Waves anodic

Thiocyanate 0.18 Anodic wave neutral or weakly alkaline medium... 

In the presence of 6-iodo-l-phenyl-l-hexyne, the current increases in the cathodic (negative potential going) direction because the hexyne catalyticaHy regenerates the nickel(II) complex. The absence of the nickel(I) complex precludes an anodic wave upon reversal of the sweep direction there is nothing to reduce. If the catalytic process were slow enough it would be possible to recover the anodic wave by increasing the sweep rate to a value so fast that the reduced species (the nickel(I) complex) would be reoxidized before it could react with the hexyne. A quantitative treatment of the data, collected at several sweep rates, could then be used to calculate the rate constant for the catalytic reaction at the electrode surface. Such rate constants may be substantially different from those measured in the bulk of the solution. The chemical and electrochemical reactions involved are... 

In the ideal case, reversible cyclic voltammograms of redoxactive films should show completely symmetrical and mirror-image cathodic and anodic waves with identical peak potentials and current levels 34-i37) pjg... 

The various oxidation states of sulfur have been determined by polarography. The electrochemical oxidation of sulfide ions in aqueous solution may lead to the production of elementary sulfur, polysulfides, sulfate, dithionate, and thiosulfate, depending on the experimental conditions. Disulfides, sulfoxides, and sulfones are typical polarographically active organic compounds. It is also found that thiols (mer-captans), thioureas, and thiobarbiturates facilitate oxidation of Hg resulting thus in anodic waves. 

Analogously we find for a solution with only red and so for the anodic wave... 

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). 

Hence the picture of the cathodic and anodic waves obtainable for a completely reversible redox couple by means of the RDE corresponds fully with that in Fig. 3.9 the value of i, i.e., the height of the sigmoidal waves, is linearly proportional to to1/2 and to C (see eqn. 3.89 and the Levich constant). If for a well chosen combination of C and E a plot of i against co1/2 deviates from a straight line passing through the origin, then in the kinetics of the electrode reaction we have to deal only with a rapid electron transfer (cf., Fig. 3.10) or even with a slow electron transfer (cf., Fig. 3.11), in which latter instance the transfer coefficient a plays an appreciable role (cf., eqns. 3.17 and 3.18). 

In the controlled (constant) potential method the procedure starts and continues to work with the limiting current iu but as the ion concentration and hence its i, decreases exponentially with time, the course of the electrolysis slows down quickly and its completion lags behind therefore, one often prefers the application of a constant current. Suppose that we want to oxidize Fe(II) we consider Fig. 3.78 and apply across a Pt electrode (WE) and an auxiliary electrode (AE) an anodic current, -1, of nearly the half-wave current this means that the anodic potential (vs. an RE) starts at nearly the half-wave potential, Ei, of Fe(II) - Fe(III) (= 0.770 V), but increases with time, while the anodic wave height diminishes linearly and halfway to completion the electrolysis falls below - / after that moment the potential will suddenly increase until it attains the decomposition potential (nearly 2.4 V) of H20 -> 02. The way to prevent this from happening is to add previously a small amount of a so-called redox buffer, i.e., a reversible oxidant such as Ce(IV) with a standard... 

For an ideal Nernstian reaction, the peak potentials of the cathodic and anodic sweeps will be the same, and equal to E0. The width of the cathodic (or anodic) wave at half peak height, A 1/2, can be found by replacing / by /p/2 = — [n2/WTe0ty8] in equation (2.39) and so obtaining the two values for 0. Using equation (2.36) then gives a AE1/2 of c. 0.09/nV, at 298 K. In practice, A 1/2 is never as small as this ideal value as a result of adsorbate adsorbate interactions these intermolecular interactions cause a smearing out of the observed redox potential, and a full treatment of this and other complications can be found in the standard texts referred to at the end of chapter 1. 

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. 

This conclusion was supported by coulometry measurements that showed that the charge passed during the growth was typically c. 9 or 10 times that under the anodic wave of the voltammogram up to the potential corresponding to the maximum doping level (cf. Schemes 3.9 and 3.10). 

MNH4C1 plus NH3, pH 8.0 Same system adjusted to pH 9.5 Anodic wave at pH 0 due to HgSe Anodic wave at pH 12 (0.01M NaOH)... 

M (NH4)2HP04 plus XM NH3 (anodic wave) 1M strong acid anodic mercury wave 90% methanol, 9.5% pyridine, 0.5% HC1 (pH 6) 0.1M NaOH (anodic mercury wave)... 

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