A Comparison Of Cyclic Voltammetry And Cyclic Staircase Voltammetry

A Comparison of Cyclic Voltammetry and Cyclic Staircase Voltammetry

Adrian W. Bott Bioanalytical Systems West Lafayette, IN 47906-1382 E-mail:
awb@bioanalytical.com

Digital potentiostats use a staircase wave form as an approximation to a linear wave form. It is shown in this article that this approximation is only valid under certain conditions. It is also shown that these conditions aremet by BAS digital instruments.

Cyclic voltammetry (CV) is arguably the most widely used technique for the characterization of redox systems. It can provide qualitative information about the number of oxidation states and their stability, as well as the rate of heterogeneous electron transfer reactions. It can also be used in combination with simulation software to calculate rates of homogeneousand heterogeneous reactions. Such studies typically involve varying the scan rate (as well as other conditions) and examining how characteristic features such as the peak currents and peak potentials change in response to these variations (1 - 3) (more sophisticated software also allows fitting of experimental data and simulated data). The wave form used for cyclic voltammetry is a linear ramp(F1), the direction of which changes at least once during the experiment. However, digital instruments cannot apply a true linear wave form, and a staircase wave form (F2) is used as an approxima-

tion. The aim of this article is to determine the conditions under which this approximation is valid and to discuss whether BAS digital instruments (the BAS 100 series and the CV-50W) meet theseconditions. As discussed in the introduction, variation of scan rate is an important element of mechanistic investigations using cyclic voltammetry. Therefore, we must examine how the scan rate is determined in the staircase wave form. Inspection of F2 shows that there are two parameters that determine the “scan rate”: the step potential (∆E) and the step period (tp) (i.e., the scan rate = ∆E/tp). Sincethere are many combinations of these two parameters that can generate a required scan rate, we need to investigate the effect, if any, of these two parameters on the current response and determine which combinations provide a current response identical to that generated by a true linear wave form. Another complication with

the staircase wave form is that the current can be sampled at differentpoints along the potential step. The sampling point is expressed in terms of the sampling parameter α, which is defined by ts/tp, where ts is the time between the application of the pulse and the current sampling; that is, if α = 1, the current is sampled at the end of the step (it should be noted that some authors have used the opposite convention). There have been a number of studies in whichthe effect of variations of ∆E, tp and α on the current response have been investigated to see if there are combinations of parameter values that provide the same current response as a linear wave form. These studies have discussed a wide range of different redox mechanisms (4 - 13) (reversible, quasi-reversible, irreversible due to coupled chemical reactions, and adsorptive species). The results ofthese studies are summarized below.
Current Separations 16:1 (1997)

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F1
Potential wave form for cyclic voltammetry.

+E High

Initial

t

Low E -E
F2
Potential wave form for staircase voltammetry.

1

2

3

Scan Segments

-E
E Step Potential Step Period

tp

t

a) Reversible electron transfer The Fe(III)/Fe(II) oxalate system has been used as an example of areversible system (7, 10, 12). There is good agreement between the linear and staircase current responses for α = 0.25 (9-12) (or 0.3 (7)), both from theory and experiment, for ∆E values up to 8 mV. However, when α = 1, ∆E values as small as 2 mV gave a significantly different response for the staircase wave form (9.6% error). The dependence of the peak current on the scan rate (υ) was also...