Square wave

Instruments are available based on sine waves, square waves tind pulses singly or in combinations. 

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.

Stripping voltammetry involves the pre-concentration of the analyte species at the electrode surface prior to the voltannnetric scan. The pre-concentration step is carried out under fixed potential control for a predetennined time, where the species of interest is accumulated at the surface of the working electrode at a rate dependent on the applied potential. The detemiination step leads to a current peak, the height and area of which is proportional to the concentration of the accumulated species and hence to the concentration in the bulk solution. The stripping step can involve a variety of potential wavefomis, from linear-potential scan to differential pulse or square-wave scan. Different types of stripping voltaimnetries exist, all of which coimnonly use mercury electrodes (dropping mercury electrodes (DMEs) or mercury film electrodes) [7, 17]. 

Anodic-stripping voltaimnetry (ASV) is used for the analysis of cations in solution, particularly to detemiine trace heavy metals. It involves pre-concentrating the metals at the electrode surface by reducmg the dissolved metal species in the sample to the zero oxidation state, where they tend to fomi amalgams with Hg. Subsequently, the potential is swept anodically resulting in the dissolution of tire metal species back into solution at their respective fomial potential values. The detemiination step often utilizes a square-wave scan (SWASV), since it increases the rapidity of tlie analysis, avoiding interference from oxygen in solution, and improves the sensitivity. This teclmique has been shown to enable the simultaneous detemiination of four to six trace metals at concentrations down to fractional parts per billion and has found widespread use in seawater analysis. 

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... 

Osteryoung J 1991 Square-wave and staircase voltammetry at small electrodes Microelectrodes Theory and Applications (Nate ASI Series) ed M I Montenegro, M A Queiros and J L Daschbach (Dordrecht Kluwer)... 

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. 

Potential-excitation signals and voltammograms for (a) normal pulse polarography, (b) differential pulse polarography, (c) staircase polarography, and (d) square-wave polarography. See text for an explanation of the symbols. Current is sampled at the time intervals indicated by the solid circles ( ). 

The factor TC/4 compensates for the square wave in the output. Tuning rules are given in Table 8-4. 

Electrically insulating materials can be analyzed in HF-plasma SNMS by applying a square-wave HF in the 100 kHz range to the sample (Fig. 3.34). Dielectric charge transfer at the start of a period shifts the surface potential to the amplitude Uhfm applied. Ar" ions are attracted from the plasma and sputter the surface until the end of At . The potential increase AU = 1-100 V caused by their charge is then converted to a positive absolute AU which is reduced to less than 1 V within <0.1 ps by the... 

Assuming that the cyclic waveform used in the previous section was sinusoidal then the effect of using a square wave is to reduce, at any frequency, the level of stress amplitude at which thermal softening failures start to occur. This is because there is a greater energy dissipation per cycle when a square wave is used. If a ramp waveform is applied, then there is less energy dissipation per cycle and so higher stresses are possible before thermal runaway occurs. 

Ferroresonant The D.C. is switched via thyristors to produce a square wave output. 

Rowlands and Bentley have provided an account of the possibilities for continuously monitoring corrosion rates by polarisation resistance measurements, and they also describe the development of a commercial instrument, which uses low-frequency square-wave current to polarise the test specimens. 

Neufeld, P. and Queenan, E. D., Frequency Dependence of Polarisation Resistance Measured with Square Wave Alternating Potential , Br. Corros. J., 5, 72-75, March (1970) Fontana, M. G., Corrosion Engineering, 3rd edn., McGraw-Hill, pp 194-8 (1986) Dawson, J. L., Callow, L. M., Hlady, K. and Richardson, J. A., Corrosion Rate Determination By Electrochemical Impedance Measurement , Conf. On-Line Surveillance and Monitoring of Process Plant, London, Society of Chemical Industry (1977)... 

In a steady state experiment the PIA signal Y is proportional to neq. Measuring the PIA with a lock-in amplifier means exciting the sample with a periodic time-dependent pump photon flux. The latter can be approximated by a square wave that switches between a constant flux and zero photons with a frequency /= 1/r. As shown in Refs. [32] and [33] the PIA signal, measured with a lock-in amplifier Y, shows the same functional dependence on p as ncq in Eq. (9.5). For the monomo-lecular (p-1) and bimolecular (//=2) case the influence of r depends on t, the lifetime of the observed states, as follows ... 

Ion chromatography (see Section 7.4). Conductivity cells can be coupled to ion chromatographic systems to provide a sensitive method for measuring ionic concentrations in the eluate. To achieve this end, special micro-conductivity cells have been developed of a flow-through pattern and placed in a thermostatted enclosure a typical cell may contain a volume of about 1.5 /iL and have a cell constant of approximately 15 cm-1. It is claimed15 that sensitivity is improved by use of a bipolar square-wave pulsed current which reduces polarisation and capacitance effects, and the changes in conductivity caused by the heating effect of the current (see Refs 16, 17). 

DC = direct current NP = normal pulse DP = differential pulse SW = square wave AC = alternating... 

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.)... 

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

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