Application of Controlled-Potential Methods

To date the most extensive application of electrochemical methods with controlled potential has been in the area of qualitative and quantitative analysis. Because a number of monographs have more than adequately reviewed the literature and outlined the conditions for specific applications, this material is not covered here. In particular, inorganic applications of polarography and 

Controlled-potential electrolysis provides the features of electrodeposition plus the ability to carry out analyses more rapidly while using smaller samples. Furthermore, by integrating the current-time curve the necessity to plate a weighable amount of substituent is eliminated. One of the most important applications of controlled-potential electrolysis is the evaluation of the number of electrons involved in the electrode reaction (n). 

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. 

Voltammetric methods also provide a convenient approach for establishing the thermodynamic reversibility of an electrode reaction and for the evaluation of the electron stoichiometry for the electrode reaction. As outlined in earlier 

TABLE 3.6 Optimum Conditions for Polarographic Determination of Inorganic and Organic Substrates a  

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Organic chemists, quite enthusiastic about possibilites of electrosynthetic methods in the thirties, have become later more sceptical because of poor economy and because of the mixtures of compounds obtained. Even though at least part of these difficulties have been overcrome by application of controlled potential electrolysis, it seems unlikely that organic chemists can be convinced of the more general applicability of electrosynthetic methods. On the other hand there are some cases in which application of electrolysis seems to be particularly useful. 

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-... 

Sine Wave Methods in the Study of Electrode Processes, Margaretha Sluyters-Rehbach and Jan El. Sluyters The Theory and Practice of Electrochemistry with Thin Layer Cells, A. T. Hubbard and F. C. Anson Application of Controlled Potential Coulometry to the Study of Electrode Reactions, Allen J. Bard and K. S. V. Santhanam... 

In the common method of electro-gravimetric analysis, a potential slightly in excess of the decomposition potential of the electrolyte under investigation is applied, and the electrolysis allowed to proceed without further attention, except perhaps occasionally to increase the applied potential to keep the current at approximately the same value. This procedure, termed constant-current electrolysis, is (as explained in Section 12.4) of limited value for the separation of mixtures of metallic ions. The separation of the components of a mixture where the decomposition potentials are not widely separated may be effected by the application of controlled cathode potential electrolysis. An auxiliary standard electrode (which may be a saturated calomel electrode with the tip of the salt bridge very close to the cathode or working electrode) is inserted in the... 

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). 

The molecular dynamics unit provides a good example with which to outline the basic approach. One of the most powerful applications of modem computational methods arises from their usefulness in visualizing dynamic molecular processes. Small molecules, solutions, and, more importantly, macromolecules are not static entities. A protein crystal structure or a model of a DNA helix actually provides relatively little information and insight into function as function is an intrinsically dynamic property. In this unit students are led through the basics of a molecular dynamics calculation, the implementation of methods integrating Newton s equations, the visualization of atomic motion controlled by potential energy functions or molecular force fields and onto the modeling and visualization of more complex systems. 

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. 

Because of the short lifetime of ions in gaseous atmospheres, even at low pressure, gas-phase IR measurements are limited to adsorption of neutral molecules. Electrochemical applications of the IR method offer the interesting possibility of providing data on the adsorption properties of charged particles (Secs. 8 and 9). In the electrochemical environment the applied potential allows ionic adsorbates to be studied under energetically controllable conditions. Otherwise the electrochemical double layer offers exceptional conditions to investigate the Stark effect on vibrational transitions by setting tunable electric fields of the order of 10 V cm at the interface. This phenomenon will be discussed in Sec. 10. 

A potential application of our ELISA method would be for the analysis of alachlor in water. To test this possibility, 208 water samples were collected for analysis from rivers and water treatment plants. Some of the samples were intentionally spiked with alachlor as controls. The samples were analyzed by ELISA without any pretreatment, and by an established GC/MS method. For the ELISA analysis, a sample size of 1.0 mL was required. The GC/MS analysis, on the other hand, required 1.0 L of sample volume. The results of the ELISA and GC/MS analyses are presented in Figure 5. The X axis displays ppb of alachlor as determined by ELISA, and the Y axis displays ppb of alachlor as determined by GC/MS. The correlation coefficient of the two methods was 0.84, and the slope of the regression line was 0.74. 

In this chapter, we provide a succinct review of some of the advances in the development and application of ab initio methods toward understanding the intrinsic reactivity of the metal and the influence of the reactive site and its environment. We draw predominantly from some of our own recent efforts. More specifically we describe (a) the chemistry of the aqueous-phase on transition metal surfaces and its influence on the kinetics and thermodynamics within example reaction mechanisms, and (b) computational models of the electrode interface that are able to account for a referenced and tunable surface potential and the role of the surface potential in controlling electro-catalytic reactions. These properties are discussed in detail for an example reaction of importance to fuel cell electrocatalysis methanol dehydrogenation over platinum(ll 1) interfaces [24,25]. 

If the electrochemistry is understood, one needs the application of surface analytical methods to learn about the chemical properties and chemical structure of passive layers. Then one has to take care that electrochemical specimen preparation has to occur with optimum control in order to get reliable results. This permits to draw clear mechanistic conclusions on the properties of the layers like their growth, reduction, changes of their composition, reactivity, degradation, and stability including realistic environmental conditions. Application of XPS, ISS, and RBS to a wide variety of pure metals and binary alloys has been described in Section 5.6. These techniques provide valuable results especially when applied together with a systematic change of the experimental parameters like the potential and time of passivation, the composition of the electrolyte, and alloy and conditions for layer degradation. 

In this monograph, an effort has been made to provide sufScient information about the theoretical background, instrumental considerations, and experimental techniques of controlled-potential coulometry so as to enable the practicing analytical chemist to make use of this method for his own particular requirements. Detailed procedures for specific analyses will not be given here, but a considerable number of typical applications to inorganic analysis have been critically treated in the final chapter. While this modest monograph is not intended in any way as a textbook it is hoped that it may serve to stimulate some interest in this interesting and useful technique. 

In addition to particle identification and sizing, the impact method can also be used to measure the concentration of nanoparticles in solution. This is best done by measuring oxidative impacts at a micro-disc electrode, typically made of carbon. The current (I)-time transient at such an electrode following the application of a potential sufficient to oxidise species under diffusion control in an n-electron process is... 

The complexity of the systems to be protected and the variety of techniques available for cathodic protection are in direct contrast to the simplicity of the principles involved, and, at present the application of this method of corrosion control remains more of an art than a science. However, as shown by the potential-pH diagrams, the lowering of the potential of a metal into the region of immunity is one of the two fundamental methods of corrosion control. 

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. ... 

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