Square Wave Voltammetry

Square wave voltammetry (SWV) is much more sensitive and has better resolution than cyclic or normal pulse voltammetry. For these reasons, SWV may be the method of choice for systems with low concentrations of electroactive material or with overlapped signals. In this technique (Table

II.1), a square potential pulse of frequency / consisting of a forward and a reverse component is superimposed on top of an increasing potential staircase and applied to the electrochemical cell (Figure 11.17)[13]. Measurements of current are timed to occur at the end of each forward and reverse pulse. These two currents are subtracted to give a difference current

Figure 11.17 Square wave voltammetry (SWV) input waveform separated into its component parts of a staircase with superimposed forward-reverse square pulse cycle superimposed on top of each step.

that has a very large faradaic to charging current ratio. The difference current output is displayed vs. the potential of the step on the staircase. The difference current response curve for a reversible electrochemical reaction is a symmetric peak, contrasting to the waves and unsymmetric peaks obtained in other voltammetric methods.

As with CV, closed form models for SWV are rarely available. However, the COOL program mentioned in Table 11.9 was designed by Janet and R. A. Osteryoung and their coworkers to analyze SWV and other types of pulse voltammetry data [13]. We illustrate this by an application of the COOL algorithm to SWV data for the reduction of Zn2+ to Zn(Hg) in M KN03 at a mercury electrode using a diffusion-kinetic model [12]. Excellent fits were obtained (Figure 11.18). Parameters and their confidence intervals were k° = {k°') = 2.64 ± 0.16 X 10"4 cm/s, a = 0.20 ± 0.02, and E\a = 1.000 ± 0.001 V vs. SCE, illustrating the precision available from the method.

Figure 11.18 shows the shapes of SWV difference current curves under experimental conditions where the reduction of Zn2+ is reversible (10 Hz) and nearly irreversible (500 Hz). The curve attains an overlapped peak appearance at the higher frequencies as a consequence of the subtraction of the forward and reverse currents [12, 13].

As in CV, a variety of coupled chemical reactions under pseudo-first-order conditions can be investigated by using the COOL program to analyze SWV data. A considerable number of electrode reactions of different types have been analyzed using this approach [13].

Figure 11.18 Square wave voltammograms for reduction of l mM Zn2+ in M KNO, at £slcp = 5 mV and £S1V = 25 mV. The points show experimental data, and the solid lines show the best fits to a diffusion-kinetic model using the COOL algorithm. The frequency in ascending order of curves is 10, 25, 100, 200, 500 Hz. (Adapted with permission from [12], copyright by Elsevier.)

Figure 11.18 Square wave voltammograms for reduction of l mM Zn2+ in M KNO, at £slcp = 5 mV and £S1V = 25 mV. The points show experimental data, and the solid lines show the best fits to a diffusion-kinetic model using the COOL algorithm. The frequency in ascending order of curves is 10, 25, 100, 200, 500 Hz. (Adapted with permission from [12], copyright by Elsevier.)

Potential vs. SCE/V

Potential vs. SCE/V

Potential vs. SCE/V

Potential vs. SCE/V

Potential vs. SCE/V

Potential vs. SCE/V

Potential vs. SCE/V

Potential vs. SCE/V

Potential vs. SCE/V

Potential vs. SCE/V

Figure 11.19 Square wave voltammograms for 5 mM azobenzene on a mercury electrode at / = 200 Hz. Left side depicts forward (positive) and reverse (negative) current, and the corresponding difference current curves are shown on the right side. £stcp = 5 mV and £sw = 25 (A, B), 50 (C, D) and 75 (E, F) mV. The points show experimental data, and the solid lines show the best fits to an adsorption-kinetic model using the COOL algorithm. (Adapted with permission from [23], copyright by the American Chemical Society.)

Potential vs. SCE/V

Potential vs. SCE/V

Figure 11.19 Square wave voltammograms for 5 mM azobenzene on a mercury electrode at / = 200 Hz. Left side depicts forward (positive) and reverse (negative) current, and the corresponding difference current curves are shown on the right side. £stcp = 5 mV and £sw = 25 (A, B), 50 (C, D) and 75 (E, F) mV. The points show experimental data, and the solid lines show the best fits to an adsorption-kinetic model using the COOL algorithm. (Adapted with permission from [23], copyright by the American Chemical Society.)

The COOL algorithm has also been applied to adsorbed molecules on electrodes and can be used to obtain kinetic and thermodynamic parameters for such systems [23]. The model predicts the variety of difference current peak shapes that can be encountered for an adsorbed system. The shapes depend on k°tp, where k° is the first-order electron transfer rate constant in s~1 and tp is the pulse width, which is inversely proportional to frequency. The shapes also depend on the square wave pulse height, £sw. These different shapes are illustrated by SWV data for azobenzene adsorbed on mercury (Figure 11.19). The forward and reverse current curves on the left give rise to the difference current curves on the right-hand side of the figure. At a pulse height £sw of 25 mV, a single peak is found, but at £sw of 75 mV two peaks result. This is a consequence of the subtraction of the forward and reverse currents, as in the case of reduction of Zn2+.

The goodness of fit of the adsorption model to the data for azobenzene on mercury is also illustrated in Figure 11.19. The azobenzene surface concentration, electrochemical transfer coefficient, and electron transfer rate constant were obtained by using the COOL algorithm. In 5 ¡iM solutions of azobenzene in acetate buffer, the surface concentration on mercury was 26 pmol cm 2, the standard potential was -0.239 V vs. SCE, the transfer coefficient was 0.51 and the rate constant was 160 s~'.

Because the charging current component is nearly eliminated from the difference current, SWV alleviates some but not all of the problems with background encountered in CV. However, as for CV, there is presently no way to account for a sloping or otherwise structured background within the COOL program package if background subtraction is not effective.

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