CONTROLLED-POTENTIAL METHODS

For chemists, the second important application of electrochemistry (beyond potentiometry) is the measurement of species-specific [e.g., iron(III) and iron(II)] concentrations. This is accomplished by an experiment in which the electrolysis current for a specific species is independent of applied potential (within narrow limits) and controlled by mass transfer across a concentration gradient, such that it is directly proportional to concentration (/ = kC). Although the contemporary methodology of choice is cyclic voltammetry, the foundation for all voltammetric techniques is polarography (discovered in 1922 by Professor Jaroslov Heyrovsky awarded the Nobel Prize for Chemistry in 1959). Hence, we have adopted a historical approach with a recognition that cyclic voltammetry will be the primary methodology for most chemists. 

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The largest division of interfacial electrochemical methods is the group of dynamic methods, in which current flows and concentrations change as the result of a redox reaction. Dynamic methods are further subdivided by whether we choose to control the current or the potential. In controlled-current coulometry, which is covered in Section IIC, we completely oxidize or reduce the analyte by passing a fixed current through the analytical solution. Controlled-potential methods are subdivided further into controlled-potential coulometry and amperometry, in which a constant potential is applied during the analysis, and voltammetry, in which the potential is systematically varied. Controlled-potential coulometry is discussed in Section IIC, and amperometry and voltammetry are discussed in Section IID. 

The simplest of the methods employing controlled current density is electrolysis at constant current density, in which the E-t dependence is measured (the galvanostatic or chronopotentiometric method). The instrumentation for this method is much less involved than for controlled-potential methods. The basic experimental arrangement for galvanostatic measurements is shown in Fig. 5.15, where a recording voltmeter or oscilloscope replaces the potentiometer. The theory of the simplest applications of this method to electrode processes was described in Section 5.4.1 (see Eqs 5.4.16 and 5.4.17). 

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. 

The investigations on electrochemistry related to sulphide mineral flotation have been widely reported for a long time. However, pilot tests and industrial plant operations of potential control flotation (PCF) of sulphide ores have made little progress due to the absence of an apphcable control potential method. The addition of oxidants and reductants will result in a big consumption of the chemicals. In the meantime, adjusting the pulp potential by external field is also of low efficiency. 

When metal ion M"+ is deposited by the controlled-current method, the electrode potential during the electrolysis changes in the order T, 2, 3, 4, 5, 6 in Fig. 5.33 and the next reduction process occurs near the end of the electrolysis. If the solution is acidic and the next reduction process is hydrogen generation, its influence on the metal deposition is not serious. However, if other metal is deposited in the next reduction process, metal M is contaminated with it. In order that two metal ions M"1+ and M "21 can be separated by the controlled-current method, the solution must be acidic and the reduction of hydrogen ion must occur at the potential between the reductions of the two metal ions. An example of such a case is the separation of Cu2+ and Zn2+ in acidic solutions. If two metal ions are reduced more easily than a hydrogen ion (e.g. Ag+ and Cu2+), they cannot be separated by the controlled-current method and the controlled-potential method must be used. 

Of these three branches, electrodeposition is the most important, especially since the introduction (in 1906) of the controlled potential method. H.j.S. Sand was the pioneer in the development of this method... 

This chapter has confined itself to a brief description of the common controlled potential methods which can be employed by the coordination chemist, but it is worth pointing out that far less sophisticated constant current methods, a DC supply and two electrodes in an undivided cell, have been used very successfully to electrosynthesize a wide range of coordination compounds, notably by anodic dissolution of a metal, i.e. metal ions are sprayed into an electrolyte solution containing an appropriate ligand.7 It must also be remembered that virtually all industrial-scale electrosyntheses are performed by controlling current density rather than potential.8 Nevertheless,... 

In early 1983, Bioanalytical Systems introduced a new class of integrated processor-driven instrumentation based on a concept first developed by Faulkner and his co-workers [1] at the University of Illinois. This unit (Figs. 6.22 and 6.23) has evolved over the years and now includes a repertoire of some 35 electrochemical techniques, including the most popular large-amplitude (Chap. 3) and small-amplitude (Chap. 5) controlled-potential methods. The unit also is capable of determining electrocapillary curves and can automatically measure and compensate for solution resistance (R in Fig. 6.5). Thus in a single instrument it is possible to utilize virtually all of the diagnostic criteria introduced in Chapters 3 and 5 and also to explore quickly which technique is optimum for... 

Diffusion to a Planar Electrode. The basic approach in controlled-potential methods of electrochemistry is to control in some manner the potential of the working electrode while measuring the resultant current, usually as a function of time. When a potential sufficient to electrolyze the electroactive species completely is applied to the electrode at (t = 0), the concentration at the electrode surface is reduced to zero and an electrode process occurs, for example... 

In addition to the analytical applications discussed above, controlled-potential methods are used for the evaluation of thermodynamic data and diffusion coefficients in both aqueous and nonaqueous solvents. Polarographic and voltammetric methods provide a convenient and straightforward means for evaluation of the diffusion coefficients in a variety of media. The requirements are that the current be diffusion-controlled, the number of electrons in the electrode reaction be known, and the concentration of the electroactive species and the area of electrodes be known. With these conditions satisfied, diffusion coefficients can be evaluated rapidly over a range of temperatures and solution conditions. 

As outlined in the theoretical section of this chapter, controlled-potential methods have extensive application in the study of the kinetics and mechanisms of the electron-transfer reaction of electrochemical processes. Furthermore, associated reactions before and after the electron-transfer process are readily studied by controlled-potential methods. For a number of systems the rate constants for these associated chemical processes can be evaluated. 

One of the most important, yet latent, applications of controlled-potential electrolysis is electrochemical synthesis. Although electrolysis has been used for more than a century to synthesize various metals from their salts, application to other types of chemical synthesis has been extremely limited. Before the advent of controlled-potential methods, the selectivity possible by classical electrolysis precluded fine control of the products. The only control was provided by appropriate selection of electrode material, solution acidity, and supporting electrolyte. By these means the effective electrode potential could be limited to minimize the electrolysis of the supporting electrolyte or the solvent. Today potentiostats and related controlled-potential-electrolysis instrumentation are commercially available that provide effective control of the potential of the working electrode to 1 mV, and a driving force of up to 100 V for currents of up to several amperes. Through such instrumentation electrochemical syn-... 

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