Potential sweep methods applications

Again, application of the principle to the simple potential-step method appears trivial or superfluous. However, it is of quite great important for other types of potential control, namely double potential step, cyclic potential step [73], and especially the linear potential sweep method [21, 22, 73]. In all these techniques, sets of data Jf ( ) /f (f) E can be obtained, thus enabling kt(E) to be determined from eqn. (100). For more details, the reader is referred to the quoted textbooks. 

An interesting point to consider is the effect of surface roughness on the range of applicability of the linear potential sweep method. The... 

A similar situation arises in the application of the linear potential sweep method, to be discussed. In this case a potential that varies linearly with time is applied by the potentiostat. A typical... 

Bard AJ, Faulkner LR. Controlled potential microelectrode techniques. Potential Sweep Methods. In Electrochemical Methods Fundamentals and Applications. New York Wiley and Sons, 1980 213-248. 

Kant, R. (2010) General Theory of Arbitrary Potential Sweep Methods on an Arbitrary Topography Electrode and Its Application to Random Surface Roughness. /, Phys. Chem C. Vol.114, No.24, Qune 2010), pp. 10894-10900, ISSN 1932-7447 Lange, R. Doblhofer, K. (1987) The Transient Response of Electrodes Coated with Membrane-Type Polymer Films under Conditions of Diffusion and Migration of the Redox Ions. /, Electroanal. Chem. Vol.237, No.l, (November 1987), pp. 13-26, ISSN 1572-6657... 

The basic theory of mass transfer to a RHSE is similar to that of a RDE. In laminar flow, the limiting current densities on both electrodes are proportional to the square-root of rotational speed they differ only in the numerical values of a proportional constant in the mass transfer equations. Thus, the methods of application of a RHSE for electrochemical studies are identical to those of the RDE. The basic procedure involves a potential sweep measurement to determine a series of current density vs. electrode potential curves at various rotational speeds. The portion of the curves in the limiting current regime where the current is independent of the potential, may be used to determine the diffusivity or concentration of a diffusing ion in the electrolyte. The current-potential curves below the limiting current potentials are used for evaluating kinetic information of the electrode reaction. 

A steady state is independent of the details of the experiment used in attaining it. Thus, under conditions where a steady state is attained, e.g., under convective conditions in an - electrochemical cell, the application of a constant current leads to a constant potential and similarly the application of a constant potential leads to the same constant current. Voltammetric steady states are most commonly reached using linear potential sweeps (or ramps) in a single or cyclic direction at a UME or RDE. A sigmoidally shaped current (l)-potential (E) voltammogram (i.e., a steady-state voltammogram) is recorded in the method known as steady-state voltammetry as shown in the Figure. Characteristics of the... 

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... 

However, it is not ideal to use cyclic or linear-sweep voltammetry as a method for analytical purposes. A more suitable method is chronoamper-ometry, which in fact is the application of a constant potential located in the limiting-current plateau and measurement of the limiting-current as a function of time. With this method, it is possible to measure continuously, and the required equipment setup becomes much more simplified. 

Electrochemical systems can be studied with methods based on impedance measurements. These methods involve the application of a small perturbation, whereas in the methods based on linear sweep or potential step the system is perturbed far from equilibrium. This small imposed perturbation can be of applied potential, of applied current or, with hydrodynamic electrodes, of convection rate. The fact that the perturbation is small brings advantages in terms of the solution of the relevant mathematical equations, since it is possible to use limiting forms of these equations, which are normally linear (e.g. the first term in the expansion of exponentials). 

A sensitivity increase and lower detection limit can be achieved in various ways with the use of voltammetric detectors rather than amperometry at fixed potential or with slow sweep. The principle of some of these methods was already mentioned application of a pulse waveform (Chapter 10) and a.c. voltammetry (Chapter 11). There is, nevertheless, another possibility—the utilization of a pre-concentration step that accumulates the electroactive species on the electrode surface before its quantitative determination, a determination that can be carried out by control of applied current, of applied potential or at open circuit. These pre-concentration (or stripping) techniques24"26 have been used for cations and some anions and complexing neutral species, the detection limit being of the order of 10-10m. They are thus excellent techniques for the determination of chemical species at trace levels, and also for speciation studies. At these levels the purity of the water and of the... 

One of the main uses of digital simulation - for some workers, the only application - is for linear sweep (LSV) or cyclic voltammetry (CV). This is more demanding than simulation of step methods, for which the simulation usually spans one observation time unit, whereas in LSV or CV, the characteristic time r used to normalise time with is the time taken to sweep through one dimensionless potential unit (see Sect. 2.4.3) and typically, a sweep traverses around 24 of these units and a cyclic voltammogram twice that many. Thus, the explicit method is not very suitable, requiring rather many steps per unit, but will serve as a simple introduction. Also, the groundwork for the handling of boundary conditions for multispecies simulations is laid here. 

In many preparative applications of EGBs the rate-determining step in product formation is the proton transfer. This is often the case when the deprotonated substrate is removed in a fast product-forming reaction (cf. Sec. II.B). For EGBs formed in situ, electrochemical methods such as cyclic voltammetry (CV), derivative cyclic voltammetry (DCV), linear sweep voltammetry (LSV), double-potential-step chronoamperometry (DPSC), and other electroanalytical methods can often be used to estimate the kinetics of proton transfer from the substrate to the EGB. When the EGBs are formed ex situ (because the acidic... 

An alternative experimental approach involves stopping the sweep beyond as before, and allowing the current to decay to a small value (so that O is depleted in the vicinity of the electrode or the concentration gradient of O is essentially zero near the electrode). Then one continues the scan and measures /p from the potential axis as a baseline (Figure 6.6.3). The application of this method requires convection-free conditions (quiet, vibration-free solutions, and shielded electrodes oriented to prevent convection from density gradients see Section 8.3.5), because the waiting time for the current decay must be 20 to 50 times the time needed to traverse a peak. 

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