Voltammetry fast scan rate cyclic

Fast scan rate cyclic voltammetry - Cyclic voltammetry executed using scan rates exceeding 105 V s 1. Presently,... 

With very fast scan rate cyclic voltammetry, an upper limit of the scan rate where standard theory prevails is given by the condition that the diffusion layer becomes equal in size to the diffuse layer (see Fig. II. 1.8). It has been estimated that this limit occurs at a scan rate of 1-2 x 10 V s [45]. In the other extreme, at very slow scan rates, natural convection is known to affect the shape of experimental cyclic voltammograms. 

Andrieux, C. P., Audebert, P., Hapiot, P., Nechtschein, M., and Odin, C., Fast scan rate cyclic voltammetry for conducting polymers electropolymerized on ultramicroelectrodes, J. Electroanal. Chem., 305, 153-162 (1991). 

Figure 7 Simple model of the concentration of vesicular events as determined by fast-scan rate cyclic voltammetry. (A) Pictorial demonstration that the area of the electrode used for oxidation/reduction of the DA species is very different for the cellular case compared with that in standard solution. (B) A head-on view depicting the difference in electrode area used in the above two cases. Bevehng a carbon fiber on a 45° angle creates an elliptical surface with major and minor radii of about 3.5 and 2.5 i,m, respectively. It is apparent that a large difference exists between the vesicular area and that of the total electrode. (Reproduced from Ana/. Chem. with permission [13].)...

Figure 9 Apparent distributions of vesicular concentrations detected by fast-scan rate cyclic voltammetry following K -stimulated exocytosis. (A) Distribution generated from control PC12 cells (n = 29 cells, 77 total events). (B) Distribution generated from L-DOPA-exposed (100 pM for 1 hr) PC12 cells (n = 11 cells, 145 total events). For each data set, the peak current of the anodic wave of all events is matched to its respective calibration plot to determine the detected concentration. All data are collected into bins having increments of 0.25 pM and plotted as the percent of the total number of release events detected. (Reproduced from Anal. Chem. with permission [13].)...

Fig. II. 1.1). Other complex reaction schemes of practical importance commonly encountered involve association reactions such as precipitation or polymerisation. These processes may also be described by a series of consecutive reaction steps. However, as a further complication in these classes of reactions, the properties of the surface of the electrode may change during the course of the process. Extensive studies aimed at elucidating very complex reaction pathways have been conducted for the electrodeposition of conducting polymers and, as might be expected, fast scan rate cyclic voltammetry has made an important contribution to an understanding of the mechanism [114]. 

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... 

The delocalised radical formed by protonation of the radical-anion is more easily reduced than the starting arene. For some polycyclic aromatic hydrocarbons, the redox potential for this radical species can be determined using a cyclic voltammetry technique [10]. Reduction in dimethylformamide is carried out to the potential for formation of the dianion. The dianion undergoes rapid monoprotonation and on the reverse sweep at a fast scan rate, oxidation of the monoanion to the radical can be observed. The radical intermediate from pyrene has E° = -1.15 V vs. see in dimethylformamide compared to E° = -2.13 V vs. see for pyrene,... 

Because the current that flows at a UME is small, we can scan the electrode potential at extremely high rates, sometimes reaching 1000000 Vs-1 or even higher [73]. Using cyclic voltammetry at such fast scan rates, we can detect in-... 

The electrochemical oxidation of NADH in aqueous solutions is seen as a single peak by cyclic voltammetry and takes place at 0.4, 0.7, and IV vs SCE at carbon, Pt, and Au electrodes, respectively (37,38). No re-reduction of NADH related intermediates is observed in cyclic voltammetry even at fast scan rates (30 V/s) (39), reflecting the high chemical irreversibility of the reaction. It was early recognized that the oxidation of NADH resulted in electrode fouling, necessitating... 

Reduction potentials for /z-superoxo//z-peroxo-cobalt(III) couples have recently been obtained by cyclic voltammetry.739,740 Decomposition or dissociation of one or the other of the components of the couple, which frustrated measurements by other techniques, may be overcome by the use of fast scan rates. Protonation of the /r-peroxo group stabilizes the complex and EB values are pH dependent below pH 3. A similar stabilization on protonation of the peroxo bridge has been noted in the Ct2+-, V2+- and Eu2+-promoted reduction705 of the [(NHj)5Co(02)Co(NH3)5]4+ ion. The protonated species has Kh 10 dm3 mol-1 at 25 °C.705... 

In linear sweep or cyclic voltammetry, high scan rates can be used to shorten the time-scale below the steady-state time-scale arising from diffusion or convective diffusion, lypically scan rates of 10-3000 Vs are used, corresponding to time-scales of 1-10 p,s, although in fast-scan CV they may reach... 

In the type of linear-sweep voltammetry discussed thus far, the potential is changed slowly enough and mass transfer is rapid enough that a steady state is reached at the electrode surface. Hence, the mass transport rate of analyte A to the electrode just balances its reduction rate at the electrode. Likewise, the mass transport of P away from the electrode is just equal to its production rate at the electrode surface. There is another type of linear-sweep voltammetry in which fast scan rates (1 V/s or greater) are used with unstirred solutions. In this type of voltammetry, a peak-shaped current-time signal is obtained because of depletion of the analyte in the solution near the electrode. Cyclic voltammetry (see Section 23D) is an example of a process in which forward and reverse linear scans are applied. With cyclic voltammetry, products formed on the forward scan can be detected on the reverse scan if they have not moved away from the electrode or been altered by a chemical reaction. 

Various polarographic techniques have been successfully used to measure elemental sulfur, S , and sulfide (H2S and HS ) concentrations in laboratory solutions [116], salt water [117], saline lake [118], freshwater [119], lake porewaters [120], salt marsh sediments [109,120], and marine porewaters [121], Using stripping square wave voltammetry, researchers have been able to measure sulfide species at nanomolar concentrations [118-120, 122,123], Recently, cyclic and linear sweep voltammetries have been used to quantify elemental sulfur S , hydrogenosulfide (HS ) and polysulfides (S ) with fast scan rates (IV s ) [124] for example, in estuarine sediments (from Rehoboth Bay, an inland bay located in Delaware), the sulfur speciation was found to change throughout a core profile, with dominant in the top layers (0-6 cm), dominant... 

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. 

The cyclic voltammetry of the complexes tranj-lRu LXJ, where L = 4NH3 or 2en, X = Cl or Br etc., illustrates the application of the technique for obtaining detailed mechanistic information on reactions occurring subsequent to the initial redox process. At moderately fast scan rates, ca. 1 V s the redox behaviour is typical of a single reversible one-electron couple corresponding to the couple At slower scan rates, ca. 0.1 V s , a wave corresponding to the same... 

Thus polarography and cyclic voltammetry of 10 -10 MSm or Gd " in 0.001 M H2SO4 showed, for the cyclic voltammetry, a two step reduction in each case, which was reversible for samarium with fast scan rates (50 V s ) but which showed no anodic waves for gadolinium. The reduction for the latter was interpreted as being similar to dysprosium where there is a two-stage reduction of water via lanthanide hydroxy species, no species being involved (equations 16 and 17). 

When the formal potentials of the adsorbed and non-adsorbed redox couples are similar, a single wave is obtained where the contribution of the electron transfer involving the adsorbed species increases with the scan rate. Thus, there is a transition from diffusional-shaped voltammograms at slow scan rates to adsorptive-shaped at fast scan rates. To understand this behaviour and illustrate the characteristics of adsorptive voltammograms let us consider the response in cyclic voltammetry of a monolayer of species A that undergoes a one-electron, fully reversible electron transfer ... 

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