Square-wave amplitude forward current

Fig. 2.15 SQWVforCriOs immersed into 0.10 M HCl shown (a) the net difference current between forward and backward pulses, and, (b) such currents and their sum. Potential scan initiated at +0.15 V in the positive direction. Potential step increment 4 mV square wave amplitude 25 mV frequency 5 Hz. Adapted from [149]...

Although the usual way of analyzing the influence of the kinetics of the electron transfer on the SWV response is based on the variation of the frequency at fixed values of the staircase and square wave amplitude, a new approach for carrying out this analysis has been proposed based on the study of the influence of the square wave amplitude sw on the current potential curves at a fixed value of the frequency (or the time pulse) [19, 33, 34], The square wave amplitude has been used rarely as a tool in mechanistic and kinetic studies. One of the main reason is that, as stated in Sect. 7.1, in SWV the current is plotted versus an index potential which is an average potential between the forward and reverse potentials (see Eq. (7.7)) and leads to a discrepancy between the plotted and actual potentials at which the current is sampled. Therefore, the role played by Esw in the process is complex. 

The second procedure is based on the effect of the square wave amplitude on the peak potential separation between the anodic and cathodic components of the SWV response. This separation depends on both the reversibility of the surface charge transfer (through co and Sw- Thus, by plotting the differences AEp = Epc — E pl>, with Ep c and EpA being the peak potentials of the forward and reverse currents measured versus the index potential, or AE p = Ef c — E p a with h p c and h p a being the peak potentials of the forward and reverse currents measured versus the real potential that is applied in each case (potential-corrected voltammograms), it is possible to obtain linear dependences between the peak potentials separation and... 

The peak currents and potentials of the forward and backward components are listed in Table II.3.2. If the square-wave amplitude is not too small nEsw > 10 mV), the backward component indicates the reversibility of the electrode reaction. In the... 

Table 2.1 Square-wave voltammetry of fast and reversible electrode reaction (1.1). The dimensionless net peak current, the ratio of peak currents of the forward and backward components, the peak potentials of the components and the half-peak width as functions of SW amplitude ...

Fig. 2.17a,b Square-wave voltammogram of Eu + (0.5 mmoldrn" ) in acidified 0.1 moldm NaC104 and its forward red) and backward blue) currents. Frequency 125 s amplitude 40 mV step potential 2 mV delay time 30 s scan direction negative (a) and positive (b) (reprinted from [23] with permission)... 

Square-wave voltammetry is a large-amplitude differential technique in which a waveform composed of a symmetric square wave, superimposed on a base staircase potential, is applied to the working electrode (8) (Fig. 3.9). The current is sampled twice during each square-wave cycle, once at the end of the forward pulse (at h) and once at the end of the reverse pulse (at t2). Since the square-wave modulation amplitude is very large, the reverse pulses cause the reverse reaction of the product (of the forward pulse). The difference between the two measurements is plotted versus the base staircase potential. 

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. 

Fig.II.3.1 Scheme of the square-wave excitation signal. E cc starting potential to delay time sw SW amplitude AE scan increment r SW period if forward current /b backward current, and ( ) points where they were sampled...

CV peak current in the backward process, Eq. (92) CV peak current in the forward process, Eq. (92) quasi-reversible LSV current, Eq. (88) reduction current, Eq. (6) ring current, Eq. (121) sampled current, Eq. (42) staircase current square-wave current, Eq. (64) steady state current transformed LSV current, Eq. (94) alternating current, Eq. (56) amplitude of the AC peak-to-peak distance in the 2nd derivative DCP current density, Eq. (2) exchange current density, Eq. (9) limiting current density, Eq. (119)... 

Square-wave voltammetry The potential-time waveform and current measuring scheme for this technique is shown in Fig. 10. The waveform consists of a symmetrical square-wave (peak to peak amplitude 2Es ) superimposed on a staircase wave of step height AE and a period t. The response current is sampled at the end of both the forward (If) and reverse (If) half cycle. A difference current dl is determined as... 

The technique of square wave voltammetry (22) (see Table 8.1) has even more to offer as a voltammetric method of probing selective chemistry, because of the speed with which a scan can be carried out. The analytical signal in this technique is the difference between the current for the forward pulse and the current for the reverse pulse. Because of the large amplitude of the square wave, for a reversible reduction, the reduced electroactive species formed at the electrode during the forward pulse is re-oxidized by the reverse pulse. Consequently, the sensitivity of this method is enhanced when compared to differential pulse voltammetry. For identical conditions, an approximately 30% improvement in signal is obtained, but when the higher scan rates that are... 

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