Limitations of electrochemical techniques

Specifics on the types and rates of microbiological attack. These must be determined by using other methods such as chemical and microbiological analysis of the solution and materials from the corrosion sites. Consideration must be given to limitations of electrochemical techniques for MIC studies, noted previously under Corrosion Testing Laboratory Tests and subsequent subsections. 

Use and Limitations of Electrochemical Techniques A major caution must be noted as to the general, indiscriminate use of all electrochemical tests, especially the use of AC and EIS test techniques, for the study of corrosion systems. AC and EIS techniques are applicable for the evaluation of very thin films or deposits that are uniform, constant, and stable—for example, thin-film protective coatings. Sometimes, researchers do not recognize the dynamic nature of some passive films, corrosion products, or deposits from other sources nor do they even consider the possibility of a change in the surface conditions during the course of their experiment. As an example, it is note-... 

Dexter, S.C., Sibert, A.W., Duquette, D.J. and Videla, H.A., 1989, Use and limitation of electrochemical techniques for investigating microbial corrosion. Proc. NACE Conf Corrosion 89. Paper 616. 

As with in vivo voltammetry, a variety of electrochemical techniques have been used for the stripping step. Because of its simplicity, linear sweep voltammetry has enjoyed widespread use however, the detection limit of this technique is limited by charging current. Differential pulse has become popular because it discriminates against the charging current to provide considerably lower detection limits. 

In the application of XAS to the study of fuel cell catalysts, the limitations of the technique must also be acknowledged the greatest of which is that XAS provides a bulk average characterization of the sample, on a per-atom basis, and catalyst materials used in low temperature fuel cells are intrinsically nonuniform in nature, characterized by a distribution of particle sizes, compositions, and morphologies. In addition, the electrochemical reactions of interest in fuel cells take place at the surface of catalyst par-... 

The use of polarographic assays for the determination of drugs in blood is the most demanding on the detection limitations of the technique. Differential pulse polarography, stripping voltammetry, and LCEC are the only electrochemical methods currently available for routine determination of drugs below 1.0 ng/mL of blood. 

The following table provides guidance in selection of electrochemical techniques by providing the relative sensitivities of various methods.1 The limit of detection of lead, defined as the minimum detectable quantity (on a mole basis), is used as the basis of comparison. 

To many analysts the major limitation of electrochemical detection for liquid chromatography (LCEC) is its limited applicability to gradient elution techniques. Amperometric electrochemical detectors exhibit both the best and the worst characteristics of solute property and bulk property detectors. While the Faradaic current arises only from the solute, the non-Faradaic current arises from... 

Virtually any electrochemical technique may be used for either analytical or mechanistic (our focus) studies. The merits and limitations of each technique and the information that can be gleaned are discussed for direct-current (d.c.) polarography, pulse polarography, alternating-current (a.c.) polarography and cyclic voltammetry. Con-trolled-potential coulometry is technically not a voltammetric technique (there is no variation of potential), and this technique is considered in 12.3.5. 

It is possible to summarize the advantages and limitations of the techniques so that they can be compared. This is done more thoroughly in Chapter 8 where Figures 8.1, 8.2 and 8.3 give a breakdown of how to select repairs but the headings below summarize the issues with respect to the electrochemical techniques discussed in this chapter. 

During the early history of electrochemical instrumentation, much of the work was directed more towards understanding the chemistry involved, rather than the development of instrumentation. By the 1920 s, however, scientists confronted by the limitations of existing techniques, particularly the fact that they were manual and required considerable skill to operate, sought to exploit the newly available electromechanical technology. One of the first improvements to arise was instruments that could automatically record an experiment. 

In all experiments, precise control or measurement of potential, charge and/ or current is an essential requirement of the experiment. In addition, modern electrochemical investigation is often supplemented with in situ spectroscopic techniques as an independent probe to monitor changes that occur at the electrode surface this introduces further design criteria. Consequently an electrochemical experiment rapidly becomes complex, and it is the aim of this chapter to examine some of the limitations of electrochemical equipment and to outline the precautions that must necessarily be taken to obtain quantitative data and to avoid erroneous results or incorrect conclusions. Features of cell design will be discussed initially, followed by a section on instrumentation. 

Pulse voltammetry techniques are characterized by a succession of potential steps. During the sequential potential steps, the rates of current decay of the capacitive (If.) and the faradaic currents (7p) are essentially different (specifically, while 4 in Equation 2.1 decays exponentially with time. Ip decreases as a function of t 2, characteristic of a diffusion-controlled electrochemical reaction). In this way, the rate of decay of If. is significantly faster than that of Ip, and thus I. is negligible at a time of 57 uQ after the potential step is imposed (where 7 uQ is the time constant, Tj-gy, for the electrochemical cell having values from microseconds to milliseconds, and R is the uncompensated resistance between reference and working electrodes). Consequently, Ip is the main contribution to the measured current I when its value is measured at the end of a potential step. The detection limits of these techniques therefore fall around 10 M making them suitable for quantitative analysis. 

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