sweep rates.

The first example uses staircase voltammetry to generate sweep rates as high as . Baur et al.⁹ used staircase voltammetry to detect biogenic amines. These high sweep rates have three advantages. One, they allow for the detection of these species while preventing the formation of an insulating layer. Two, they allow extracellular detection of these amines in the brain. And three, they discriminate against chemical events after the initial electron transfer.

In another example, Heering et al.¹⁰ used SCV to measure electron transfer rates of an adsorbed species. The researchers showed that SCV becomes independent of step amplitude and similar to CV at high sweep rates () if the current is sampled at of the step period. The researchers determined the rate constant and n for adsorbed yeast cytochrome c peroxidase. Values obtained from SCV were comparable to more traditional methods such as chronoamperometry (CA).

1. Faulkner, L. R.; Bard, A. J. Polarography and Pulse Voltammetry, Electrochemical Methods: Fundamentals and Applications, 2^nd ed.; Wiley: New Jersey, 2000, 275-278.

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

3. Bilewicz, R.; Osteryoung, R. A.; Osteryoung, J. Anal. Chem. 1986, 58, 2761-2765.

4. Bilewicz, R.; Wikiel, K.; Osteryoung, R.; Osteryoung, J. Anal. Chem. 1989, 61, 965-972.

5. Stojek, Z.; Osteryoung, J. Anal. Chem., 1991, 63, 839–841.

6. Christie, J. H.; Osteryoung, R. A. Anal. Chem. 1976, 48, 869-872.

7. Kalapathy, U.; Tallman, D. E. Anal. Chem., 1992, 64, 2693–2700.

8. Murphy, M. M.; O’Dea, J. J.; Arn, D.; Osteryoung, J. G. Anal. Chem., 1990, 62, 903–909.

9. Baur, J. E.; Kristensen, E. W.; May, L. J.; Wiedemann, D. J.; Wightman, R. M. Anal. Chem., 1988, 60, 1268–1272.

10. Heering, H. A.; Mondal, M. S.; Armstrong, F. A. Anal. Chem., 1999, 71, 174-182.