Time-dependent current

The analysis of chronoamperometry data is based on the Cottrell equation, which defines the current-time dependence for linear diffusion control ... 

Figure 5.6 Chronoamperometry, semi-infinite condition, and current-time dependence which follows the Cottrell equation.

The current-time dependence of a cyclic voltammogram at a microelectrode for diffusion-controlled conditions is given by the equation... 

For short times (below milliseconds or even microseconds) the current decreases with Vf. Potential- and current-time dependence are shown schematically in Figure 6.18. 

In such measurements the limitation by the double-layer charging is a serious problem. The charging peak can dominate the current-time dependence at short times. 

Figure 6. Leakage current time dependence for constant applied voltage.

The change of the slope of the current-time dependencies due to the dendritic growth initiation will be treated here in somewhat simplified way. 

Typical log (current)-time dependencies obtained for Cu electrodeposition from 0.20 M Q1SO4 in 0.50 M H2SO4 at overpotentials belonging to the limiting diffusion current plateau are shown in Fig. 2.14a. According to the above discussion, it is clear that the intersection points of the two linear dependencies determine the induction time of dendritic growth initiation [38]. 

The diffusion-controlled current-time dependence according to the Eq. (23) is illustrated in Fig. 11. It increases in time to a maximum value at the end of the drop-life. The smallest change of the current in time appears closely before the fall of the drop. The maximum current is given by the Eq. (24) where the increasing time was replaced by the drop time... 

Fig. 62. Scheme of potential-time and current-time dependences in anodic stripping analysis, (a) Ed potential during the deposition period E 1/2 and E"/2 half-wave potentials of two test substances, Ef the final potential t p rest period, tj stripping period. (B) Current-time dependence during the LSV stripping step, IJ, and I" peak heights of the test substances. 

Double-potential step chronoamperometry This method was proposed in 1965 by Schwarz and Shain [18] for the investigation of follow-up reactions especially for the mechanism. During the first potential pulse the product B is produced at a stationary electrode under diffusion-controlled conditions for a timed interval tp. During this interval substance B diffuses into the solution and simultaneously undergoes a chemical reaction. Then, the potential is suddenly switched to a value where B is converted back into A. The backward current indicates the amount of B which has not reacted and can be related to the rate constant kf. The forward current-time dependence is given by the Cottrell equation... 

Fig. 2.17 - (a) The potential-time profile for differential pulse polarography. Also shown schematically, (b) the charging current-time dependence (c) the Faradaic current-time dependence, and (d) the time-dependence of the total current. 

The generation/collection (G/C) modes constitute a different SECM procedure that expands the applicability of the technique to a wide range of situations, hi these modes, the collector (either tip or substrate) works as an amperometric sensor that collects the products produced at the generator surface (either substrate or tip, respectively). Thus, the collector potential is controlled to electrochemically reaet with the generator-produced species. Typical collector responses used in G/C experiments are (a) voltammetric curves, where the collector potential is swept, and (b) diffusion-controlled limiting current vs. time curves. In contrast to the feedback mode where steady-sate responses are monitored, in G/C experiments, the current-time dependence is an important set of data to evaluate. The timescale of most of G/C transient experiments is much wider, possibly up to 100 sec. Moreover, as the tip-substrate distances increase, typical coupling and distortion of transient responses are not significant. 

CURRENT-TIME DEPENDENCE AT CONSTANT POTENTIAL (POTENTIOSTATIC REGIME)... 

Current-time dependence of a rotating Pt-split-ring-Fe-disk electrode in IM HCIO4 after HF addition to 0.1 M, 2h prepassivation at 0.1 V in IM HCIO4, Fe + detection at ring 1. (From Lochel, B.P. and Strehblow, H.-H.,... 

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