Fast-scan electrochemical techniques

Electrochemical techniques, fast-scan, 46 163 Electrochemistry, 36 341-342, 368, see also Dynamic electrochemistry, FeOI3S proteins... 

Fast-scan electrochemical techniques, 46 163 F-Centers, and mercury chalcogenide halides, 23 356-357... 

If M is unstable then ipb/fpf will be less than unity. Its magnitude will depend upon the scan rate, the value of the first-order constant k, and the conditions of the experiment. At fast scan rates the ratio ipb/ ip, may approach one if the time gate for the decomposition of M is small compared with the half-life of M-, (In 2jk). As the temperature is lowered, the magnitude of k may be sufficiently decreased for full reversible behaviour to be observed. The decomposition of M- could involve the attack of a solution species upon it, e.g. an electrophile. In such cases, ipb/ipf, will of course be dependent upon the concentration of the particular substrate (under pseudo-first-order conditions, k is kapparent). Quantitative cyclic voltammetric and related techniques allow the evaluation of the rate constants for such electrochemical—chemical, EC, processes. At the limit, the electron-transfer process is completely irreversible if k is sufficiently large with respect to the rate of heterogeneous electron transfer the electrochemical and chemical steps are concerted on the time-scale of the cyclic voltammetric experiment.1-3... 

The information that can be obtained with electrochemical detectors is not restricted to quantification. Instead of the conventional use of electrochemical detectors in amperometric mode at fixed potential, electrode arrays with each electrode held at different values of fixed potential can be used, in order to build up chronovoltammograms, three-dimensional current-voltage-time profiles. A 32-microband electrode array has been described for this purpose and applied to phenolic compounds [17] and which permits studying the electrode reaction mechanism at the same time as identification and quantification are carried out. Alternatively, fast voltammetric techniques such as fast-scan cyclic voltammetry or square wave voltammetry can be used to create chronovoltammograms of the eluted components. 

The time range of the electrochemical measurements has been decreased considerably by using more powerful -> potentiostats, circuitry, -> microelectrodes, etc. by pulse techniques, fast -> cyclic voltammetry, -> scanning electrochemical microscopy the 10-6-10-1° s range has become available [iv,v]. The electrochemical techniques have been combined with spectroscopic ones (see -> spectroelectrochemistry) which have successfully been applied for relaxation studies [vi]. For the study of the rate of heterogeneous -> electron transfer processes the ILIT (Indirect Laser Induced Temperature) method has been developed [vi]. It applies a small temperature perturbation, e.g., of 5 K, and the change of the open-circuit potential is followed during the relaxation period. By this method a response function of the order of 1-10 ns has been achieved. 

One-electron oxidation of phenyl iron(III) tetraarylpor-phyrin complexes with bromine in chloroform at —60°C produces deep red solutions whose H and H NMR spectra indicate that they are the corresponding iron(IV) complexes. For the low-spin aryl Fe porphyrins the electron configuration is (dxyf(dxz,dyzf, with one tt-symmetry unpaired electron, and for the low-spin aryl Fe porphyrins the electron configuration is d, yf- d, zAyzf with two TT-symmetry unpaired electrons. The aryl Fe porphyrins are thermally unstable, and upon warming convert cleanly to A-phenylporphyrin complexes of Fe by reductive elimination. This process has been investigated by electrochemical techniques, by which it was shown that the reversible (at fast scan rates) one-electron oxidation of a-aryl complexes of PFe was followed by an irreversible chemical reaction that yielded the Fe complex of the A-phenylporphyrin, which could then be oxidized reversibly by one electron to yield the Fe complex of the A-phenylporphyrin. (If the Fe complex of the N-phenylporphyrin is instead reduced by one electron, the Fe complex of the A-phenylporphyrin is formed reversibly at... 

Although multiple electrochemical techniques exist, those used in freely moving animals are chronoamperometry, differential normal-pulse voltammetry, and fast-scan cyclic voltammetry. Excellent comparisons between these can be found in literature, particularly Troyer et al.[5,7,30] and Robinson el al.[8] and therefore will not be diskussed here. 

Many interesting processes occurring at the liquid/liquid interface involve coupled homogeneous chemical reactions. In principle, electrochemical methods used for probing complicated mechanisms at metal electrodes (61) can be employed at the ITIES. However, many of these techniques (e.g., rotating ring-disk electrode or fast-scan cyclic voltammetry) are hard to adapt to liquid/liquid measurements. Because of technical problems, few studies of multistep processes at the ITIES have been reported to date (1,62). 

Yet another option is being elaborated by Fraser Armstrong and his group at Oxford through fast-scan electrochemical techniques, which have now advanced to the point where they are capable of probing... 

One advantage of the cyclic voltammetry technique is that, to some extent at least, the lifetime of the experiment may be controlled through the scan rate, i.e. the rate at which the potential range is scanned and then reversed. This means that if one of the components of the couple is not very stable it may still be possible, by increasing the scan rate, to observe reversibility and hence determine °. Thus the n-superoxo/p-peroxo-dicobalt(III) couples in the complexes of the type [(NH3) Co()i-02)Co(NH3)5] + + can be determined readily even though the peroxo complex is not very stable and decomposes rapidly under conditions where the superoxo is stable. However, the scan rate must not be increased too much as at fast scan rates the rate of the electrochemical process becomes slow relative to the scan rate and irreversible behaviour (A p>57/n mV) will be obtained. In fact, this is the basis of a method for obtaining the heterogeneous rate constants of reversible couples. 

Electrochemical techniques have also been applied to determining metal-metal bond energies in dinuclear organometallic molecules (44). Using a combination of redox equilibration experiments with reagents of known potentials and fast scan cyclic voltammetry, it is possible to obtain E° data for the kinds of reactions shown in Equations 33 and 34, respectively. The result is the free energy (but not the... 

The primary electrochemical technique employed in these investigations is background-subtracted fast-scan cyclic voltammetry. For DA, the waveform is... 

As it can provide some of the most basic electrochemical information related to the reactivity of the selected analyte (peak potential and peak current) most instruments that perform amperometry can also perform some of the most basic voltammetric techniques. These techniques determine the current as a function of the potential applied to the WE (in a conventional three-electrode cell) and can be performed with relatively simple instrumentation [105,106]. As different signals can be combined in the input ports of the instrument, multiple variations of the technique have been developed including cyclic voltammetry, linear sweep voltammetry, linear sweep stripping voltammetry, stripping voltammetry [107, 108], fast-scan cyclic voltammetry [109], square-wave voltammetry [110],and sinusoidal voltammetry [111]. 

The time range of the electrochemical measurements has been decreased considerably by using more powerful -> potentiostats, circuitry, -> microelectrodes, etc. by pulse techniques, fast -> cyclic voltammetry, -> scanning electrochemical microscopy the s range... 

Impedance spectroscopy or electrochemical impedance spectroscopy is a powerful electrochemical technique used to investigate the binding events that occur at the electrode surface. The same three electrode systems comprising of a WE, RE and CE are utilized for the EIS experiment. Electrochemical impedance is usually measured by applying an AC sine wave potential with low amplitude (5—10 mV peak to peak) superimposed on a DC potential to the electrochemical system. The AC signal scans the frequency domain, allowing the individual excitation of different processes with different time constants. Therefore, slow processes like chemical reactions and fast reactions Hke ionic conduction can be studied independently this way. 

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