Voltammetry, square-wave

Square wave amplitude l/square wave frequency 

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 major advantage of square-wave voltammetry is its speed. The effective scan rate is given by / AEs. The term / is the square-wave frequency (in Hz) and AEs is the step height. Frequencies of 1-100 cycles per second permit the use of extremely fast potential scan rates. For example, if AEs = 10 mV and /= 50 Hz, then the effective scan rate is 0.5 V/s. As a result, the analysis time is drastically reduced a complete voltammogram can be recorded within a few 

FIGURE 3-8 Square-wave wavefonn showing the amplitude, Esw step height, AE square-wave period, r delay time, Td and current measurement times, 1 and 2. (Reproduced with permission from reference 9.) 

FIGURE 3-9 Square-wave voltaimnograms for reversible electron transfer. Curve A forward cinrent. Curve B reverse current. Ciurve C net current. (Reprodnced with permission from reference 9.) 

As it happens in DPV, if A Sw is small (about 50fn mV), the peak-potential for a reversible process virtually coincides with the formal electrode potential. 

In addition, for a reversible process the width of the peak at half height, AEp/2, is given by  

Like DPV, OSWV is very effective in solving almost overlapping processes. With respect to DPV, OSWV can use higher scan rates (from a few hundreds of mV s-1 to a few V s-1). The typical scan rate (which is given by the product SWfrequency X A.Ebase) is around 0.2 V s-1 (which is the typical scan rate of cyclic voltammetry). This allows one to compete with the eventual presence of chemical complications coupled to the electron transfers. 

Rudjer Boskovic Institute, 10001 Zagreb, Croatia e-mail mlovric irb.hr 

is one-half of the peak-to-peak amplitude, and the potential increment AE is the step height of the staircase waveform. The scan rate is defined as AE/r. Relative to the scan direction, AE, forward and backward pulses can be distinguished. The currents are measured at the end of each pulse and the difference between the currents measured on two successive pulses is recorded as a net response. Additionally, the two components of the net response, i.e., the currents of the forward and backward series of pulses, respectively, can be displayed as well [6, 27-30]. The currents are plotted as a function of the corresponding potential of the staircase waveform. 

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 A Pp and the half-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 Pp does not vanish [6, 32, 33]. 

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 

The net peak current depends linearly on the square root of the frequency  

As with DPV, one of the major advantages of SWV is its ability to cancel non-Faradaic currents as a result of the capacitance being effectively constant at both points 1 and 2. A further advantage of SWV over other pulse techniques 

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Figure Bl.28.5. Applied potential-time wavefonns for (a) nomial pulse voltannnetry (NPV), (b) differential pulse voltannnetry (DPV), and (e) square-wave voltammetry (SWV), along with typieal voltannnograms obtained for eaeh method.

Osteryoung J and O Dea J J 1986 Square wave voltammetry Electroanalytical Chemistry ed A J Bard (New York Dekker)... 

O Dea J, Wo]ciechowski M and Osteryoung J 1985 Square wave voltammetry at electrodes having a small dimension... 

One aspect that reflects the electronic configuration of fullerenes relates to the electrochemically induced reduction and oxidation processes in solution. In good agreement with the tlireefold degenerate LUMO, the redox chemistry of [60]fullerene, investigated primarily with cyclic voltammetry and Osteryoung square wave voltammetry, unravels six reversible, one-electron reduction steps with potentials that are equally separated from each other. The separation between any two successive reduction steps is -450 50 mV. The low reduction potential (only -0.44 V versus SCE) of the process, that corresponds to the generation of the rt-radical anion 131,109,110,111 and 1121, deserves special attention. 

Describe and draw clearly the waveform employed in square-wave voltammetry. Explain how the current is measured. 

Sodium-silicate glass, 151 Sol-gel films, 120, 173 Solid electrodes, 110 Solid state devices, 160 Solvents, 102 Speciation, 84 Spectroelectrochenristry, 40 Spherical electrode, 6, 8, 9, 61 Square-wave voltammetry, 72, 92 Staircase voltammetry, 74 Standard potential, 3 Standard rate constant, 12, 18 Stripping analysis, 75, 79, 110 Supporting electrolyte, 102 Surface-active agents, 79... 

Determined by direct electrochemistry at a glassy carbon electrode (cyclic, differential pulse, or square-wave voltammetry). 

The redox properties of a series of heterometal clusters were assessed by electrochemical and FPR measurements. The redox potentials of derivatives formed in D. gigas Fdll were measured by direct square wave voltammetry promoted by Mg(II) at a vitreous carbon electrode, and the following values were determined 495, 420,... 

Molecular Characterization It has been repotted that o-qulnones oxidize ascorbic acid In homogeneous solutions (25). Surface qulnones have also been reported to exist on activated carbon surfaces (16). However, cyclic voltarammetry Is not sufficiently sensitive to allow an unambiguous Identification of the reversible wave ascribed to surface qulnones (16). Therefore, differential pulse voltammetry (DPV) and square wave voltammetry were employed. 

The Model 384B (see Fig. 5.10) offers nine voltammetric techniques square-wave voltammetry, differential-pulse polarography (DPP), normal-pulse polar-ography (NPP), sampled DC polarography, square-wave stripping voltammetry, differential pulse stripping, DC stripping, linear sweep voltammetry (LSV) and cyclic staircase voltammetry. 

The Model 303A static mercury drop electrode (SMDE) has received continuous design improvements (cf., Fig. 3.28 and associated explanation), e.g., the larger drop size afforded by the 303A design has yielded a further increase in sensitivity and the flip of a switch converts it into an extremely stable HMDE for stripping and/or square-wave voltammetry. ... 

Cyclic voltammetry, square-wave voltammetry, and controlled potential electrolysis were used to study the electrochemical oxidation behavior of niclosamide at a glassy carbon electrode. The number of electrons transferred, the wave characteristics, the diffusion coefficient and reversibility of the reactions were investigated. Following optimization of voltammetric parameters, pH, and reproducibility, a linear calibration curve over the range 1 x 10 6 to 1 x 10 4 mol/dm3 niclosamide was achieved. The detection limit was found to be 8 x 10 7 mol/dm3. This voltammetric method was applied for the determination of niclosamide in tablets [33]. 

Detection of damage caused to DNA by niclosamide in schistosomiasis was investigated using an electrochemical DNA-biosensor. It showed for the first time clear evidence of interaction of niclosamide with DNA and suggested that niclosamide toxicity can be caused by this interaction, after reductive activation. The electrochemical reduction and oxidation of niclosamide involved the use of cyclic, differential, and square-wave voltammetry, at a glassy carbon electrode. It enabled the detection limit of 8 x 10-7 M [34]. 

Alemu et al. [35] developed a very sensitive and selective procedure for the determination of niclosamide based on square-wave voltammetry at a glassy carbon electrode. Cyclic voltammetry was used to investigate the electrochemical reduction of niclosamide at a glassy carbon electrode. Niclosamide was first irreversibly reduced from N02 to NHOH at —0.659 V in aqueous buffer solution of pH 8.5. Following optimization of the voltammetric parameters, pH and reproducibility, a linear calibration curve over the range 5 x 10 x to 1 x 10-6 mol/dm3 was achieved, with a detection limit of 2.05 x 10-8 mol/dm3 niclosamide. The results of the analysis suggested that the proposed method has promise for the routine determination of niclosamide in the products examined [35]. 

To date, a few methods have been proposed for direct determination of trace iodide in seawater. The first involved the use of neutron activation analysis (NAA) [86], where iodide in seawater was concentrated by strongly basic anion-exchange column, eluted by sodium nitrate, and precipitated as palladium iodide. The second involved the use of automated electrochemical procedures [90] iodide was electrochemically oxidised to iodine and was concentrated on a carbon wool electrode. After removal of interference ions, the iodine was eluted with ascorbic acid and was determined by a polished Ag3SI electrode. The third method involved the use of cathodic stripping square wave voltammetry [92] (See Sect. 2.16.3). Iodine reacts with mercury in a one-electron process, and the sensitivity is increased remarkably by the addition of Triton X. The three methods have detection limits of 0.7 (250 ml seawater), 0.1 (50 ml), and 0.02 pg/l (10 ml), respectively, and could be applied to almost all the samples. However, NAA is not generally employed. The second electrochemical method uses an automated system but is a special apparatus just for determination of iodide. The first and third methods are time-consuming. 

Luther et al. [92] have described a procedure for the direct determination of iodide in seawater. By use of a cathodic stripping square-wave voltammetry, it is possible to determine low and sub-nanomolar levels of iodide in seawater, freshwater, and brackish water. Precision is typically 5% (la). The minimum detection limit is 0.1 - 0.2 nM (12 parts per trillion) at 180 sec deposition time. Data obtained on Atlantic Ocean samples show similar trends to previously reported iodine speciation data. This method is more sensitive than previous methods by 1-2 orders of magnitude. Triton X-100 added to the sample enhances the mercury electrode s sensitivity to iodine. 

Rozan, T. F., Benoit, G. and Luther, G. W. (1999). Measuring metal sulfide complexes in oxic river waters with square wave voltammetry, Environ. Sci. Technol., 33, 3021-3026. 

This is a dynamic electrochemical technique, which can be used to study electron transfer reactions with solid electrodes. A voltammo-gram is the electrical current response that is due to applied excitation potential. Chapter 18b describes the origin of the current in steady-state voltammetry, chronoamperometry, cyclic voltammetry, and square wave voltammetry and other pulse voltammetric techniques. 

CA—chronoamperometry, CV—cyclic voltammetry, SWV—square wave voltammetry, ASV—anodic stripping voltammetry, SHE—standard hydrogen electrode. 

The realization that current sampling on a step pulse can increase the detection sensitivity by increasing the faradaic/charging ratio is the basis for the development of various pulse voltammetric (or polarographic) techniques. Also, the pulses can be applied when it is necessary and can reduce the effect of diffusion on the analyte. Figure 18b. 11 shows the waveform and response for three commonly used pulse voltammetric techniques normal pulse voltammetry (NPY), differential pulse voltammetry (DPV), and square-wave voltammetry (SWV). 

After tq is passed, the second step starts by scanning the potential from Ed to a potential when all the deposited metals are re-oxidized (the reverse of reaction 25). The oxidation current recorded as a function of potential is the anodic stripping voltammogram (ASV). A typical ASY of three metals (Cd, Pb, and Cu) deposited on a mercury film electrode is shown in Fig. 18b.12b. The sensitivity of ASY can be improved by increasing the deposition time and by using the pulse technique to record the oxidation current. ASV in Fig. 18b. 12b was obtained by using the square wave voltammetry. In most cases a simple linear or step ramp is sufficient to measure sub-ppm level of metals in aqueous solution. The peak current of a linear scan ASV performed on a thin mercury film electrode is given by... 

Appendix Understanding cyclic voltammetry and square-wave voltammetry 84... 

Hg. 64 The potential-time wave for square-wave voltammetry. This wave may be obtained by superimposing a square wave with constant pulse height ( squai(.) and width (1/(2/) / is the wave frequency) on a staircase wave with constant increment... 

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