Electrochemical instrumentation

Because the chief electrochemical variables are all analog quantities (at least in the ranges of normal interest), our first concerns are with circuitry for controlling and measuring voltages, currents, and charges in the analog domain. The circuit elements best suited to these jobs are operational amplifiers. We must explore their properties before we can understand the way in which the amplifiers are assembled into instruments. 

Operational amplifiers are devices with special properties, and are almost always found as packaged integrated circuits. We have no interest in the contents of the amplifier our concern is strictly with its behavior as a unit in a circuit. 

The two input terminals are labeled with signs in the manner depicted there. The top one is called the inverting input and the bottom one is the noninverting input. The fundamental property of the amplifier is that the output, e, is the inverted, amplified voltage difference e, where e is the voltage of the inverting input with respect to the noninverting input. That is. 

The names of the inputs come from a different way of looking at e. We could picture the system as having two independent inputs, e- and +, both measured with respect to ground. The output is then 

In most discussions of circuitry, we assume ideal behavior, because it simplifies the approach. For most electrochemical applications, available devices perform so well that nonidealities are negligible. However, in demanding circumstances, nonideal properties may have to be recognized. 

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Electrochemical measurements are made in an electrochemical cell, consisting of two or more electrodes and associated electronics for controlling and measuring the current and potential. In this section the basic components of electrochemical instrumentation are introduced. Specific experimental designs are considered in greater detail in the sections that follow. 

The nature of electrochemical instruments makes them very attractive for decentralized testing. For example, compact, battery-operated voltammetric analyzers, developed for on-site measurements of metals (9,10), readily address the growing needs for field-based environmental studies. Similarly, portable (hand-held) instruments are being designed for decentralized clinical testing (11). 

Finally, the recording of many signals from the output of the analytic and electrochemical instrumentation requires a reliable multi-pen recorder or an equivalent recording system based on a data acquisition card and appropriate software. The recorded signals are normally in the range of a few mV to 10V. The use of reliable temperature controllers and thermocouples is also crucial for the success of the experiments. A lot of suppliers of such equipment can be easily found and will not be reported here. 

Electrochemical Measurements. The electrochemical instrumentation included (a) a PAR model 263A potentiostat, (b) a PAR PowerCV software for data acquisition and analysis, and (c) a Dell Pentium IV computer. 

Different experimental approaches were applied in the past [6, 45] and in recent years [23, 46] to study the nature of the organic residue. But the results or their interpretation have been contradictory. Even at present, the application of modem analytical techniques and optimized electrochemical instruments have led to different results and all three particles given above, namely HCO, COH and CO, have been recently discussed as possible methanol intermediates [14,15,23,46,47]. We shall present here the results of recent investigations on the electrochemical oxidation of methanol by application of electrochemical thermal desorption mass spectroscopy (ECTDMS) on-line mass spectroscopy, and Fourier Transform IR-reflection-absorption spectroscopy (SNIFTIRS). 

Electrochemical Instrumentation. For the Ru complexes, a 1 cm diameter platinum disk brazed onto a brass holder was used as a working electrode. It was masked with ChemGrip (a teflon based epoxy) except for the upper face. Prior to use, it was polished with 1 micron diamond paste (Buehler) and rinsed with water, acetone and methanol. The working electrode for each Os complex was the uppermost platinum layer of a platinum/carbon layered synthetic microstructure (LSM) (Energy Conversion Devices). The LSM consisted of 200 layer pairs of carbon and platinum whose thicknesses were 24.4 and 17.0 A, respectively and where platinum was the outermost layer. The LSM was placed in 1.0 M H2SO4 and cleaned... 

It must be however underlined that, in measuring the peak-to-peak separation, a departure of 10-20 mV from the theoretical value (especially at relatively high scan rates) does not compromise the criterion of reversibility, in that the eventual presence of solution resistances not adequately compensated by the electrochemical instrumentation (see Chapter 3, Section 2) tends to lay down the forward/ reverse peaks system, thus increasing the relative Aisp value. 

There are many publications which discuss not only the principles of electrochemistry, but also its practical aspects (electrodes, cells, solvents, supporting electrolytes, and so on).1-8 In addition, all the electrochemical instrumentation manufacturers have catalogues of commercial products which allow proper experiments to be carried out. Therefore, the present discussion will be limited to point out the minimal basic knowledge required to set up an electrochemical experiment. 

One must keep in mind that modern electrochemical instrumentation compensates for the potential drop i (Rn + Rnc) through the use of appropriate circuitry (positive feedback compensation). This adds a supplementary potential to the input potential of the potentiostat (equal to the ohmic drop of the potential), which is generated by taking a fraction of the faradaic current that passes through the electrochemical cell, such that in favourable cases there will be no error in the control of the potential. However, such circuitry can give rise to problems of reliability in the electrochemical response on occasions when an overcompensation is produced. 

More often the passive layer is broken down locally and then the steel is said to be attacked by localized corrosion, the most important forms being pitting, crevice corrosion, and corrosion cracking. Most often the localized corrosion is caused by halogen ions such as chloride, bromide, and iodide. Pitting or pitting corrosion is seen as small pinholes on the surface of the steel. This section describes electrochemical instrumental methods to investigate and measure this form of corrosion attack. 

A uniform approach to trace the analysis and evaluation of electrode kinetics of the Cd(II)/Cd(Hg) system with fast Fourier transform electrochemical instrumentation was presented by Schiewe et al. [34]. 

Electrochemical Instrumentation - Roles of Operational Amplifiers and Microcomputers... 

The previous sections outlined various electrochemical techniques. For most of these techniques, instruments are commercially available and we can use them conveniently. The common features of modern electrochemical instruments are that operational amplifiers and microcomputers play important roles in them. These are discussed in this section. 

Operational amplifiers, which are the main components of an analog computer, were first used in electrochemical instrumentation at the beginning of the 1960s [26]. Because they are extremely useful in measuring and controlling the electrode potentials and the currents that flow at the electrodes, electrochemical instruments were completely modernized by their introduction. Today, most electrochemical instruments are constructed using operational amplifiers. Knowledge of operational amplifiers will help the reader to understand electrochemical instruments and to construct a simple apparatus for personal use. 

Potential and Current Controls in Electrochemical Instrumentation by Use of Operational Amplifiers... 

Commercial processor-based electrochemical instruments are available in two forms. In the first configuration, a general-purpose laboratory or personal computer is interfaced to the analog instrumentation. In the other approach, the package is integrated in such a way that the processor is dedicated to the electrochemical experiments. Several dedicated, processor-based pulse polarographs... 

The goal of this chapter was to present an overview of electrochemical instrumentation, not to carry the reader to the point of instrument construction. Most universities have one or more courses in electronics at an adequate level to build rudimentary apparatus. The nature of electrochemistry is such that some knowledge of electronics is extremely useful to good science. 

Note Each year the August 15 issue of Analytical Chemistry contains extensive listing of suppliers of electrochemical instruments, electrodes and accessories, with current phone numbers. 

The use of a tubular carbon electrode (TCE) for the electrochemical oxidative determination of ascorbic acid [159], L-dopa [160], and methyldopa [161] in dosage forms has been described. The flow system, electrode assembly, and electrochemical instrumentation required for these assays are shown in Figure 26.11. The method is based on continuous analysis in flowing streams... 

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