Potentiostat selection

The determination of polarisation curves of metals by means of constant potential devices has contributed greatly to the knowledge of corrosion processes and passivity. In addition to the use of the potentiostat in studying a variety of mechanisms involved in corrosion and passivity, it has been applied to alloy development, since it is an important tool in the accelerated testing of corrosion resistance. Dissolution under controlled potentials can also be a precise method for metallographic etching or in studies of the selective corrosion of various phases. The technique can be used for establishing optimum conditions of anodic and cathodic protection. Two of the more recent papers have touched on limitations in its application and differences between potentiostatic tests and exposure to chemical solutions. ... 

The selective net loss of a component such as zinc, aluminium or nickel from copper-base alloys sometimes occurs when these alloys corrode. Early studies of the phenomenon were done by simple immersion. More recently, however, the potential-pH dependence of de-alloying has been examined , and it appears that this approach can provide a much more detailed understanding of the mechanism. Future experimental work is expected to include potentiostatic and potentiodynamic techniques to a much greater extent. 

Dissolution kinetics are influenced by pH, potential and the ions present in the test solution, and this forms the basis of selective metallographic etching techniques that have been used for some time . The potentiostat is often used to hold the potential of a multi-phase alloy constant at a level... 

Steady state measurements of NO decomposition in the absence of CO under potentiostatic conditions gave the expected result, namely rapid self-poisoning of the system by chemisorbed oxygen addition of CO resulted immediately in a finite reaction rate which varied reversibly and reproducibly with changes in catalyst potential (Vwr) and reactant partial pressures. Figure 1 shows steady state (potentiostatic) rate data for CO2, N2 and N2O production as a function of Vwr at 621 K for a constant inlet pressures (P no, P co) of NO and CO of 0.75 k Pa. Also shown is the Vwr dependence of N2 selectivity where the latter quantity is defined as... 

Electrodeposition of metals can be performed under different electrochemical modes. In the work mentioned in Ref. [18], it was performed in potentiostatic mode. The potential value for formation of platinum nanoparticles is —25 mV vs. SCE the deposition is performed from 2.5 mM solution of H2[PtCl6] in 50 mM KCl. The size of nanoparticles formed depends on the reduction charge. Continuous monitoring of the charge in potentiostatic mode is provided by different potentiostats, for example, by Autolab-PG-stat (EcoChemie, The Netherlands). Conditions for deposition of other metals should be selected according to their electrochemical properties. 

The disadvantages described above in terms of the irreversibility of the polyion response stimulated further research efforts in the area of polyion-selective sensors. Recently, a new detection technique was proposed utilizing electrochemically controlled, reversible ion extraction into polymeric membranes in an alternating galvanostatic/potentiostatic mode [51]. The solvent polymeric membrane of this novel class of sensors contained a highly lipophilic electrolyte and, therefore, did not possess ion exchange properties in contrast to potentiometric polyion electrodes. Indeed, the process of ion extraction was here induced electrochemically by applying a constant current pulse. 

Controlled potential methods have been successfully applied to ion-selective electrodes. The term voltammetric ion-selective electrode (VISE) was suggested by Cammann [60], Senda and coworkers called electrodes placed under constant potential conditions amperometric ion-selective electrodes (AISE) [61, 62], Similarly to controlled current methods potentiostatic techniques help to overcome two major drawbacks of classic potentiometry. First, ISEs have a logarithmic response function, which makes them less sensitive to the small change in activity of the detected analyte. Second, an increased charge of the detected ions leads to the reduction of the response slope and, therefore, to the loss of sensitivity, especially in the case of large polyionic molecules. Due to the underlying response mechanism voltammetric ISEs yield a linear response function that is not as sensitive to the charge of the ion. 

Experimental CFTs were determined by Brigham and Tozer in three different ways based on chloride solutions two electrochemical and one chemical. In electrochemical testing in sodium chloride (NaCl) solution, the methods involved recording the current while either increasing the potential at suitable selected temperatures (apotentiodynamic test) or increasing the temperature at suitable fixed potentials (a potentiostatic test). Alternatively, a potential was established by the redox couple (Fe(in)/Fe(n) in a simple immersion test in ferric chloride solution. 

To determine the potential-mdependent CPT by the potentiostatic method, it is necessary to select a potential placed between the pitting potential and the transpassive potential for the relevant stainless steel (Fig. 13). A suitable choice of potential is 700 mV SCE, and in order to obtain compatibility for a range of stainless steels, the polarization was always... 

From an exhaustive potentiostatic electrolysis, the product(s) formed at the selected electrode potential can be isolated. Preparative and analytical techniques are available to determine the composition of the product mixture and the structure of its components. Mechanistic reasoning will often allow defining the reaction steps. Even more information about the reaction can be gained from electrolysis experiments at various defined potentials, for example, after each peak in the cyclic voltammogram of the substrate. 

In some problematic cases, there will be no obvious limits available for the choice of the current density in galvanostatic operation. Concurrent reactions take place, resulting in a poor selectivity. But here the potentiostatic operation also cannot demonstrate its advantages, and probably the simpler galvanostatic operation may be applied. To find relatively suitable operation conditions, an experimental optimization of the current density should be carried out, perhaps including parameters... 

In a recent publication, Schafer and coworkers point out the utility of the electrode as a reagent which is effective in promoting bond formation between functional groups of the same reactivity or polarity [1]. They accurately note that reduction at a cathode, or oxidation at an anode, renders electron-poor sites rich, and electron-rich sites poor. For example, reduction of an a, -un-saturated ketone leads to a radical anion where the )g-carbon possesses nucleophilic rather than electrophilic character. Similarly, oxidation of an enol ether affords a radical cation wherein the jS-carbon displays electrophilic, rather than its usual nucleophilic behavior [2]. This reactivity-profile reversal clearly provides many opportunities for the formation of new bonds between sites formally possessing the same polarity, provided only one of the two groups is reduced or oxidized. Electrochemistry provides an ideal solution to the issue of selectivity, given that a controlled potential reduction or oxidation is readily achieved using an inexpensive potentiostat. 

In this paper we report the application of bimetallic catalysts which were prepared by consecutive reduction of a submonolayer of bismuth promoter onto the surface of platinum. The technique of modifying metal surfaces at controlled electrode potential with a monolayer or sub-monolayer of foreign metal ("underpotential" deposition) is widely used in electrocatalysis (77,72). Here we apply the theory of underpotential metal deposition without the use of a potentiostat. The catalyst potential during promotion was controlled by proper selection of the reducing agent (hydrogen), pH and metal ion concentration. 

In [53], oscillatory wave patterns observed during electrochemical dissolution of a nickel wire in acidic media was reported. It was shown that space-averaged potential or current oscillations are associated with the creation of an inhomogeneous current distribution, and that the selection of a specific spatial current pattern depends on the current control mode of the electrochemical cell. In the almost potentiostatic (fixed potential) mode of operation, a train of traveling pulses prevails, whereas antiphase oscillations occur in the galvanostatic (constant average current) mode. 

For their characterization, electrochromic compounds are initially tested at a single working electrode under potentiostatic control using a three-electrode arrangement. Traditional characterization techniques such as cyclic voltammetry, coulometry, chronoamperometry, all with in situ spectroscopic measurements, are applied to monitor important properties [27]. From these results, promising candidates are selected and then incorporated into the respective device. 

It should be anticipated that there will not be a smooth transition from these idealized, simple systems into the real world. Some precautions and pitfalls have been cited, but usually in a parenthetical manner that lacked proper emphasis. The systems selected to illustrate the general principles of potentiostatic control have shown what can be expected under ideal conditions, but real systems have additional parameters that may tilt the balance from graceful control to chaos. Cell design is of paramount importance, and a guide to transfer characteristics of cells is included in the bibliography. To bring the information in this chapter effectively into use, it is necessary to acknowledge the role that cells play... 

In general, controlled-current electrolyses need less expensive equipment. Only a controlled-current source in combination with a coulomb integrator is necessary. Therefore, in industry, electroorganic reactions are always performed at a fixed current density. In the laboratory, it is advisable to start with controlled-potential electrolyses using a potentiostat and a three-electrode electrolysis cell (Fig. 22.8). In this way, the reaction can be controlled at the redox potential of the substrate determined analytically, and the selectivity of the process can be studied at different potentials. After determination of the selectivity controlling factors, it is usually possible to change over to current control by proper selection of the current density and the concentration of the substrate. Using a continuous process, the concentration can be fixed at the desired value. Thus, selectivity can also be obtained under these conditions. 

Instrumentation for selected aspects of electroanalytical chemistry is covered in Chapters 6-8. Although computers have made a tremendous impact on electroanalytical instrumentation, many aspects of these chapters are timeless. The basic configurations of a potentiostat have not changed since the early 1960s, although the electronic components themselves are dramatically different Learn to build your own potentiostat in Chapter 6, then see how to fine-tune it in Chapter 7. 

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