Waves cathodic

Fig. 4.7 Voltammetric behavior of a Au(lll) electrode immersed in 1 mM Se02 in 0.100 M HCIO4 supporting electrolyte. The five major cathodic waves corresponding to Se deposition are labeled C1-C5, respectively. The scan rate was 0.100 V s. (Reprinted from [82], Copyright 2009, with permission from Elsevier)...

Figure 6.7 shows a typical special feature of the polarization curves. In the case of reversible reactions (curve 1), the anodic and cathodic branches of the curve form a single step or wave. In the case of irreversible reactions, independent, anodic and cathodic, waves develop, each having its own inflection or half-wave point. The differences between the half-wave potentials of the anodic and cathodic waves will be larger the lower the ratio fH. ... 

The term "half-wave potential relates to the symmetrical wave that one obtains when E is plotted against i in eqn. 3.15 (cathodic wave) and eqn. 3.16... 

The above considerations also apply to the ion of an amalgamating metal with the reversible equilibrium M"+ + ne M(Hg) at a stationary mercury electrode such as an HMDE (hanging mercury drop) or an MTFE (mercury thin-film) with the restriction, however, that the solution can contain only ox, so that merely the cathodic wave (cf., eqn. 3.15) represents a direct dependence of the analyte concentration, whilst the reverse anodic wave concerns only the clean-back of amalgam formed by the previous cathodic amplitude. When one or both of the electrodic reactions is or becomes (in the case of a rapid potential sweep) irreversible, the cathodic wave shifts to a more negative potential and the anodic wave to a more positive potential (cf., Fig. 3.10) this may even result in a complete separation of the cathodic and anodic waves (cf., Fig. 3.11). 

Another situation occurs in the case of a totally irreversible electrode reaction, notwithstanding the solubility of ox and red for an irreversible cathodic wave it means that the current i is determined only by the first term on the right-hand side in eqn. 3.8, so that... 

The first cathodic wave was studied by cycling the potential across it at various scan rates and the peak potentials were found to increase as indicative of a reversible, diffusion-controlled system, with ° = — 1.43 V vs. SCE. However, at sweep rates 20mV/s the peak anodic current is much smaller than expected which was interpreted by the authors as indicating that the reduced species undergoes a subsequent chemical reaction, i.e. an EC process. 

The authors then examined the voltammograms obtained for two consecutive sweeps between 0 V and — 2.1 V (see Figure 3.57(a)). The first complete scan shows a very similar voltammogram to that observed in Figure 3.56 down to —2.1V, with cathodic waves at —1.47V and —1.98V. On the second sweep these two waves have decreased substantially and a new wave appears near —1.65 V and —0.23 V. Initiating the second sweep in the negative direction at — 1.45 V (see Figure 3.57(b)) shows that the new cathodic wave is the counterpart to the anodic wave at — 1.57 V. The authors attributed these features to reversible couples with ° at —1.62 V and -0.19V. 

The situation is different on GC where only one irreversible cathodic wave is observed at —1.6 V, associated with disintegration of the compound, and one anodic wave on the reverse scan, caused by stripping of the lead released by the reduction ... 

A complicated reaction pattern is also observed with dichlorotetraphenylditin87. The electrochemistry of this compound compound on Hg electrodes involves formation of intermediate SnHg compounds by reduction (see also Reference88). The polarogram of Ph2ClSn—SnClPh2 (in methanol/LiCl, on Hg) shows an anodic peak and two cathodic waves at —0.4, —0.55 and —1.35 (vs SCE). The oxidation involves between one and two electrons as determined by coulometry, and the proposed reactions are ... 

Reduction of the dimer appears to take place in two stages. In the first cathodic wave, a single-electron reduction generates a radical which decomposes in a chemical step in an ECE sequence ... 

The oxidation in the last two reactions explains the anodic peak. Under the second cathodic wave an overall four-electron reduction yields a diphenyltin polymer and diphenyltin hydride ... 

The cyclic voltammogram for a silver electrode in 0.1M LiC104 acetonitrile solution is shown in Figure 1 (curve a). At a potential of -1.5 V, cathodic current due to the reduction of Li+ ions commences. The upd of lithium has been reported previously by Kolb et al. for positive potential sweeps after substantial lithium reduction (i) however, due to the reactivity of the metallic lithium with impurities in solution, the adsorbed layer formed on the negative potential sweep is not as stable as other upd monolayers (i). An additional cathodic wave due to the reduction of lithium is observed at approximately -2.5V, and on the return sweep the lack of an anodic wave is indicative of the reactivity of the chemisorbed atoms. 

The nanostructured Au and AuPt catalysts were found to exhibit electrocatalytic activity for ORR reaction. The cyclic voltammetric (CV) curves at Au/C catalyst reveal an oxidation-reduction wave of gold oxide at +200 mV in the alkaline (0.5 M KOH) electrolyte but little redox current in the acidic (0.5 M H2SO4) electrolyte. Under saturated with O2, the appearance of the cathodic wave is observed at -190 mV in the alkaline electrolyte and at +50 mV in the acidic electrolyte. This finding indicates that the Au catalyst is active toward O2 reduction in both electrolytes. From the Levich plots of the limiting current vs. rotating speed data, one can derive the electron transfer number (w). We obtained n = 3.1 for ORR in 0.5 M KOH electrolyte, and 2.9 for ORR in 0.5 M H2SO4 electrolyte. The intermittent n-value between 2 and 4 indicates that the electrocatalytic ORR at the Au/Ccatalyst likely involved mixed 2e and 4e reduction processes. 

In acidic or slightly basic aqueous media, Ai-hydroxyindoles can be prepared after electrolysis of a-(o-nitrophenyl) ketones vide supra Scheme 9) [13], at a working potential corresponding to the first cathodic wave. In an acidic medium, indoles are directly obtained at a working potential corresponding to the second cathodic wave. 

As a result, an insolnble transparent blue polymer film forms on the electrode. Electrochemical oxidation of the film in acetonitrile initiates a rapid color change from bine to pale gray, while redaction to the first or second cathodic waves causes the film to become pale green or orange, respectively. These electrochromic effects are stable and reversible when air and water are excluded, even after 30,000 rednction cycles. The material has potential uses in electrochromic or electroluminescent devices. 

Anionic and Cationic Carbonyls. The polarographic behaviour of Et N-[Fe(CO)3NO] at dropping mercury and stationary platinum electrodes has been studied. Two anodic waves and one cathodic wave were observed and the following reactions were suggested ... 

The more cathodic wave is due to further reduction of the radical intermediate formed at the potential of the first wave. 

The solution electrochemical properties of 15 and 16 were studied. The one-electron wave for the core unit of 15 at negative potential provides a very convenient internal standard, which assists calculations of the number of electrons involved in the redox processes at the pieriphery. The voltammogram of 15 shows a relative intensity ratio of 6 0.3 between the anodic and cathodic waves which are fully chemically reversible (/// = ca. 1.0) (Figure 1). The A p value of the ferrocene signal (55 mV) indicates the six ferrocene groups are electronically equivalent, and their oxidation potentials are essentially the same in compounds 15 and 16 ( =... 

Kinetics and mechanism of electroreduction of Zn(II) ammonia and hydroxy-ammonia complexes on the DME were investigated by Kravtsov and coworkers [93]. The reduction of Zn(II) on DME in solution of pH 9.2-12 and [NH3] = 0.05-2 M occurred in one irreversible diffusion-limited cathodic wave. 

Fig. 8 Cathodic peak potential (empty symbols) and cathodic wave charge (filled symbols) as a function of holding time at +0.1 V in 0.1 M KOH ethanolic solutions containing 10 pM (o, ), lOOpM ( , ), and 1 mM (0, ) decanethiol. Inset shows the initial stage of the above relations. All data concerning reductive charge contain 5% error [125].

KC1, which bathed the CdS film. This system was also investigated by cyclic voltammetry both in the dark and under illumination. Starting at about — 0.9 V, the dark cathodic current exhibited a peak at — 1.15 V due to Cd2+ reduction and then rose to — 1.4 V as a result of hydrogen production. The observed anodic peak at — 0.85 V was attributed to the stripping of cadmium deposits in the lattice (Cd ). Cyclic voltammetry subsequent to illumination resulted in the appearance of cathodic waves at — 1.0 V and — 1.3 V, at the expense of that at — 1.15 V. The anodic peak broadened, as is indicative of photocorrosion. 

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