Square wave pulse voltammetry applications

Differential pulse voltammetry (DPV) is essentially an instrumental manipulation of chronoamperometry. It provides very high sensitivity because charging current is almost wholly eliminated. More important for CNS applications, it often helps to resolve oxidations which overlap in potential. The method combines linear potential sweep and square-wave techniques. The applied signal is shown in Fig. 16A and consists of short-duration square-wave pulses (<100 msec) with constant amplitude (typically 20 or 50 mV) and fixed repetition interval, superimposed on a slow linear potential scan. The Fapp waveform can be generated with a laboratory-built potentiostat, but most DPV work is done with a commercial pulse polarograph (see Appendix). The inset of Fig. 16A shows an enlargement of one pulse. The current is measured just before the pulse... 

The detection limit for TLV has been improved substantially by using differential pulse and square-wave voltammetry (Chap. 5). For example, detection limits in the 10 8 M range and below have been demonstrated in thin-layer cells requiring less than 100 /xL of sample [61,62]. One practical application of twin-electrode thin-layer cells is in the automatic electrochromic rearview mirror for automobiles. A cell with optically transparent electrodes is placed in front of a mirrored surface. At night, electrolysis in the cell to generate colored material can rapidly reduce glare from following vehicles. 

This chapter analyzes the subtractive techniques Differential Multipulse Voltammetry (DMPV), Differential Staircase Voltammetry (DSCVC), and Square Wave Voltammetry (SWV). Of these, the most employed SWV will be analyzed in more detail. Interesting alternatives to DSCVC and SWV are Differential Staircase Voltcoulometry (DSCVC) and Square Wave Voltcoulometry (SWVC), which are based on the analysis of the difference of converted faradaic charge signals obtained between two successive potential pulses when a staircase potential or a square wave potential is applied [4, 5], which is very useful for the study of surface-confined redox species. There exists, however, a book in this series devoted entirely to the theory and application of SWV [6], so in some of the reaction mechanisms analyzed, the reader will be directed to this title for a more thorough treatment of the SWV response. 

Applications of pulse techniques in electrochemistry have been predominantly in the area of analysis, relying on the linear dependence of peak height on potential, although recently their use in mechanistic studies, particularly square-wave voltammetry, has begun to be exploited. The reason for their use in analysis is intimately linked with the low detection limits that are attainable, particularly in combination with pre-concentration techniques, as will be seen in Chapter 14. Finally, since nowadays the pulse sequences are generally controlled and responses analysed using microprocessors, the development of new waveforms for particular situations is now a much easier task than it was even a decade ago. 

The use of pulse techniques for electroanalytical determinations has been much publicized, and is applicable to both solid electrodes and the HMDE/SMDE. The development in recent years of square wave voltammetry (SWV)39 widens the possibilities beause of its rapidity (Section 10.9) it is especially useful because the time necessary to do an experiment is only 2 s, which means that a SMDE drop in the dropping mode can also be used for micromolar determinations. Progress obtained with pulse techniques40,41 has meant that applications of a.c. voltammetry have been few, but there is no theoretical reason for this. The very low detection limits achieved in stripping voltammetry result not only from the pre-concentration step but also from the use of pulse waveforms in the determination step. 

We will consider five subtopics tast polarography and staircase voltammetry, normal pulse voltammetry, reverse pulse voltammetry, differential pulse voltammetry, and square wave voltammetry. Tast polarography, normal pulse voltammetry, and differential pulse voltammetry form a sequence of development rooted historically in polarography at the DME. To illustrate the motivating concepts, we will introduce each of these methods within the polarographic context, but in a general way, applicable to both the DME and SMDE. Then we will turn to the broader uses of pulse methods at other electrodes. Reverse pulse voltammetry and square wave voltammetry were later innovations and will be discussed principally outside the polarographic context. 

If the concentrations of the redox compounds in the solution or at the electrode surface are low, and better sensitivities are needed than those optimal for LSV and SCV, differential pulse (DPV), normal pulse (NPV) and Osteryoung square-wave voltammetries (OSWV) are more suitable [9a, 9b]. They allow better elimination of the capacitive/background currents and, therefore, the measurement of smaller faradic signals becomes easier. This is achieved either by sampling the current at the end of each pulse (OSWV, NPV) (Fig. 10.5.IE and F) or twice at the end and before pulse application (DPV) (Fig. 10.5.ID). 

Several coulometric and pulse techniques are used in electroanalytical chemistry. Rather low detection limits can be achieved, and kinetic and transport parameters can be deduced with the help of these fast and reliable techniques. Since nowadays the pulse sequences are controlled and the data are collected and analyzed using computers, different pulse programs can easily be realized. Details of a wide variety of coulometric and pulse techniques, instrumentation and applications can be found in the following literature controlled current coulometry [6], techniques, apparatus and analytical applications of controlled potential coulometry [7], coulostatic pulse techniques [8], normal pulse voltammetry [9], differential pulse voltammetry [9], and square-wave voltammetry [10]. 

Whilst these techniques are used almost entirely for analytical purposes [22, 23] their application to kinetic measurements has been described, and the theory for data analysis is available [24]. The major limitation is that with commercial instruments the current measurements are usually made about 50 ms after the application of the potential step. Such instruments are therefore only suitable for the study of relatively slow kinetics. More information on these techniques is available elsewhere [22,23]. Recently a new pulse method has been developed, square wave voltammetry, which is again intended largely for analytical purposes. The potential-time waveform applied to the electrode is shown in Fig. 2.19, and pairs of current measurements are made on each period of the square wave. These are at time piT, late in the forward pulse and designated /fonvard>... 

Electrochemical techniques can provide exquisite information with relatively simple experiments, even for more complex systems. A recent report by Tommos has demonstrated the applicability of such electrochemical analyses to assess the role of PCET (concerted or otherwise) in the model protein 3Y using a Pourbaix diagram (Fig. 20) [154, 155]. In this system, the tyrosine residue is positioned inside a protein matrix in a desolvated and well-structured environment Voltammetric study of its oxidation displays a reversible square-wave and differential pulse voltammogram under basic conditions. The tyrosine residue in question exhibits a potential of 0.910 and 1.070 against NHE at pH 8.5 and 5.5, respectively [156]. Based on expected rate constants for side reactions associated with square-wave voltammetry [157, 158], the authors initially suspected the radical species in question must have a lifetime of at least 30 ms in collaboration with Hammarstrom, transient absorption spectroscopy has placed the half-life somewhere between 2 and... 

For reversible systems there is no special reason to use these techniques, unless the concentration of the electrochemical active species is too low to allow application of DCP or cyclic voltammetry. For a reversible electrochemical system, the peak potentials in alternating current voltammetry (superimposed sinusoidal voltage perturbation) and in square-wave voltammetry will be equal to the formal potential, i.e., JBp = E However, in differential pulse voltammetry, there is a systematic deviation according to ... 

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