Repetitive square-wave pulse

Thus, the potential varies linearly with time in this region, and the capacitance can be calculated from the slope according to Eq. (8.18). Usually a repetitive square-wave pulse is applied, as seen in Figure 8.6, from which both the capacitance and the solution resistance can be determined. 

Another comphcation arises from the fact that, in contrast to conventional electrodes, CP-coated electrodes often undergo changes in their oxidation state and structure in the course of metal electrodeposition. This is usually the case when driving metal-ion reduction by means of the most fiequently applied electrochemical techniques - cyclic voltammetry, multistep potential procedures, repetitive square-wave potential (or pulse potentiostatic) techniques, and galvanostatic reduction (Figure 7.3). In spite of the difficulty in... 

Repetitive square-wave potential techniques switch the potential continuously between the strongly reductive value necessary for the nucleation of the metal particles and a more positive one that is chosen to promote reoxidation of the CP material and thus recuperation of its conducting state, and/or unproved penetration of metal complex anions in the CP layer. Metal complex anions that are used as sources of metal reduction become partially consumed, but also expulsed as doping anions in the course of the reductive dedoping pulse. The size of the electrodeposited metal particles has been found to depend essentially on the frequency of the potential pulses [37,169] (Table 7.3). In fact, the data summarized in Table 7.3 show that by appropriate adjustment of the corresponding parameters, all of the currently exploited electrochemical techniques may result in the deposition of metal NPs in CPs. 

A single pulse or a repetitive square wave may give both DC and AC effects. In both cases, the duration of the pulse or square wave is an important variable (see rheobase and chronaxie). 

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... 

Sinusoidal excitation provides only one harmonic at the modulation frequency. In contrast, pulsed light provides a large number of harmonics of the excitation repetition frequency. The harmonic content, the number of harmonics and their amplitude, is determined by the pulse width and shape.(25) For example, a train of infinitely short pulses provides an infinite number of harmonics all with equal amplitude. A square wave provides only three modulation frequencies with sufficient amplitude to be usable. Equation (9.74) gives the harmonic content of a train of rectangular pulses R(t) of D duty cycle (pulse width divided by period) and RP peak value ... 

Figure 4.5. Square-wave voltammetric detection of 100 )iL of a micromolar mixture of copper, lead, nickel, cobalt, and cadmium. The square wave was 30 Hz with a pulse size of -70 millivolts and a step size of -2- i millivolts. Sweep repetition was 2.3 seconds with each sweep on a fresh mercury drop electrode.

By using nonsinusoids as excitation waveforms, a system is excited at several frequencies simultaneously. If the system is linear, the response of each sine wave can be added. If the system is nonlinear, new frequencies are created influencing the frequency spectrum. The square wave to the left of Figure 8.7 has no DC component. One of the ramps in the middle is used in scanning devices such as polarographs. As drawn, the waveform has a DC component. The pulse to the right has a DC component dependent on the repetition frequency. 

Pulsating overpotential consists of a periodic repetition of overpotential pulses of different shapes. Square-wave PO is defined in the same way as PC except that the overpotential pulsates between the amplitude value and zero instead of current density. Non-rectangular pulsating overpotential is defined by the amplitude of the overpotential, rj, frequency, and overpotential waveform [7]. 

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