Square-wave amplitude voltammetry

Table 3.1 Diagnostic criteria for characterizing lead pigments via voltammetry of microparticles using deposits of the pristine pigments on parafiSn-impregnated graphite electrodes. Data from square-wave voltammograms at a potential step increment of 4 mV, square-wave amplitude of 25 mV, and frequency of 15 Hz. All potentials refer to AgQ (3M NaCl)/Ag. Electrolyte, 0.50 M acetate buffer, pH 4.85...

Equation (7.116) indicates that the charge-potential curves for reversible processes are only dependent on the square wave amplitude Sw and are independent of the frequency / = 1 jh and the staircase amplitude AEs. As a consequence, they are superimposable on those obtained at any differential electrochemical technique, such as DSCVC, provided that the differences between the successive potential pulses coincide (AE = 2 sw)- Moreover, when this difference is much less than RT/F (i.e., less than 25 mV at T = 198 K), the responses obtained in Cyclic Voltammetry (CV), Alternating Current Voltammetry, Potentiometric Stripping Analysis (PSA) and also in any Reciprocal Derivative Chronopotentiometry (RDCP) fulfill [5, 74, 75] ... 

In square-wave voltammetry of diffusion-controlled, reversible, and totally irreversible electrode reactions, the peak current is a linear function of the square root of frequency, but its relationship with square-wave amplitude is not linear. 

FIGURE 11.1 Square-wave voltammetries (SQWVs) for BTP MCM immersed into (a) 0.10 M Bu4NPFs/MeCN and (b) 0.05 M I A ( () + 0.10 M Bu4NPFyMeCN. Potential scan initiated at 0.45 V in the negative direction. Potential step increment, 4 mV square-wave amplitude, 25 mV frequency, 15 Hz. (Adapted from Domenech et al., 2005. J. Electroanal. Chem. 577, 249-262, with permission from Elsevier.)... 

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. 3.8 Square-wave voltammetry of simvastatin microparticles in 0.09 M NaC104, pH 7. Net response (/net) and its forward (If) and backward (I, ) components. Frequency is 150 Hz, amplitude is 50 mV and potential increment is 2 mV (reprinted from [188] with permission)...

Fig. 1. Waveform used in square-wave voltammetry (SWV) showing the amplitude, Esw step height, AE square-wave period, T delay time, T6 and current measurement times, 1 and 2. Reproduced 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. 

Fig. 9 Catalytic square wave voltammetry of PDDA/ds-DNA (/Mb/ds-DNA)2 films on rough PG electrodes before and after incubations at 37°C with 2% styrene (no styrene in controls) and 0.2 mM H202 in aerobic buffer (SWY amplitude 25 mV frequency 15 Hz step height 4 mV PDDA=polydiallyldi-methylammonium ion). After incubation, electrodes were washed and placed into pH 5.5 buffer containing 50 pM Ru(bpy)3+ for the SWY analysis. (From Ref. [15] with permission. Copyright American Chemical Society.)...

Fig. 16.6. Examples of BIA voltammetry, illustrated for the oxidation of 2 mM K4Fe(CN)6 in 0.4 M K2S04 electrolyte at a Pt electrode, dispension flow rate 24.5 p.Ls , cell parameters as in Fig. 16.5. (a) Consecutive injections of 16 p.L during a linear potential sweep, scan rate 10 mVs l (b) Background-subtracted cyclic voltammogram recorded during injection, scan rate 2 Vs-1 (c) Background-subtracted square wave (SW) voltammogram recorded during injection SW amplitude 50 mV, SW increment 2 mV, frequency 100 Hz.

AC voltammetry — Historically the analysis of the current response to a small amplitude sinusoidal voltage perturbation superimposed on a DC (ramp or constant) potential [i]. Recent applications invoke large amplitude perturbation (sinusoidal, square wave or arbitrary wave... 

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. 

Another electrochemical technique being used in protein studies is square-wave voltammetry. The usefulness of this method is often ascribed to its ability to factor out the double-layer charging current but for protein molecules confined to a film, the advantages are the increased sensitivity and additional kinetic information that can be obtained by varying the frequency and pulse amplitude. The data are more difficult to extract than cyclic voltammetry, and we will not attempt to elaborate on this aspect, although studies that have included square-wave voltammetry will be mentioned later in this chapter. 

The dimensionless net peak current A p primarily depends on the product nEsw [31]. This is shown in Table II.3.1. With increasing nfsw the slope BA pIdnEsv, continuously decreases, while the half-peak width increases. The maximum ratio between Ahalf-peak width appears for nEv, = 50mV [6]. This is the optimum amplitude for analytical measurements. If sw = 0, the square-wave signal turns into the signal of differential staircase voltammetry, and A[Pg.124]

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... 

Fig. 65. Comparison of three voltammetric stripping responses under equal accumulation conditions. (A) linear sweep voltammetry, (B) differential pulse voltmametry, (C) square-wave technique. Conditions 5 x 10 M vitamin K3 in 0.3 M HCIO3, static MDE, accumulation potential, Eacc=-0.1V (vs. Ag/AgCl), tacc = 60s, rest time tr=10s. Linear sweep rate v = 0.02Vs , differential pulse scanning 0.01 Vs , SW-scanning 0.20Vs pulse amplitudes dEop = JEsw = 0.02 V, SW frequency 100 Hz. Adapted according to [162].

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... 

Sher, A.A., Bond, A.M., Gavaghan, D.J. et al. (2005) Fourier transformed large amplitude square-wave voltammetry as an alternative to impedance spectroscopy evaluation of resistance, capacitance and electrode kinetic effects via an heuristic approach. Electroanalysis, 17, 1450-1462. 

The excitation potential wave-form for square wave voltammetry (SWV) is shown in Figure 2.10. The perturbation consists of a square wave having constant amplitude. 

Similar to EIS, SWV (square-wave voltammetry) is another frequency-dependent electrochemical technique that could also be used in label-free Faradaic immunosensing [167]. In this case, a train of potential pulses is superimposed on a staircase potential signal with the latter centered between a cathodic pulse and an anodic pulse of the same amplitude. During each cathodic pulse, the analyte diffuses to the electrode surface and it is immediately reduced. During the anodic pulse, analyte that was just reduced is reoxidized. The current is sampled just before and at the end of each pulse and the current difference between these two points is then plotted against the staircase potential in a SW voltammogram. A linear potential scan in SWV is faster than EIS record and a familiar peak-shaped signal is more easily interpreted. 

---