Sweeping technique

Potential Sweep Method, In the transient techniques described above, a set of measurements of the potential for a given current or the current for a given potential is measured in order to construct the current-potential function, i = f(E). For example, the Tafel lines shown in Figure 6.20 were constructed from a set of galvanostatic transients of the type shown in Figure 6.18. In the potential sweep technique, i = f(E), curves are recorded directly in a single experiment. This is achieved by sweeping the potential with time. In linear sweep voltammetry, the potential of the test electrode is varied linearly with time (Fig. 6.23a). If the sweep rate is... 

Designations such as upfield/highfiekT or "downfield/lowfield" are meaningless in a pulse Fourier transform experiment, because, in contrast to the older Continuous Wave ( Sweep-") technique all nuclei experience exactly the same pulse frequency, i.e., the same excitation field. Thus, it is better to say that a nucleus is shielded or deshielded, respectively, and that a signal is shifted to lower or higher frequency, respectively487. Since, however, the term .vyu-upheld rule is well established in the literature, it is used in this section. 

The first observation of Zn NMR in zinc metal was by Abart and coworkers in a measurement at 4.2 K, using a field sweeping technique, which yielded a value of = 12 73(4) MHz. No subsequent NMR observation has been reported. A measure of the temperature dependence of the Zn nuclear quadrupole coupling in zinc metal has been obtained from time differential perturbed angular correlation (TDPAC) measurements using an excited state of Zn (/ = 9/2 605 keV). However, the use of liquid helium temperatures and exotic short-lived isotopes precludes the adoption of these techniques for general material characterization. 

In the linear sweep technique, a recording of the current during the potential sweep (say, from 0.0 V on the normal hydrogen scale to 1.2 V positive to it in a 1 M H2 S04 solution) completes one act of the basic experiment. However, and hence the title of this part of the chapter, the electronics can be programmed so that when the electrode potential reaches 1.20 V, it begins a return sweep, going from 1.2 to 0.00 V, NHS. Completion of the two sweeps and back to the starting point is one act in what is called cyclic voltammetry.16 The current is displayed on a cathode ray oscilloscope screen on an X Y recorder, and it is normal to cany out not one but several and often many cycles. Much information is sometimes contained in the difference between the second and other sweeps in comparison with the first (Fig. 8.10). 

Historically, the potential sweep technique and cyclic voltammetry were developed for analysis (as successors to polarography) and much of the theoretical development is concerned with the situation under conditions of diffusion control, for that is where the analytical applications are most readily made. In many of these approaches, the underlying assumption is that the electron transfer that must necessarily occur at the interface is a fast process and plays little part in determining the dependence of the observed current upon potential or upon the concentration of the reactant. However, these assumptions may not always apply. 

Thus, potential sweep techniques and cyclic voltammetry are excellent tools for... 

A complete comprehension of Single Pulse electrochemical techniques is fundamental for the study of more complex techniques that will be analyzed in the following chapters. Hence, the concept of half-wave potential, for example, will be defined here and then characterized in all electrochemical techniques [1, 3, 8]. Moreover, when very small electrodes are used, a stationary current-potential response is reached. This is independent of the conditions of the system prior to each potential step and even of the way the current-potential was obtained (i.e., by means of a controlled potential technique or a controlled current one) [9, 10]. So, the stationary solutions deduced in this chapter for the current-potential curves for single potential step techniques are applicable to any multipotential step or sweep technique such as Staircase Voltammetry or Cyclic Voltammetry. Moreover, many of the functional dependences shown in this chapter for different diffusion fields are maintained in the following chapters when multipulse techniques are described if the superposition principle can be applied. 

In this section, we will show that the stationary responses obtained at microelectrodes are independent of whether the electrochemical technique employed was under controlled potential conditions or under controlled current conditions, and therefore, they show a universal behavior. In other words, the time independence of the I/E curves yields unique responses independently of whether they were obtained from a voltammetric experiment (by applying any variable on time potential), or from chronopotentiometry (by applying any variable on time current). Hence, the equations presented in this section are applicable to any multipotential step or sweep technique such as Staircase Voltammetry or Cyclic Voltammetry. 

This chapter offers a study of the application of the multipulse and sweep techniques Cyclic Staircase Voltammetry (CSCV) and Cyclic Voltammetry (CV) to the study of more complex electrode processes than single charge transfer reactions (electronic or ionic), which were addressed in Chap. 5. 

This technique could be simplified (l) using low temperature (25 C) gas-sweeping techniques for removal of Oe(lV) from 12 N SCI eolns. and (2) by absorbing Ge(l7) from the gas stream on an anion exchange resin (Dowex-1X8, 30-100 mesh). It was studied by using Ge77(l2 hr) As ikO hr) tracer. 

The NMR parameters for titanium metal deduced in earlier studies by field sweeping techniques (Narath 1967, Ebert et al. 1986) have been confirmed by more recent room temperature FT NMR (Bastow et al. 1998a). The value of Xq deduced from the ( /2, 2) satellite transitions was used in an accurate simulation of the central transition, which required an axial Knight shift of 70 10 ppm. The Ti NMR spectra of a number of titanium aluminide alloys and TiAg have also been reported... 

The nature and structure of surface intermediates in hydrocarbon adsorption has been investigated using galvanostatic (constant current) and potential sweep techniques (7, 10, 172-174 or radiotracer methods (175. Niedrach s (172, 173 galvanostatic results with C1-C4 alkanes and with ethylene indicate the existence of common, partially oxidized surface species, despite differences in the initially adsorbed hydrocarbons. Methane adsorption is very slow, but higher saturated hydrocarbons adsorb faster and at similar rates. Potentiostatic adsorption followed by an anodic potential sweep gives two peaks [Fig. 14 (174 corresponding to different adsorbed species. The intermediate responsible for the peak at low potentials (0.7-... 

The above experiment is called a frequency sweep experiment. Equivalent results are obtained if the radio frequency is held constant and the magnetic field is changed. This is called the field sweep technique. 

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