Chronopotentiogram

A typical chronopotentiogram is shown in Fig. 18. When the slopes corresponding to the intervals shown in Fig. 18 are designated Sq, Si, and S2, the diffusion coefficient is calculated from ... 

Figure 18. Atypical chronopotentiogram variation of potential with time.

Here the second term on the right-hand side becomes zero when <1/2 = t/4, while the first term agrees with the polarographic value (cf., eqn. 3.37a) hence for the chronopotentiogram we preferably write... 

In the chronopotentiogram of mixtures (see Fig. 3.58), the reactive components will yield different inflection points if their standard potentials show a sufficient difference (at least 0.1 V). To take a simple example, let us consider a reversible electrode process for both ox and ox2 in a solution of the supporting electrolyte. Then eqn. 3.72 is simply valid for the first reacting oxj with up to the first inflection point however, beyond this point the last traces of exj... 

These results are plausible since according to Sand a two-fold concentration of a component yields a four-fold transition time. Now, these features show, in contrast to the net separation and pure additivity of polarographic waves and their diffusion-limited currents as concentration functions, that in chrono-potentiometry the transition times of components in mixtures are considerably increased by the preceding transition times of any other more reactive component, which complicates considerably the concentration evaluation of chronopotentiograms. 

Figure 3.42 Chronopotentiograms for reduction of carbon dioxide in dimethyl sulphoxide at a mercury electrode. Percentage by volume of C02 in N2 to saturate solution is noted on the curves. From Haynes and Sawyer (1967). Copyright 1967, American Chemical Society. Reprinted...

Figure 10.10. Change of transition time for reduction of Pb and Sn due to adsorption of peptone 1, chronopotentiogram in the absence of peptone 2, chronopotentiogram in the presence of 4 g/L peptone Erp, the rest potential. (From Ref. 8, with permission from the American Electroplaters and Surface Finishers Society.)...

In spite of its implicit character, this expression is useful for kinetic purposes as it enables us to calculate kf(E) for any point on a measured E, t curve (the chronopotentiogram), being the response to a current step perturbation. 

The applicability of chronopotentiometry in kinetic studies will, of course, be limited by the presence of the double layer. As the faradaic process causes E to change most rapidly both at short and long times, deviation from the theoretical behaviour described above will be most severe at the extreme parts of the chronopotentiogram. It may be worth noting that eqn. (25) provides the possibility of an internal check since, on variation of j, a certain pair of E, kt values will be found at different values of tin/r1/2. 

Chronopotentiometry is the electrochemical technique in which a current step is impressed across an electrochemical cell containing unstirred solution [1-5]. The resulting potential response of the working electrode versus a reference electrode is measured as a function of time (thus, chronopotentiometry), giving a chronopotentiogram as shown in Figure 4.3. 

Figure 4.3 Chronopotentiometry. (A) Current excitation signal. (B) Potential response signal (chronopotentiogram).

If the polarity of the applied current in an ordinary chronopotentiometry experiment is reversed during the recording of the chronopotentiogram, the product R of the initial electrochemical reaction may now undergo the reverse reaction to give a current-reversal chronopotentiogram, as shown in Figure 4.5 [1-5]. A reverse transition time xr will result when the concentration of R becomes zero at the electrode surface (see Fig. 4.2C). Such reverse potential-time curves can be treated quantitatively for reversible and irreversible couples. 

Figure 4.6 Current-reversal chronopotentiogram for the oxidation of 1 mM PAP in 0.1 M H2S04. Pt electrode, i = 100 jtA/cm2. [From Ref. 9, adapted with permission. Copyright 1960 American Chemical Society.]...

In derivative chronopotentiometry, the potential response signal of a normal chronopotentiometry experiment is electronically differentiated, and this rate of change of potential with time, dE/dt, is recorded as a function of time, as shown in Figure 4.9 [12]. The minimum in a derivative chronopotentiogram is quantitatively related to the transition time. Thus for a reversible couple,... 

Figure 4.11 Chronopotentiogram for the reduction of Fe(IH) at a platinum electrode, r represents the transition time for the reduction process and t the transition time on current reversal for the oxidation of the reduction product.

TABLE 4.2 Diagnostic Criteria of Chronopotentiograms for Various Kinetic Schemes... 

Fig. 5.7. Variation of potential with time (chronopotentiogram) in a system controlled by diffusion and with application of a constant current, x is the transition time. Er/4 is the potential when t = t/4 (Section 10.5).

Fig. 10.5. Chronopotentiometry in a reversible system for 0 + e —>R (only O present in bulk solution), (a) Chronopotentiogram (b) Plot of...

Figure 25 illustrates three cases of lithiated-graphite electrodes in solutions [87], (Typical chronopotentiograms are also presented.)... 

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