Applied potential waveform

The special case of square-wave voltammetry (SWV) is worth noting separately from other alternating current techniques because it is both more rapid and more sensitive than DPP/DPV. In SWV, the applied potential waveform is a staircase with constant step height on which is superimposed an asymmetrical forward and reverse voltage pulse of constant amplitude and very short duration, typically less than 10 ms. Thus, the entire polarogram may be run in about approximately 1 s, with the enhanced sensitivity of the method owing to sampling of the current at the end of both the forward and reverse directions of the pulse. 

Figure 4. Applied-potential waveform used in differential-pulse polarography. [Reproduced with permission from J. Flato, Anal. Chenu, 44, 75A (1972).]...

Three broad classifications of electrochemical methods are used in this chapter. Po-tentiometric methods include zero-current potentiometry and methods in which current of controlled magnitude is apphed to the working electrode, such as in potentiometric stripping analysis (PSA). Amperometric methods consider all techniques in which current is measured these include constant-potential amper-ometry and amperometric measurements made in response to a variety of applied potential waveforms in voltammetric methods. Impedimetric methods comprise a final classification in these methods, faradaic currents are generally absent, and impedance, conductance, or capacitance is the measured property. 

Fig. 3. Schematic potentiostat for electrochemical experiments. Signal—operational amplifier section for generating applied potential waveform control—control operational amplifier output—current-measuring operational amplifier with output to recording system. See text for further explanation.

Fig. 16. Differential pulse voltammetry. (A) Applied potential waveform. Circular inset shows one pulse enlarged currents ii and i2 measured just before and at end of pulse. (B) Differential pulse voltammogram 170 jjlM dopamine in pH 7.4 phosphate buffer, sweep rate 10 mV/sec. At in arbitrary units. Oxidation current is plotted downward in accord with standard electroanalytical conventions. Oxidative differential pulse voltammograms are frequently plotted with 180° rotation about the horizontal axes of Fig. 16B. One should always clearly label the type of current and direction of scan (oxidative) for all voltammetric figures. The oxidative direction of potential sweep should increase from right to left.

Although a wide range of applied potential waveforms have been employed, only a few are of mainstream significance these, along with typical volt-ammograms and some comments are presented in Table 8.1. 

Notes Only a small fraction of a complete applied potential waveform scan is shown for differential pulse and square wave voltammetry. The entire measured signal for a single analyte is shown. 

Before discussing the voltammogram obtained with the triangular waveform of figure 16.3, which is simply a plot of the observed current intensity versus the applied potential, it is useful to describe some experimental details of a cyclic voltammetry experiment [335-337] and to recall some basic theory of dynamic electrochemistry [180,332], A typical cell (figure 16.4) consists of... 

An alternative and more recent electroanalytical tool is square-wave voltammetry (which is probably now employed more often than normal or differential pulse voltammetry). In this technique, a potential waveform (see Figure 6.26) is applied to the working electrode. Pairs of current measurements are then made (depicted on the figure as t and f2) these measurements are made for each wave period ( cycle ), which is why they are drawn as times after to (when the cycle started). The current associated with the forward part of the pulse is called /forward, while the current associated with the reverse part is /reverse- A square-wave voltammogram is then just a graph of the difference between these two... 

Conditions flow injection measurements at the 1 x lO" M level differential pulse waveform scan rate 10 mV/s pulse amplitude 50 mV 0.05 M phosphate buffer (pH 4) applied potential -0.2 V flow-rate 1.0 mL/min. 

Sinusoidal voltammetry (SV) is an EC detection technique that is very similar to fast-scan cyclic voltammetry, differing only in the use of a large-amplitude sine wave as the excitation waveform and analysis performed in the frequency domain. Selectivity is then improved by using not only the applied potential window but also the frequency spectrum generated [28]. Brazill s group has performed a comparison between both constant potential amperometry and sinusoidal voltammetry [98]. 

The general features of the potential waveform applied and of the output currents of these double pulse techniques are shown. 

Let us consider now a potential waveform consisting of a potential pulses sequence E, E2,. .., Ep, with each pulse of the sequence Ep being applied to an electrode of any geometry G over the interval 0 < tp < rp. The total time of the experiment is... 

This change is external and directly imposed by the particular form of the potential waveform applied. 

In order to show the distribution of the applied potential between the outer and the inner interface in the case of systems with two polarized interfaces, the potential time waveform used in SWV is depicted in Scheme 7.5. The applied potential, E (red line), and the outer ( out, blue line) and inner potentials ( "", green line) have been plotted. 

Scheme 7.5 Potential-time waveform of SWV obtained from Eq. (7.5) ( , red line), and its distribution between the outer interface ( °ul, dark blue line) and the inner interface ( "", green line). The three index potentials (the outer index potential, out,mdex, the inner index potential, """ index, and the membrane index potential, mdex) are also included (blue line, dark green line, and black line, respectively). Inset figure Distribution of the applied potential red line), between the outer and the inner interfaces (dark blue line and green line, respectively). jnitiai = —450mV,...

In this section, a brief discussion will be presented about the solution for the current-potential response of different charge transfer processes taking place at a planar electrode when a cyclic linear sweep potential is applied. This procedure has been discussed in detail in references [1-3] and only a short deduction will be provided here. The potential waveform can be written as... 

A sensitivity increase and lower detection limit can be achieved in various ways with the use of voltammetric detectors rather than amperometry at fixed potential or with slow sweep. The principle of some of these methods was already mentioned application of a pulse waveform (Chapter 10) and a.c. voltammetry (Chapter 11). There is, nevertheless, another possibility—the utilization of a pre-concentration step that accumulates the electroactive species on the electrode surface before its quantitative determination, a determination that can be carried out by control of applied current, of applied potential or at open circuit. These pre-concentration (or stripping) techniques24"26 have been used for cations and some anions and complexing neutral species, the detection limit being of the order of 10-10m. They are thus excellent techniques for the determination of chemical species at trace levels, and also for speciation studies. At these levels the purity of the water and of the... 

Fig. 3 Illustration of the potential ( ) waveform applied and the current (/) response during double potential step chronoamperometry...

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