Controlled potential, electrolysis voltammetry

Anodic stripping voltammetry consists of two steps (Figure 11.37). The first is a controlled potential electrolysis in which the working electrode, usually a hanging mercury drop or mercury film, is held at a cathodic potential sufficient to deposit the metal ion on the electrode. For example, with Cu + the deposition reaction is... 

Cyclic voltammetry, square-wave voltammetry, and controlled potential electrolysis were used to study the electrochemical oxidation behavior of niclosamide at a glassy carbon electrode. The number of electrons transferred, the wave characteristics, the diffusion coefficient and reversibility of the reactions were investigated. Following optimization of voltammetric parameters, pH, and reproducibility, a linear calibration curve over the range 1 x 10 6 to 1 x 10 4 mol/dm3 niclosamide was achieved. The detection limit was found to be 8 x 10 7 mol/dm3. This voltammetric method was applied for the determination of niclosamide in tablets [33]. 

As mentioned in the introduction to controlled potential electrolysis (Section 2.3), there are various indirect methods to calculate the number of electrons transferred in a redox process. One method which can be rapidly carried out, but can only be used for electrochemically reversible processes (or for processes not complicated by chemical reactions), compares the cyclic voltammetric response exhibited by a species with its chronoamperometric response obtained under the same experimental conditions.23 This is based on the fact that in cyclic voltammetry the peak current is given by the Randles-Sevcik equation ... 

Controlled potential electrolysis (cpe) of alkanes in acetonitrile/0.1 M TBABF4 yields acetamidoalkanes (Table 3). Voltammetry and coulometry indicate a 2e-oxidation to a carbenium ion that subsequently reacts with the nitrogen atom of acetonitrile in a Ritter reaction (Eq. 6) [20]. 

Cathodic reduction of Ai-(2-iodophenyl)-A -alkylcinnamides under deaerated conditions forms l-alkyl-3-benzylindolin-2-ones regioselectively (70-85%), presumably in a radical or anionic S-exo-trig process. The mechanism has been explored by the use of cyclic voltammetry, controlled-potential electrolysis (cpe), and deuterium labeling [116]. 

The cyclic voltammetry of ketene imines of the type 52 (X = H, p-CHj, P-CH3O, p-Br) exhibited two irreversible waves in dichloromethane at a Pt-anode between 0.90-1.25 and 1.63-2.0 V vs Ag/AgCl, respectively. The controlled potential electrolysis at the first wave gave tetracyclic (53), bicyclic (54), and tricyclic (55) products [83] (Scheme 28). 

The ferrocenyl dendrimers were electrodeposited in their oxidized forms onto the electrode surfaces (platinum, glassy-caibon, and gold) either by controlled potential electrolysis or by repeated cycling between the appropriate anodic and cathodic potential limits therefore the amount of electroactive material electrode-posited can be controlled with the electrolysis time or the number of scans. The electrochemical behavior of films of the polyfeirocenyl dendrimers was studied by cyclic voltammetry in fresh CH2CI2 and CHjCN solutions containing only supporting electrolyte. 

The electrode reaction of triamterene 15 was elucidated by means of DCP, Tast polarography, cyclic voltammetry, microcoulometry, controlled potential electrolysis, and spectroscopy (ultraviolet/visible (UVA is), NMR). Two steps of reduction independent of pH were observed two-electron reduction of 15 resulted in the formation of 17. The first reduction wave of 15 was assumed to be due to irreversible two-electron reduction forming unstable 16, which tautomerized to 17, and the second reduction wave was ascribed to two-electron reduction of 17 to the tetrahydro product, 18 (Scheme 2). 

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. 

The redox chemistry of [NbO(TPP)(MeCOO)] was investigated by means of cyclic voltammetry and controlled potential electrolysis the reduction of Nbv to NbUI was found to occur prior to the reduction of the ligand. The redox potentials were measured for the [Nbv0(TPP)(MeC00)]/[NbIV0(TPP)l/[Nbm(TPP)]+/[NbI (TPP)] systems and found to be -0.94, 1.1 and -1.48 V respectively.334... 

The electrochemistry of heteropolymolybdates parallels that of the tungstates but with the following differences the reduction potentials are more positive and the primed species (metal-metal bonded ) are much less stable. Scheme 7 applies for or-fSiMo O ]4-. Species in parentheses are detectable only by rapid scan cyclic voltammetry, and XVIII decomposes rapidly at 0°C. The reduced anions such as II and IV are easily obtained by controlled potential electrolysis or by careful chemical reduction, e.g. with ascorbate. The use of metal ion reductants generally leads to other reactions, (equation 7). The reduced anions slowly isomerize (equation 8). The isomerization can be followed polarographically (all S potentials are more positive) or by NMR spectroscopy. By this means / isomers of most Keggin and Dawson molybdates have been prepared. 

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. 

Cyclic voltammetry and controlled-potential electrolysis are the techniques that have been used to investigate the electrochemistry of oxo-chromium and oxo-molybdenum corrolates. The data have been related to those obtained for similar porphyrin complexes. Redox potentials are reported in Table 17. 

The electrochemical properties of all these bis-adducts have been studied in reasonable detail, using either cyclic voltammetry (CV) or controlled potential electrolysis (CPE) [8], Reductive electrolysis of ester-containing methanofullerenes results in the removal of the adducts in a versatile and useful reaction, initially called the retro-Bingel reaction (Figure 1). Additional work resulted in the discovery of an intramolecular electrochemically induced isomerization of C6o-bis-adducts. Exhaustive reduction with one electron per molecule resulted in seven regio-isomers regardless of which pure bis-adduct regioisomer was electrolyzed. Recently, it has been observed that, in addition to the malonates, electrochemical reduction of other methano-adducts can also lead to removal of the addends,... 

The electrochemical behavior of 14+ is particularly clean and interesting, since only the 4- and the 5-coordinate geometries can be obtained on translating the metal-complexed ring from the phen site to the terpy site)841 The electrochemically induced molecular motions (square scheme1851), similar to those represented in Figure 10 but now involving stopped compounds, can be monitored by cyclic voltammetry (CV) and controlled potential electrolysis experiments)851... 

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. 

For the organic chemist, product studies in the widest sense, ie., including stereochemical aspects, isotope effects, etc. fall most natural in the study of electro-organic reactions. However, there are also some simple electrochemical techniques which are extremely useful in the design of electrochemical syntheses and can be set up in any laboratory for a modest cost. These methods — which are the ones to be discussed here - include different kinds of voltammetry, controlled potential electrolysis, and coulometry, andigive information as to the nature of the electro-active species, the possible nature of intermediates involved and their reactions with reagents present, and the number of electrons involved in the process. 

This relationship holds for any electrochemical process that involves semiinfinite linear diffusion and is the basis for a variety of electrochemical methods (e.g., polarography, voltammetry, and controlled-potential electrolysis). Equation (3.6) is the basic relationship used for solid-electrode voltammetry with a preset initial potential on a plateau region of the current-voltage curve. Its application requires that the electrode configuration be such that semiinfinite linear diffusion is the controlling condition for the mass-transfer process. 

The mono-cationic species [M(sdt)2]+ (sdt = stilbeneditholate M = Ni, Pd, Pt) has been observed by cyclic voltammetry (CV) at )/2 1.2 V (vs. Fc/Fc+, ferrocene/ferrocenium) (41). The [M(sdt)2]07+ couple is irreversible since the cationic species is highly reactive, which decomposes to give other radical species on the time scale of controlled potential electrolysis even at —40 °C (41). 

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