Electrochemical potential coulometry

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. 

Coulometric methods of analysis are based on an exhaustive electrolysis of the analyte. By exhaustive we mean that the analyte is quantitatively oxidized or reduced at the working electrode or reacts quantitatively with a reagent generated at the working electrode. There are two forms of coulometry controlled-potential coulometry, in which a constant potential is applied to the electrochemical cell, and controlled-current coulometry, in which a constant current is passed through the electrochemical cell. 

From this equation we see that increasing k leads to a shorter analysis time. For this reason controlled-potential coulometry is carried out in small-volume electrochemical cells, using electrodes with large surface areas and with high stirring rates. A quantitative electrolysis typically requires approximately 30-60 min, although shorter or longer times are possible. 

The way in which these alternatives with their particular measuring characteristics are carried out can be best described by (1) controlled-potential coulometry and (2) coulometric titration (controlled-current coulometry). Both methods require an accurate measurement of the number of coulombs consumed, for which the following instrumental possibilities are available (a) chemical coulometers, (b) electrochemical coulometers and (c) electronic coulometers. 

It exhibits a single oxidation process (Ea = + 0.59 V, vs. SCE) affected by some adsorption problems. These adsorption phenomena, which typically affect the electrochemical response of these derivatives, sometimes make it difficult to ascertain by controlled potential coulometry the effective number of electrons involved in the oxidation step. In this case, the (approximate) number of electrons involved per molecule of dendrimer, nd, can be roughly calculated by comparing the cyclic voltammetric responses of the dendrimer with that of the ferrocene monomer using the following empirical equation.27,40... 

Often the first step in the electrochemical characterization of a compound is to ascertain its oxidation-reduction reversibility. In our opinion, cyclic voltammetry is the most convenient and reliable technique for this and related qualitative characterizations of a new system, although newer forms of pulse polarography may prove more suitable for quantitative determination of the electrochemical parameters. The discussion in Chapter 3 outlines the specific procedures and relationships. The next step in the characterization usually is the determination of the electron stoichiometry of the oxidation-reduction steps of the compound. Controlled-potential coulometry (discussed in Chapter 3) provides a rigorously quantitative means for such evaluations. 

We must note that in many cases the electrochemical recognition has been limited to the simple evaluation of the redox potentials of the electron transfer steps, together with a superficial examination of the chemical stability of the complexes in various oxidation states. This is of little help to synthetic chemists, who would have to prepare redox congeners for complete chemical, physico-chemical, and structural characterization. Controlled potential coulometry and macroelectrolysis tests must be routinarily performed, both to define the number of electrons involved in each redox change and to obtain redox congeners, even if in low quantity, at least for a preliminary determination of their stability and of their spectroscopic properties. 

Virtually any electrochemical technique may be used for either analytical or mechanistic (our focus) studies. The merits and limitations of each technique and the information that can be gleaned are discussed for direct-current (d.c.) polarography, pulse polarography, alternating-current (a.c.) polarography and cyclic voltammetry. Con-trolled-potential coulometry is technically not a voltammetric technique (there is no variation of potential), and this technique is considered in 12.3.5. 

When pcrtechnctate is electrochemically reduced in aqneous alkaline solution in the presence of gelatin using the techniques of controllcd-potential coulometry, chronoamperometry, and double potential step chronoamperometry, at the mercury electrode surface the technetate ion TcO " is reported to be produced ... 

Figure 2 shows cyclic voltammograms and observed redox potentials [Eobs° = (E,° + E2° )/2] for [W2(n-SPh)2(CO)g] - (15) and [W2( -PPh2)2(CO)8] (18) at a sweep rate of 0.1 V s. The overall two-electron character of the reactions has been confirmed by controlled potential coulometry and comparative voltammetric peak current measurements. Only single reduction and oxidation peaks are observed at sweep rates ranging from 0.005 to 1000 V s" thus, the one-electron intermediate in reactions 1 and 2 is not detectable by simple electrochemical means. 

Controlled-potential coulometry has also found some use in the study of basic electrochemistry. It is not always obvious how many electrons are involved in a newly studied electrochemical reaction, e.g., in polarography. Thus, coulometry at controlled potential, in which a known quantity of the substance is electrolyzed and Q is measured, is often used to determine values for n and thereby help elucidate electrode mechanisms for a wide variety of compounds, both organic and inorganic. Very slow chemical reactions coupled with the electrochemical reaction may also be studied by controlled-potential coulometry [4] other electrochemical techniques usually are suitable only for much faster chemical reactions, with time scales of jusec to sec. 

Lewis TD (1961) Columetric methods in analysis a review. Analyst 86 494—506 Bard AJ (2001) Electrochemical methods, fundamentals and applications. Wiley, New York Harrar JE (1987) Analytical controlled-potential coulometry. TrAC Trend Anal (Them 6(6) 152-157... 

In this section the theory and methodology of electro-analytical chemistry are explored. Chapter 22 provides a (general foundation for the study of subsequent chapters in this section. Terminology- and conventions of electrochemistry as well as theoretical and practical aspects of the measurement of electrochemical potentials and current s are. presented. Chapter 23 comprises the many methods and applications of potentiometry. and constant-potential coulometry and constant-current coulornetrv are discussed in Chapter 2 4. The many facets of the important and widely used technique of voltammetry are presented in ( hapter 2.5. which concludes the section. 

Electrochemical oxidation of catechol in the presence of carbohydrazide in aqueous solution using carbon electrode resulted in benzimidazole derivatives by cyclic voltammetry and controlled-potential coulometry techniques [20] (Scheme 49). A direct electron transfer mechanism occurred during the process on the surface of carbon. The investigators proposed that the Michael adduct formed between the carbohydrazide and o-quinone (Eqn (2)) leads to the formation of l,3-diamino-5,6-dihydroxy-lH-benzo[d]imidazole-2(3H)-one 52. 

Mercury itself serves as one of the most commonly used electrode materials for controlled-potential coulometry but there is absolutely no reason why this technique cannot be applied to the analytical determination of mercury as well. A great deal of electrochemical and kinetic information regarding the oxidation and reduction of various mercury species is already available in the literature so that only the optimum conditions with regard to potential and solvent remain to be determined. Much of this work has been carried out by Tanaka (154-158) who obtained quantitative deposition of mercury on platinum electrodes at potentials of 0.15 V, 0.40 V, 0.40 V, —0.05 V, and —0.80 V vs. SCE in media consisting of 0.3 n hydrochloric acid-0.14 m hydroxylamine hydrochloride, 0.4 n sulphuric... 

In potentiometry, the potential of an electrochemical cell under static conditions is used to determine an analyte s concentration. As seen in the preceding section, potentiometry is an important and frequently used quantitative method of analysis. Dynamic electrochemical methods, such as coulometry, voltammetry, and amper-ometry, in which current passes through the electrochemical cell, also are important analytical techniques. In this section we consider coulometric methods of analysis. Voltammetry and amperometry are covered in Section 1 ID. 

Electrochemical methods covered in this chapter include poten-tiometry, coulometry, and voltammetry. Potentiometric methods are based on the measurement of an electrochemical cell s potential when only a negligible current is allowed to flow, fn principle the Nernst equation can be used to calculate the concentration of species in the electrochemical cell by measuring its potential and solving the Nernst equation the presence of liquid junction potentials, however, necessitates the use of an external standardization or the use of standard additions. 

Electrochemical Detectors Another common group of HPLC detectors are those based on electrochemical measurements such as amperometry, voltammetry, coulometry, and conductivity. Figure 12.29b, for example, shows an amperometric flow cell. Effluent from the column passes over the working electrode, which is held at a potential favorable for oxidizing or reducing the analytes. The potential is held constant relative to a downstream reference electrode, and the current flowing between the working and auxiliary electrodes is measured. Detection limits for amperometric electrochemical detection are 10 pg-1 ng of injected analyte. 

Electrical methods of analysis (apart from electrogravimetry referred to above) involve the measurement of current, voltage or resistance in relation to the concentration of a certain species in solution. Techniques which can be included under this general heading are (i) voltammetry (measurement of current at a micro-electrode at a specified voltage) (ii) coulometry (measurement of current and time needed to complete an electrochemical reaction or to generate sufficient material to react completely with a specified reagent) (iii) potentiometry (measurement of the potential of an electrode in equilibrium with an ion to be determined) (iv) conductimetry (measurement of the electrical conductivity of a solution). 

Analytical methods based upon oxidation/reduction reactions include oxidation/reduction titrimetry, potentiometry, coulometry, electrogravimetry and voltammetry. Faradaic oxidation/reduction equilibria are conveniently studied by measuring the potentials of electrochemical cells in which the two half-reactions making up the equilibrium are participants. Electrochemical cells, which are galvanic or electrolytic, reversible or irreversible, consist of two conductors called electrodes, each of which is immersed in an electrolyte solution. In most of the cells, the two electrodes are different and must be separated (by a salt bridge) to avoid direct reaction between the reactants. 

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