Introduction potentiostat

The solution to reference electrode instabiUty is the introduction of a third or auxiUary electrode. This particular electrode is intended to carry whatever current is required to keep the potential difference between the working and reference electrodes at a specified value, and virtually all potentiostats (instmments designed specifically for electrochemistry) have this three-electrode configuration. Its use is illustrated in Figure 3. 

In this section an attempt is made to give a more detailed introduction to experimental procedures, as well as to some of the ideas where the use of the potentiostat has helped in the understanding of corrosion processes. 

The potentiostat is particularly useful in determining the behaviour of metals that show active-passive transition. Knowledge of the nature of passivity and the probable mechanisms involved has accumulated more rapidly since the introduction of the potentiostatic technique. Perhaps of more importance for the subject at hand are the practical implications of this method. We now have a tool which allows an operational definition of passivity and a means of determining the tendency of metals to become passive and resist corrosion under various conditions. 

Although the practical utility of a small potentiostatic perturbation is not great, it is worth treating this case from a methodological point of view and as an introduction to the discussion of the a.c. methods. 

As described in the introduction, submicrometer disk electrodes are extremely useful to probe local chemical events at the surface of a variety of substrates. However, when an electrode is placed close to a surface, the diffusion layer may extend from the microelectrode to the surface. Under these conditions, the equations developed for semi-infinite linear diffusion are no longer appropriate because the boundary conditions are no longer correct [97]. If the substrate is an insulator, the measured current will be lower than under conditions of semi-infinite linear diffusion, because the microelectrode and substrate both block free diffusion to the electrode. This phenomena is referred to as shielding. On the other hand, if the substrate is a conductor, the current will be enhanced if the couple examined is chemically stable. For example, a species that is reduced at the microelectrode can be oxidized at the conductor and then return to the microelectrode, a process referred to as feedback. This will occur even if the conductor is not electrically connected to a potentiostat, because the potential of the conductor will be the same as that of the solution. Both shielding and feedback are sensitive to the diameter of the insulating material surrounding the microelectrode surface, because this will affect the size and shape of the diffusion layer. When these concepts are taken into account, the use of scanning electrochemical microscopy can provide quantitative results. For example, with the use of a 30-nm conical electrode, diffusion coefficients have been measured inside a polymer film that is itself only 200 nm thick [98]. 

In situ polymerization, and electrochemical polymerization in particular [22], is an elegant procedure to form an ultra thin MIP film directly on the transducer surface. Electrochemical polymerization involves redox monomers that can be polymerized under galvanostatic, potentiostatic or potentiodynamic conditions that allow control of the properties of the MIP film being prepared. That is, the polymer thickness and its porosity can easily be adjusted with the amount of charge transferred as well as by selection of solvent and counter ions of suitable sizes, respectively. Except for template removal, this polymerization does not require any further film treatment and, in fact, the film can be applied directly. Formation of an ultrathin film of MIP is one of the attractive ways of chemosensor fabrication that avoids introduction of an excessive diffusion barrier for the analyte, thus improving chemosensor performance. This type of MIP is used to fabricate not only electrochemical [114] but also optical [59] and PZ [28] chemosensors. 

Before the introduction of potentiostats in the early 1960s, the study of electrode processes was done mainly with two-electrode systems in which the functions of the reference and the counter electrode were unified in one simple electrode. Such an electrode is a non-polarisable electrode with a relatively large surface to be sure that it can conduct a certain amount of current due to occurring electrochemical processes. 

A discussion of the instrumental aspects of voltammetry and leading references to the original literature can be found in some of the monographs already cited in the introduction [1-4]. The essential units are the potentiostat, a triangular waveform generator, and a recording device. The latter is most conveniently a digital oscilloscope or a transient recorder. Commercial equipment with the units combined into one instrument controlled by a PC is available from a number of manufacturers. Also, homebuilt instrumentation... 

As mentioned in the introduction, the electrical nature of a majority of electrochemical oscillators turns out to be decisive for the occurrence of dynamic instahilities. Hence any description of dynamic behavior has to take into consideration all elements of the electric circuit. A useful starting point for investigating the dynamic behavior of electrochemical systems is the equivalent circuit of an electrochemical cell as reproduced in Fig. 1. The parallel connection between the capacitor and the faradaic impedance accounts for the two current pathways through the electrode/electrolyte interface the faradaic and the capacitive routes. The ohmic resistor in series with this interface circuit comprises the electrolyte resistance between working and reference electrodes and possible additional ohmic resistors in the external circuit. The voltage drops across the interface and the series resistance are kept constant, which is generally achieved by means of a potentiostat. 

We have concentrated on explaining the control over drop lifetime that may be exercised by adjustment of the head of mercury. You will learn later that more advanced techniques depend on the absolute reproducibility of this droptime. This necessitates the introduction of a mechanical tapping device controlled by the potentiostat which can produce drops with lifetimes of say 0.5,1 or 2 s with great precision. Since this facility is built into modern instrumentation it is now also routinely used for dc polarography. 

There are many apphcations where multichannel impedance measurement systems are particularly useful. The throughput of testing in a laboratory can be increased either by the use of multiplexed systems where multiple cells are connected and are automatically tested in sequence by, for example, a potentiostat and FRA, or by the use of true parallel measurement systems where each cell has access to its own potentiostat and FRA. The parallel system, of course, is the more efficient method since aU cells can be tested simultaneously however the equipment required for this is more expensive since there are separate potentiostats and FRAs on every channel. The introduction of multichannel systems, however, has seen a reduction... 

With continued improvement in the performance of operational amplifiers, operational characteristics of potentiostats have also advanced. The original vacuum tube op-amps were surpassed by the introduction of solid state devices, first discrete transistor based devices and then by integrated circuits. Increasing the level of integration of electronic circuits has resulted in the... 

Both potentiostats and waveform generators have benefited substantially in recent years from the introduction of operational amplifiers, as well as from the availability of desktop computers. Electrochemical cell design is now wellestablished both for static systems and for detectors situated in flowing reagent streams. 

A survey of the literature on electropolymerization covering the past few years indicates a growing use of modem electroanalytical techniques to investigate the nature of the electrode processes involved of particular interest are linear and cyclic voltammetry together with the use of the rotating disc and ring-disc electrodes. This development is most likely associated with the introduction of reliable, stable potentiostats incorporating solid-state electronics. 

With regard to the capacitive currents, the galvanostatic and potentiostatic techniques are superior because the double layer charging can be accelerated in these cases. The attraction of impedance measurements lies in the high accmacy of the data thus obtained. They are particularly useful for systems in which adsorption of the reactants or of intermediates plays an important role. The discussion of such cases is, however, outside the scope of this introduction. 

The electrochemical polymerization of PEDOT allows the introduction of a wide range of counterions since the latter can be added in the form of salts to the reaction mixture. The choice of counterions is limited only by their solubility and stability under the reaction conditions. A list of selected counterions used in the electrochemical polymerization is shown in Table 7.1 together with the methods used and the conductivities obtained. Different polymerization methods such as potentiostatic, galvanostatic, and repetitive multisweep result in different PEDOT films with differing properties. The in situ photoelectropolymerization uses light as well as a constant potential. Brief descriptions of polymerization methods can be found in the description of Table 7.1. 

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