Peak voltammogram

Fig. 5.14 Peak voltammogram (curve 1, j t) — E dependence) and convoluted voltammogram (curve 2, F(t) — E dependence) for the reversible case...

A transformation of the peak voltammogram to the sigmoidal shape shown in the preceding section, Fig. 5.13, is achieved by the convolution analysis method proposed by K. Oldham. The experimental function j(t) = j[T(E — Ej)/v] is transformed by convolution integration... 

Electrode reactions yielding the differential peak voltammograms shown in Figure 3... 

Ep/2 is the so-called half peak potential, measured on peak voltammograms as the potential at half the peak current. 

If -Eapp is scanned linearly, the current response does not have a limiting plateau as in flowing solution, but reaches a maximum and then decays. This is called a peak voltammogram and is shown in Fig. 14A. In... 

Fig. 14. Peak voltammograms of dopamine at graphite electrodes. (A) Linear potential sweep (200 mV/sec), 500 pM dopamine in pH 7.4 phosphate buffer (B) cyclic voltammogram of same solution as A. Dotted line—see text.

If compounds oxidize reversibly, then peak voltammograms are quite sharp and narrow in shape, and if the peak potentials of two or more substances are separated by more than about 150 mV, each can be determined separately. But shapes of irreversible or partially reversible voltammograms are sensitive to the electrode material and its surface state. Since few compounds in real life oxidize reversibly, peak voltammograms are often broadened from their theoretical narrow shape. 

If two different substances give well-separated peak voltammograms, applying a potential just past the first peak will give i sec data exclusively for the first (most easily oxidized) component. If two or more compounds oxidize at the same potential, or their peak voltammograms overlap seriously, tj sec will be the sum of the currents from each component. However, chronoamperometry is inherently no less selective than other electroanalytical methods. One must always know a priori or have reasonable verification from some independent observation that only one component is undergoing electrolysis. 

The current during the stripping step is monitored as a function of potential, giving rise to peak-shaped voltammograms similar to that shown in Figure 11.37. The peak current is proportional to the analyte s concentration in the solution. 

In view of the limitations referred to above, and particularly the influence of electrode characteristics upon the peaks in the voltammogram, some care must be exercised in setting up an apparatus for stripping voltammetry. The optimum conditions require ... 

Voltaic cells 64. 504 Voltammetry 7, 591 anodic stripping, 621 concentration step, 621 mercury drop electrode, 623 mercury film electrode, 623 peak breadth, 622 peak current, 622 peak potential, 622 purity of reagents, 624 voltammogram, 622 D. of lead in tap water, 625 Volume distribution coefficient 196 Volume of 1 g of water at various temperatures, (T) 87... 

C o(0, /)/CR(0, /) = 1/10,n = 1). The decrease in Co(0, t) is coupled with an increase in the diffusion layer thickness, which dominates the change in the slope after Co(0, /) approaches zero. The net result is a peak-shaped voltammogram. Such... 

The cyclic voltammogram is characterized by several important parameters. Four of these observables, the two peak currents and two peak potentials, provide the basis for the diagnostics developed by Nicholson and Shain (1) for analyzing the cyclic voltammetric response. 

For multielectron-transfer (reversible) processes, the cyclic voltammogram consists of several distinct peaks if the E° values for the individual steps are successively higher and are well separated. An example of such a mechanism is the six-step reduction of the fullerenes C60 and C70 to yield the hexaanion products and C7q. Such six successive reduction peaks are observed in Figure 2-4. 

For quasi-reversible systems (with 10 1 > k" > 10 5 cm s1) the current is controlled by both the charge transfer and mass transport. The shape of the cyclic voltammogram is a function of k°/ JnaD (where a = nFv/RT). As k"/s/naD increases, the process approaches the reversible case. For small values of k°/+JnaD (i.e., at very fast i>) the system exhibits an irreversible behavior. Overall, the voltaimnograms of a quasi-reversible system are more drawn-out and exhibit a larger separation in peak potentials compared to those of a reversible system (Figure 2-5, curve B). 

---