Electrolysis Controlled-current techniques

Controlled-current electrolysis in flowing solution has been extremely useful for analytical purposes. The prevalent techniques are constant-current coulometry and coulometric titrations, which are discussed in Chapter 25. 

The principle of controlled current electrolysis has been known since the beginning of this century.21 However, the utilization of this form of electrochemistry remained dormant for 50 years until three groups of investigators illustrated its many advantages for analytical and physicochemical measurements.22 24 Several works describe this technique in detail,25"27 and other re-... 

A number of embellishments of controlled-current electrolysis have been proposed for specialized systems, but discussion of these is beyond the scope of this chapter. Those interested in pursuing this material are referred to discussions of programmed-current techniques, 4 single-step and double-step gal-vanostatic methods,35 and chronocoulometiy.36... 

The methods can be classified by the controlled parameter (E or i) and by the quantities actually measured or the process carried out. Thus in controlled-potential techniques the potential of the working electrode is maintained constant with respect to a reference electrode. Since the potential of the working electrode controls the degree of completion of an electrolytic process in most cases, controlled-potential techniques are usually the most desirable for bulk electrolysis. However, these methods require potentiostats with large output current and voltage capabilities and they need stable reference electrodes, carefully placed to minimize uncompensated resistance effects. Placement of the auxiliary electrode to provide a fairly uniform current distribution across the surface of the working electrode is usually desirable, and the auxiliary electrode is often placed in a separate... 

Controlled potential techniques have shown to be useful tools to characterize the electrochemical behavior of organic compounds. Particularly, cyclic, convolution and square wave voltammetries, controlled potential electrolysis, and digital simulation provide valuable information, which allows elucidating the electrochemical reaction mechanism, and the determination of thermodynamic, kinetic and diffusion parameters (Bard Faulkner, 2001). In addition, the square wave voltammetry is particularly a fast and sensitive technique to detect and quantify a given substrate considering its ability to discriminate against capacitive currents (Osteryoimg O Dea, 1987 Mirceski, Komorsky-Lovric Lovric, 2007). 

Controlled-potential electrolysis (CPE) represents an improvement over the previous constant-potential method this is attained by the application of an emf across the electrodes that yields a cathodic potential as negative as is acceptable in view of current density limitations and without taking the risk that the less noble metal is deposited hence the technique requires non-faradaic control of the cathodic potential versus the solution. 

Electrogravimetry, which is the oldest electroanalytical technique, involves the plating of a metal onto one electrode of an electrolysis cell and weighing the deposit. Conditions are controlled so as to produce a uniformly smooth and adherent deposit in as short a time as possible. In practice, solutions are usually stirred and heated and the metal is often complexed to improve the quality of the deposit. The simplest and most rapid procedures are those in which a fixed applied potential or a constant cell current is employed, but in both cases selectivity is poor and they are generally used when there are... 

Generally, irrespective of the technique for which they are used, electrochemical cells are constructed in a way which minimizes the resistance of the solution. The problem is particularly accentuated for those techniques which require high current flows (large-scale electrolysis and fast voltammetric techniques). When current flows in an electrochemical cell there is always an error in the potential due to the non-compensated solution resistance. The error is equal to / Rnc (see Chapter 1, Section 3). This implies that if, for example, a given potential is applied in order to initiate a cathodic process, the effective potential of the working electrode will be less negative compared to the nominally set value by a amount equal to i Rnc. Consequently, for high current values, even when Rnc is very small, the control of the potential can be critical. 

Among electrochemical techniques,cyclic voltammetry (CV) utilizes a small stationary electrode, typically platinum, in an unstirred solution. The oxidation products are formed near the anode the bulk of the electrolyte solution remains unchanged. The cyclic voltammogram, showing current as a function of applied potential, differentiates between one- and two-electron redox reactions. For reversible redox reactions, the peak potential reveals the half-wave potential peak potentials of nonreversible redox reactions provide qualitative comparisons. Controlled-potential electrolysis or coulometry can generate radical ions for smdy by optical or ESR spectroscopy. 

An interesting study [52] of the protonation kinetics and equilibrium of radical cations and dications of three carotenoid derivatives involved cyclic voltammetry, rotating-disk electrolysis, and in situ controlled-potential electrochemical generation of the radical cations. Controlled-potential electrolysis in the EPR cavity was used to identify the electrode reactions in the cyclic volt-ammograms at which radical ions were generated. The concentrations of the radicals were determined from the EPR amplitudes, and the buildup and decay were used to estimate lifetimes of the species. To accomplish the correlation between the cyclic voltammetry and the formation of radical species, the relative current from cyclic voltammetry and the normalized EPR signal amplitude were plotted against potential. Electron transfer rates and the reaction mechanisms, EE or ECE, were determined from the electrochemical measurements. This study shows how nicely the various measurement techniques complement each other. 

A polarogram of the type shown in Fig. 2 immediately tells us that the substrate is the electroactive species in a certain potential region below that of the anodic limit of the SSE. The potential at half the plateau value of the current is denoted the half-wave potential (E,, 2) of the substrate and is a measure of how easily the compound is oxidized. With a knowledge of the half-wave potential of the substrate it is now easy to link products isolated from macroelectrolyses with the electrode processes possible in the system. This is done by the technique of controlled potential electrolysis (Sect. 4.4). From a peak polarogram, the peak potential (IT ) may be used in the same way asEx, 2. 

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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