Scans, cyclic

The technique employed by Lamy and colleagues was rapid-scan cyclic voltammetry in extremely dry DMF. In order to try and increase the lifetime of the C02 species the experiments were performed in the presence of active alumina suspensions. Aylmer-Kelly et al1973 had calculated the rate constant for reaction of the radical with water as a fast 5.5dm1 niol 1 s 1 and it was also hoped that reducing the solvation of the radical by water would increase the coulombic repulsion between radicals and so reduce dimerisation). 

Fig. 5 Linear sweep and cyclic voltammetry (a) dotted lines five profiles respectively at various typical excitation signal (b) current response times, increasing time shown by arrows] for a and concentration profiles [(c) forward scan cyclic voltammetric experiment.

Similar irreversible processes have been previously observed by Aurbach et al. on LiNi02 ° and Croce et al. on LiNio.75Coo.2502 ° by slow scan cyclic volta-mmetric (SSCV) measurements. However, in their EIS studies the former authors did not observe any obvious potential-dependence for the resistance associated with the surface layer ° but rather an invariant resistance in the range between 3.6 and 4.2 V. This latter observation is in direct disagreement... 

Figure 51. Cathodic and anodic stability of LiBOB-based electrolytes on metal oxide cathode and graphitic anode materials Slow scan cyclic voltammetry of these electrode materials in LiBOB/EC/EMC electrolyte. The scan number and Coulombic efficiency (CE) for each scan are indicated in the graph. (Reproduced with permission from ref 155 (Eigure 2). Copyright 2002 The Electrochemical Society.)...

In the case of dissociative electron transfer to aromatic compounds, electron transfer is not necessarily concerted with bond dissociation. The substrate 7t-radical-anion may be an intermediate whose existence can be demonstrated by fast scan cyclic voltammetry in aptotic solvents. At fast scan rates, reversible electron transfer occurs. At slower scan rates, die anodic peak height falls and a second reversible electron transfer step appears due to formation of the radical-anion of the compound formed by replacement of the substituent by hydrogen. Cleavage of the... 

Fig. 2 Negative-scan cyclic voltammogram of5 x 10 M [Mn(CO)2(dppe)2] PFe (l + PFe-) in THF, containing 0.3 M TBACIO4 at V = 0.5 V (reprinted with permission from Ref 43, Copyright 1988 American Chemical Society).

Figure 2.2 shows the first-scan cyclic voltammogram obtained by a platinum electrode for a 1.0 mM K4[Fe(CN)g] plus O.IOM KCl solution in water. Here, as in all figures in this chapter, potentials are referred to the AgCl (3 M NaCl)/Ag electrode. Generally, the first recorded cycles differ slightly, and only after 3-4 cycles do the CVs approach a constant response. In this experiment, the potential scan is... 

In conventional electrochemistry in solution, quantitation of analytes can be obtained by using several techniques. Thus, exhaustive electrolysis provides an absolute quantitation of an electroactive component in the sample. Voltammetric measurements (linear potential scan, cyclic, pulse, and square-wave techniques) can be used for determination of analytes in solution via calibration because peak currents (and peak areas) are usually proportional to the analyte concentration under fixed electrochemical conditions. 

On the contrary, the radical cation of anthracene is unstable. Under normal volt-ammetric conditions, the radical cation, AH +, formed at the potential of the first oxidation step, undergoes a series of reactions (chemical -> electrochemical -> chemical -> ) to form polymerized species. This occurs because the dimer, tri-mer, etc., formed from AH +, are easier to oxidize than AH. As a result, the first oxidation wave of anthracene is irreversible and its voltammetric peak current corresponds to that of a process of several electrons (Fig. 8.20(a)). However, if fast-scan cyclic voltammetry (FSCV) at an ultramicroelectrode (UME) is used, the effect of the follow-up reactions is removed and a reversible one-electron CV curve can be obtained (Fig. 8.20(b)) [64], By this method, the half-life of the radical cat-... 

U seful methods for detecting short-lived cationic radicals are fast-scan cyclic voltammetry at a UME (Section 8.4.2) and cyclic voltammetry at low temperatures (Section 8.4.3). It is preferable to prepare electrolyt-... 

Tab. 8.9 Examples of the rate constants of electrode reactions and subsequent reactions determined by rapid-scan cyclic voltammetry [51 d, 66e]...

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. 

Reduction of the solution temperature allows transition from steady-state to peak-shaped response simply by way of the marked diminution of D at low temperatures. Figure 16.5 shows slow-scan cyclic voltammograms obtained at two microdisk electrodes as a function of solution temperature. Between -120 and -140°C there is a particularly clear transition for the 25-pm-diameter electrode as the diffusion-layer thickness becomes less than the disk radius. Also illustrated here is the immense decrease in the limiting currents that is seen over this range of temperatures due to the 100-fold decrease in D. 

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]. 

Attention has turned recently to the mechanistic details underlying these processes. Probably the most significant development in this area in recent years is the discovery that the ketyl radical produced by electrochemical reduction of benzaldehyde in buffered neutral ethanol is harder to reduce than benzaldehyde itself46. All previous discussions had assumed that the ketyl radical would be reduced as quickly as it is formed, but fast scan cyclic voltammetry demonstrated the existence of a short-lived intermediate, apparently the ketyl radical. Computer simulation of the voltammograms showed that the radical dimerizes at a rate ca 106 1VD1 s 1. 

Sinusoidal voltammetry (SV) is an EC detection technique that is very similar to fast-scan cyclic voltammetry, differing only in the use of a large-amplitude sine wave as the excitation waveform and analysis performed in the frequency domain. Selectivity is then improved by using not only the applied potential window but also the frequency spectrum generated [28]. Brazill s group has performed a comparison between both constant potential amperometry and sinusoidal voltammetry [98]. 

Another voltammetric method, sinusoidal voltammetry (SV), was also employed. This was a frequency-based electrochemical method, which was found to be more sensitive than the usual constant potential (DC) amperometric detection method. SV has been achieved to detect on-chip separated catecholamines. This method is very similar to fast-scan cyclic voltammetry (CV), except that a... 

Fig. 14.34. Voltammetry of epinephrine. Background (A, solid line) and signal containing (A, dashed line) currents generated during fast-scan cyclic voltammetry (300 V/s) at a carbon fiber microelectrode r = 5 pm). A background subtracted cyclic voltammogram (B) is produced from the traces shown in A. (Reprinted from Wightman, et al. Chemical Communication, Interface, 5(3) 22, Fig. 2,1996. Reproduced by permission of the Electrochemical Society, Inc.)...

The use of fast scan cyclic voltammetry has already been described (Section 8.6). In general, microelectrodes, in some cases modified by electrocatalysts, are making it possible to learn about biological events on the scale of a single cell. Among the more important achievements (Wightmann, 1996) is the monitoring of dopamine released after stimulation from neurons in the intact brain and involved in neurotransmission. 

The detection of (specifically) dopamine is hindered by the presence in the extracellular fluid of several compounds having redox potentials close to that of dopamine. The technique most likely to succeed here is fast scan cyclic voltammetry (Section 8.6) because the voltamogram provides characteristics that are indicative of the individual compound being monitored. The microelectrodes used have radii of 5 pm, but even this is not small enough to be able to determine dopamine from just one cell. The reacting compounds come from several nerve endings. Nevertheless, the fast scan cyclic voltammetry technique her sufficient time and resolution to allow information to be obtained on the part played by dopamine in neurotransmission in the brain. For example, it answers such questions as does the released dopamine stay at the synapse or does it diffuse in the extracellular fluid to contact other neurons ... 

Fig. 4.2 Cathodic scan cyclic voltammograms of near-Lewis neutral and LiCl-buffered [EMIM]Cl/AICI3 ionic liquids at a W working electrode. Scan rate lOOmVs-1. (a) Negative scan limit —2.2V (b) negative scan limit —2.7V.

Fast-scan cyclic voltammetry (10 mVs-1 to 106 Vs-1) was used to measure the rate constants of C—X cleavage, which are extremely fast. The technique was applied to measure rate constants of the order of submicrosecond half-lives. For example, the radical anions generated at the electrode surface were determined to have a half-life ranging from less than 100 ns in the case ofp-bromoacetophenone to 70 ms for m-nitrobenzyl chloride189. The method complements the redox catalytic method developed by Saveant and co workers190. 

The EQCM has been most commonly used simultaneously to quasisteady state techniques like slow scan cyclic voltammetry. In this way mass changes during electrolysis can be obtained from A/(Am/.4) vs. potential curves, while A/(AmA4) vs. charge density curves allow evaluation of the number of Faraday exchanged per mole of electro-active species by use of Faraday s law of electrolysis. 

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. 

In normal high pressure liquid chromatography, typical sample volumes are 20-200 p.L this can become as little as 1 nL in capillary HPLC. Pretreatment of the sample may be necessary in order to protect the stationary phase in the column from deactivation. By employing supercritical fluids such as carbon dioxide, pretreatment can be bypassed in many instances so that whole samples from industrial and environmental matrices can be introduced directly into the column. This is due to the fact that the fluid acts as both extraction solvent and mobile phase. Post-column electrochemistry has been demonstrated. For example, fast-scan cyclic voltammo-grams have been recorded as a function of time after injection of microgram samples of ferrocene and other compounds in dichloromethane solvent and which are eluted with carbon dioxide at pressures of the order of 100 atm and temperatures of 50°C the chromatogram is constructed as a plot of peak current vs. time [18]. 

Other forms of voltammetry are as follows (1) fast-scan cyclic voltammetry useful in neuroelectrochemistry (2) nanosecond voltammetry for a 5-pm disk working microelectrode with RC < 1 gs, scan rates of 2.5 MV/s allow for fast kinetics measurements (3) differential-pulse voltammetry with staircase pulses, potential resolutions of 0.04 V and detection limits of 10 8M can be attained (4) anodic (cathodic) stripping voltammetry traces... 

Transient technique — A technique whose response is time dependent and whose time dependence is of primary interest, e.g., -> chronoamperometry, -> cyclic voltammetry (where current is the transient), -> chronopotentiometry and -> coulostatic techniques (where voltage is the transient). A transient technique contrasts with steady-state techniques where the response is time independent [i]. Some good examples are cyclic voltammetry [i, ii] (fast scan cyclic voltammetry), the indirect-laser-induced-temperature-jump (ILIT) method [iii], coulostatics [i]. The faster the transient technique, the more susceptible it is to distortion by -> adsorption of the redox moiety. 

Garris PA, Wightman RM (1995) Regional differences in dopamine release, uptake and diffusion measured by fast-scan cyclic voltammetry. In Boulton A, Baker G, Adams RN (Eds), Neuromethods, Vol. 25, pp. 179-220. Humana Press Inc. 

These considerations highlight the fact that microdialysis and voltammetry, particularly fast-scan cyclic voltammetry, estimate two quite different aspects of DA transmission characterized by different temporal and spatial constants. Microdialysis estimates steady-state levels of extracellular DA and changes in these levels taking place on a minute scale away from DA release sites. Fast-scan voltammetry estimates changes in DA-like signals taking place on a subsecond scale near DA release sites. 

Robinson DL, Venton BJ, Heien ML, Wightman RM (2003) Detecting subsecond dopamine release with fast-scan cyclic voltammetry in vivo. Clin Chem 9(10) 1763—1773. 

These kinetic results are interesting in that they are consistent with the physical reality of the thinned diffusion-layer model introduced above. Moreover it is evident that sonovoltammetry enables fast rate constants to be measured under steady-state conditions at conventionally dimensioned electrodes otherwise these would only be accessible via transient measurements such as fast-scan cyclic voltammetry or using steady-state microelectrode methodology. 

---