Coulometry cyclic voltammetry

Polarography, chronopotentiometry, coulometry, cyclic voltammetry ip>c/ip>a 1 ESR, polarography... 

The electronic adsorption spectra for the complexes [Ir(OH)6]", where n = 0-2, have been resolved and peak maxima locations, molar extinction coefficients, oscillator strengths, and band half-widths calculated.44 Bands have been assigned in the main part to be one-electron MLCT transitions. Spectrophotometrically determined rate constants for the OH reduction of the IrVI and Irv complexes at 25 °C in 3M NaOH are (2.59 0.09) x 10—3 s—1 and (1.53 0.05) x 10 4 s 1 respectively. The activation energy for the reduction, Irv—>IrIV, is nAkcalmoC1. Cyclic voltammetry and potentiostatic coulometry of [Ir(OEI )r,]2 in 3M NaOH on a Pt electrode show that during the electro-oxidation compounds of Irv and IrVI are formed.45... 

Radiometry is often combined with electrochemical techniques, such as cyclic voltammetry and coulometry. In this way, the number of electrons involved in the oxidation of adsorbed species can be obtained. 

Table 6 Electrochemical reduction of organotellurium(IV) compounds and their corresponding organotellurium (II) compounds hy cyclic voltammetry and coulometry...

In principle, E° can be determined by the widely used electroanalytical techniques (e.g. polarography, cyclic voltammetry [25]). The combination of the techniques is also useful. It has been demonstrated recently where potentiom-etry, coulometry, and spectrophotometry have been applied [26]. The case of the cyclic voltammetry is examined below. 

Lobacz et al. [52] have described partial adsorption ofTl+-cryptand (2,2,2) complex on mercury electrode. From voltocoulom-etry, cyclic voltammetry, and chrono-coulometry, it has been deduced that electroreduction of this complex proceeds via two parallel pathways from the solution and from the adsorbed states, which are energetically close. Also, Damaskin and coworkers [53] have studied adsorption of the complexes of alkali metal cations with cryptand (2,2,2) using differential capacity measurements and a stationary drop electrode. It has been found that these complexes exhibit strong adsorption properties. Novotny etal. [54] have studied interfacial activity and adsorptive accumulation of U02 " "-cupferron and UO2 - chloranilic acid complexes on mercury electrodes at various potentials in 0.1 M acetate buffer of pH 4.6 and 0.1 M NaCl04, respectively. 

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. 

It should also be recalled that a full electrochemical, as well as spectroscopic and photophysical, characterization of complex systems such as rotaxanes and catenanes requires the comparison with the behavior of the separated molecular components (ring and thread for rotaxanes and constituting rings in the case of catenanes), or suitable model compounds. As it will appear clearly from the examples reported in the following, this comparison is of fundamental importance to evidence how and to which extent the molecular and supramolecular architecture influences the electronic properties of the component units. An appropriate experimental and theoretical approach comprises the use of several techniques that, as far as electrochemistry is concerned, include cyclic voltammetry, steady-state voltammetry, chronoampero-metry, coulometry, impedance spectroscopy, and spectra- and photoelectrochemistry. 

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. 

Electrochemistry of [VO(acac)2] in DMSO has been studied by cyclic voltammetry and controlled-potential coulometry at a platinum electrode.516 [VO(acac)2] is irreversibly reduced by one electron at —1.9 V vs. SCE (saturated calomel electrode) to a stable Vm product. In the presence of an excess of ligand, [VO(acac)2] is reduced by two electrons to [V(acac)3] with the V111 species mentioned above and [V(acac)3] as intermediates. The one-electron oxidation of [VO(acac)2] at +0.81 V vs. SCE gives a product. 

Many of the electroanalytical techniques that are routinely employed in conventional solvents, such as, chronoamperometry, chronocoulometry, chronopotentiometry, coulometry, cyclic (stationary electrode) voltammetry, rotating electrode voltammetry, and pulse voltammetry, have also been applied to molten salts. Some of these techniques are discussed next with special attention to their employment in molten salts. References to noteworthy examples appearing in the literature are included. Background information about these techniques is available elsewhere in this book. 

Cyclic voltammetry has gained widespread usage as a probe of molecular redox properties. I have indicated how this technique is typically employed to study the mechanisms and rates of some electrode processes. It must be emphasized that adherence of the CV responses to the criteria diagnostic of a certain mechanism demonstrates consistency between theory and experiment, rather than proof of the mechanism, since the fit to one mechanism may not be unique. It is incumbent upon the experimenter to bring other possible experimental probes to bear on the question. These will often include coulometry, product identification, and spectroelectrochemistry. 

Chloramphenicol, a broad-spectrum antibiotic, has probably received more attention in the polarographic literature than any other pharmaceutical. The aromatic nitro group is quite easily reduced. Studies [76] employing polarog-raphy, cyclic voltammetry, and constant-potential coulometry have suggested that... 

Recent studies describe the use of cyclic voltammetry in conjunction with controlled-potential coulometry to study the oxidative reaction mechanisms of benzofuran derivatives [115] and bamipine hydrochloride [116]. The use of fast-scan cyclic voltammetry and linear sweep voltammetry to study the reduction kinetic and thermodynamic parameters of cefazolin and cefmetazole has also been described [117]. Determinations of vitamins have been studied with voltammetric techniques, such as differential pulse voltammetry for vitamin D3 with a rotating glassy carbon electrode [118,119], and cyclic voltammetry and square-wave adsorptive stripping voltammetry for vitamin K3 (menadione) [120]. 

Recently a series of dialkylpyrrolidinium (Pyr+) cations have been studied in our laboratory 7-9). These cations are reduced at relatively positive potentials and could be investigated electrochemically as low concentration reactants in the presence of (C4H9)4N+ electrolytes. Using cyclic voltammetry, polarography and coulometry, it was shown that Pyr+ react by a reversible le transfer. The products are insoluble solids which deposit on the cathode and incorporate Pyr+ and mercury from the cathode. Both the cation and the metal can be regenerated by oxidation. Quantitative analysis of current-time transients, from potential step experiments, showed that the kinetics of the process involve nucleation and growth and resemble metal deposition. 

Controlled potential electrolysis or coulometry can be used to generate radical ions in quantities sufficient for study by appropriate techniques such as optical or EPR spectroscopy. This method is routinely applied to characterize radical anions and has also been used extensively for studying radical cations. However, the application of eoulometric techniques to the study of strained ring compounds is severely limited, even more than the application of cyclic voltammetry, by the limited stability of their one-electron oxidation products. 

The cleavage of primary amines in aprotic solvents, such as acetonitrile or DMF, was studied by cpe, controlled potential coulometry, and cyclic voltammetry 463 The results indicate that at low anode potential the primarily formed radical cation RCH2NH2 dissociates to an alkyl cation RCH2+ and an amino radical NH2 , which subsequently is oxidized to nitrogen. At higher anode potentials path a)ofEq. (219), leading to an aldehyde and an ammonium ion, preponderates. 

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. 

With respect to chemical steps prior to the electron-transfer step, chrono-potentiometiy offers a convenient technique. The methods of measurement and the quantitative relationships are outlined in Chapter 4. Post-electron-transfer reactions to the electron-transfer step are most conveniently characterized by cyclic voltammetry (see Chapter 3). Although the techniques of cyclic voltammetry and chronopotentiometiy both provide a means for the qualitative detection of adsorption processes at an electrode, the coulostatic method and chrono-coulometry are the methods of choice for quantitative measurements of adsorption. 

There are two methods of electrochcmically reducing acetylenes, namely, direct charge transfer to the triple bond from the cathode and the electrolytic generation of an intermediate which attacks the acetylene. The first method (direct reduction) has the advantage that mechanistic studies using, for example, cyclic voltammetry and coulometry can be carried out, while the second method (indirect reduction) appears to offer more scope for product control and has been more extensively investigated. 

The electrochemical behaviour of trinuclear clathrochelate [MiD3(ttnM)2] and [CoDm3(dienM)2] cations has been investigated by cyclic voltammetry, polarography, and coulometry. The data obtained are summarized in Table 41. 

Part IV is devoted to electrochemical methods. After an introduction to electrochemistry in Chapter 18, Chapter 19 describes the many uses of electrode potentials. Oxidation/reduction titrations are the subject of Chapter 20, while Chapter 21 presents the use of potentiometric methods to obtain concentrations of molecular and ionic species. Chapter 22 considers the bulk electrolytic methods of electrogravimetry and coulometry, while Chapter 23 discusses voltammetric methods including linear sweep and cyclic voltammetry, anodic stripping voltammetry, and polarography. 

Figure 3.51 Cyclic voltammetry (v = 1 Vs-1) of CoPI (see insert) modified graphite surface in neat (dotted line) and 02-saturated (solid line) 1 M NaOH aqueous solutions. The surface concentration of CoPI as determined from coulometry was about 4 x 10-10 mol cm-2.

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. 

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