Electrochemical methods cyclic voltammetry

Then appears linear sweep rate voltammetry in which the electrode potential is a linear function of time. The current-potential curve shows a peak whose intensity is directly proportional to the concentration of electroactive species. If the potential sweep takes place in two directions, the method is named cyclic voltammetry. This method is one of the most frequently used electrochemical methods for more than three decades. The reason is its relative simplicity and its high information content. It is very useful in elucidating the mechanisms of electrochemical reactions in the case where electron transfer is coupled... 

Oxidation at solid electrodes. Oxidation of cysteine, cystine and both forms of glutathione at a platinum electrode was studied extensively by Pradac, Koryta and Ossendorfova [158-160]. The oxidation of these substances occurs by reaction of the adsorbed radical with a surface oxide. Therefore, the calibration curve approaches a limiting value at higher concentrations. Reproducible results are obtained at pH 7.2 using cyclic voltammetry. This method makes it impossible to estimate these substances in blood serum [161, 162] and in some body organs [161,163]. Cysteine as electrochemical indicator can also be determined in vivo after intravenous injection, as for example in the assessment of the blood supply to the kidney [164-166]. 

Yet when applied to current reversal techniques, such as double-step chronampero-metry of cyclic voltammetry, these methods require that an appreciable current be observed during the backward perturbation, that is, for t > 0, in potentiostatic methods or after the potential scan inversion in cyclic voltammetry. This requires that the characteristic time 0 of the method is adjusted to match the half-life ti/2 of the electrogenerated intermediate. Today, owing to the recent development of ultramicroelectrodes, 0 can be routinely varied from a few seconds to a few nanoseconds [102]. Yet with basic standard electrochemical equipment, 0 is usually restricted from the second to the low millisecond range. Thus for experimental situations involving faster chemical reactions, current rever-... 

The charges that we are concerned with here are the electronic charges (electrons and holes). For charges of this type to be transported across the interface, electrochemical reactions must take place. In the presence of a membrane that is impermeable to ions, what will happen then Here, the membrane must serve at least two functions (1) pathway for electronic charges, and (2) electrode surface for chemical transformation (reduction and oxidation or redox reactions). To probe such a membrane, we will describe a well-established cyclic voltammetry (CV) method of electrochemists, which was applied to the BLM system in 1984 [20-24, 32, 36]. 

In spite of the instrumental simplicity of cyclic voltammetry, the method is surprisingly versatile and an experienced researcher can obtain a considerable amount of information on the basis of just recording a single CV. A large number of different electroanalytical techniques have been applied to the research on conducting polymers, from the chronoamperometric method to AC impedance methods, However, the methods rest only on the electrochemical characteristics of the polymer and naturally the information obtained is rather restricted. 

Such effects are observed inter alia when a metal is electrochemically deposited on a foreign substrate (e.g. Pb on graphite), a process which requires an additional nucleation overpotential. Thus, in cyclic voltammetry metal is deposited during the reverse scan on an identical metallic surface at thermodynamically favourable potentials, i.e. at positive values relative to the nucleation overpotential. This generates the typical trace-crossing in the current-voltage curve. Hence, Pletcher et al. also view the trace-crossing as proof of the start of the nucleation process of the polymer film, especially as it appears only in experiments with freshly polished electrodes. But this is about as far as we can go with cyclic voltammetry alone. It must be complemented by other techniques the potential step methods and optical spectroscopy have proved suitable. 

For the in situ characterization of modified electrodes, the method of choice is electrochemical analysis by cyclic voltammetry, ac voltammetry, chronoamperometry or chronocoulometry, or rotating disk voltametry. Cyclic voltammograms are easy to interpret from a qualitative point of view (Fig, 1). The other methods are less direct but they can yield quantitative data more readily. 

Method Abs, chemical reduction, monitored by absorption spectroscopy CD, chemical reduction, monitored by CD spectroscopy CD/OTTLE, electrochemical reduction using an optically transparent thin layer (OTTLE) cell, monitored by CD spectroscopy CV, cyclic voltammetry EPR, chemical reduction, monitored by EPR. 

The use of direct electrochemical methods (cyclic voltammetry Pig. 17) has enabled us to measure the thermodynamic parameters of isolated water-soluble fragments of the Rieske proteins of various bci complexes (Table XII)). (55, 92). The values determined for the standard reaction entropy, AS°, for both the mitochondrial and the bacterial Rieske fragments are similar to values obtained for water-soluble cytochromes they are more negative than values measured for other electron transfer proteins (93). Large negative values of AS° have been correlated with a less exposed metal site (93). However, this is opposite to what is observed in Rieske proteins, since the cluster appears to be less exposed in Rieske-type ferredoxins that show less negative values of AS° (see Section V,B). 

Cyclic voltammetry and other electrochemical methods offer important and sometimes unique approaches to the electroactive species. Protein organization and kinetic approaches (Correia dos Santos et al. 1999, Schlereth 1999) can also be studied by electrochemical survey. The electron transfer reaction between cytochrome P450scc is an important system for... 

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]. 

Alemu et al. [35] developed a very sensitive and selective procedure for the determination of niclosamide based on square-wave voltammetry at a glassy carbon electrode. Cyclic voltammetry was used to investigate the electrochemical reduction of niclosamide at a glassy carbon electrode. Niclosamide was first irreversibly reduced from N02 to NHOH at —0.659 V in aqueous buffer solution of pH 8.5. Following optimization of the voltammetric parameters, pH and reproducibility, a linear calibration curve over the range 5 x 10 x to 1 x 10-6 mol/dm3 was achieved, with a detection limit of 2.05 x 10-8 mol/dm3 niclosamide. The results of the analysis suggested that the proposed method has promise for the routine determination of niclosamide in the products examined [35]. 

In order to determine the electrochemical properties of the solvent, the electrode process in molten carbamide and in carbamide-MeCl (where Me - NH4, K) mixtures on inert electrodes (platinum, glassy carbon) were investigated using cyclic voltammetry. The electrode reaction products were analysed by spectroscopic methods. The adsorbtion of carbamide- NH4CI anodic product was investigated by differential capacity method. 

Having defined our near electrode region, we turn now to consider the various techniques that can be employed in the in situ investigation of the reactions that occur within it. The various methods that can be employed will each provide different types of information on the processes occurring there. As has already been discussed, cyclic voltammetry is the most common technique first employed in the investigation of a new electrochemical system. However, in contrast to the LSV and CV of adsorbed species, the voltammetry of electroactivc species in solution is complicated by the presence of an additional factor in the rate, the mass transport of species to the electrode. Thus, it may be more useful to consider first the conceptually more simple chronoamperometry and chronocoulometry techniques, in order to gain an initial picture of the role of mass transport. 

The heterogeneous rates of electron transfer in eq 7 were measured by two independent electrochemical methods cyclic voltammetry (CV) and convolutive potential sweep voltammetry (CPSV). The utility of the cyclic voltammetric method stems from its simplicity, while that of the CPSV method derives from its rigor. 

An alternative electrochemical method has recently been used to obtain the standard potentials of a series of 31 PhO /PhO- redox couples (13). This method uses conventional cyclic voltammetry, and it is based on the CV s obtained on alkaline solutions of the phenols. The observed CV s are completely irreversible and simply show a wave corresponding to the one-electron oxidation of PhO-. The irreversibility is due to the rapid homogeneous decay of the PhO radicals produced, such that no reverse wave can be detected. It is well known that PhO radicals decay with second-order kinetics and rate constants close to the diffusion-controlled limit. If the mechanism of the electrochemical oxidation of PhO- consists of diffusion-limited transfer of the electron from PhO- to the electrode and the second-order decay of the PhO radicals, the following equation describes the scan-rate dependence of the peak potential ... 

Chemical reactivity of unfunctionalized organosilicon compounds, the tetraalkylsilanes, are generally very low. There has been virtually no method for the selective transformation of unfunctionalized tetraalkylsilanes into other compounds under mild conditions. The electrochemical reactivity of tetraalkylsilanes is also very low. Kochi et al. have reported the oxidation potentials of tetraalkyl group-14-metal compounds determined by cyclic voltammetry [2]. The oxidation potential (Ep) increases in the order of Pb < Sn < Ge < Si as shown in Table 1. The order of the oxidation potential is the same as that of the ionization potentials and the steric effect of the alkyl group is very small. Therefore, the electron transfer is suggested as proceeding by an outer-sphere process. However, it seems to be difficult to oxidize tetraalkylsilanes electro-chemically in a practical sense because the oxidation potentials are outside the electrochemical windows of the usual supporting electrolyte/solvent systems (>2.5 V). 

The one-electron reduction potentials, (E°) for the phenoxyl-phenolate and phenoxyl-phenol couples in water (pH 2-13.5) have been measured by kinetic [pulse radiolysis (41)] and electrochemical methods (cyclic voltammetry). Table I summarizes some important results (41-50). The effect of substituents in the para position relative to the OH group has been studied in some detail. Methyl, methoxy, and hydroxy substituents decrease the redox potentials making the phe-noxyls more easily accessible while acetyls and carboxyls increase these values (42). Merenyi and co-workers (49) found a linear Hammett plot of log K = E°l0.059 versus Op values of substituents (the inductive Hammett parameter) in the 4 position, where E° in volts is the one-electron reduction potential of 4-substituted phenoxyls. They also reported the bond dissociation energies, D(O-H) (and electron affinities), of these phenols that span the range 75.5 kcal mol 1 for 4-amino-... 

According to Eq. (27), Stromme et al.125,126 developed systematically the peak-current method to determine the fractal dimension of the electrode surface by using cyclic voltammetry. It must be recalled that this method is valid when the recorded current is limited by diffusion of the electroactive species to and away from the electrode surface. Since the distribution of the reaction sites provides extensive information about the surface geometry, the fractal dimension of the reaction site distribution may agree with the fractal dimension of the electrode surface which is completely electrochemical-active. In addition, it is well known that this method is insensitive to the IR drop in the electrolyte.126... 

As mentioned in potentiostatic current transient method, when the fractal dimension is determined by using diffusion-limited electrochemical technique, the diffusion layer length acts as a yardstick length.122 In the case of cyclic voltammetry, it was... 

How electron transfer kinetics may be investigated by means of an electrochemical method such as cyclic voltammetry is the question we address now, starting with the case where the reactants are immobilized on the electrode surface, as in the beginning of Section 1.2. The key equations are those that relate the surface concentrations rA and rB to the current. The first of these expresses the Faradaic consumption of A and production of B as the current flows ... 

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