Chronopotentiometric transient

Fig. 12K Development of the concentration profile with time during a constant current (chronopotentiometric) transient. T = 40 ms, D - 6x10 cm Is. The distance, x, is given in dimensionless form in the bottom scale and in micrometers in the top scale. The transition time X corresponds to the time taken for the concentration at the surface to be reduced to zero.

Fig. I3K Chronopotentiometric transient, showing the meaning of T and of Reduction of 2 inM Cd in 1.0 M KNOy at a mercury electrode, i = 0.16 mAlcm. From Gileadi, Kirowa-Eisner and Penciner, Interfacial Electrochemistry - An Experimental Approach" Addison Wesley, Publishers 1975, with permission.

It is always interesting to plot the effect of the chronoamperometric transient at a certain prefixed potential (Figure 12.3) and to check the stability of the modified layer with a chronopotentiometric transient (Figure 12.4) by the open circuit potential response. The break of the (3-oxide growth on the platinum is observed after 250 s (Figure 12.4) when the current density becomes stable (Figure 12.3). 

Figure 2 Schematic chronopotentiometric transients in the presence of a diffusional redox probe for (a) An electrode of low resistance, (b) A modified electrode exhibiting high resistance.

Figure 20(A) shows the E-t chronopotentiometric transients upon the buildup of the layered assembly shown in Fig. 18. The assembly of the primer, (26), on the surface is accompanied with an overpotential for the reduction of the redox-probe as a result of its electrostatic repulsion. The hybridization with the target DNA, (27), and the subsequent coupling of the biotinylated-oligonu-cleotide, (28), further repel the redox-probe [Fe(CN)g] , thus enhancing the overpotential for the electron transfer at the electrode. The buildup of avidin and the biotinylated liposomes insulates the electrode interface and results in an addi-...

Figure 14.9 Chronopotentiometric transient (j = const.) for a reversible reaction, showing the meaning oft and of 1/4. (t = 40ms j = 0.16mAcm Cb = 2 mM Cd " in TOM KNO3).

For the individual types of transient measuring techniques, special names exist but their terminology lacks uniformity. The potentiostatic techniques where the time-dependent current variation is determined are often called chronoamperometric, and the galvanostatic techniques where the potential variation is determined are called chronopotentiometric. For the potentiodynamic method involving linear potential scans, the term voltammetry is used, but this term is often used for other transient methods as well. 

In a recent communication, Fenner, Van Dyke, and Martin report a variant of the chronopotentiometric current pulse technique, which involves examining the time variation of the open-circuit electrode potential. The open-circuit potential recorded after applying a current pulse varied in a linear manner with The charge transport diffusion coefficient is evaluated by comparing simulated and experimental oc transients. Martin and coworkers show that the concentration of diffusing species at any distance x in the film at any time t after terminating the current pulse is given by... 

FIGURE 1.53. Typical experimental current step chronopotentiometric plots recorded for poly(pyrrole) electrodes in 0.2Af Et4NBp4 supporting electrolyte, (a) Overpotential/time transients for current step experiments recorded at various initial potentials, initial potentials were (a) 0.4 V, (b) 0.2 V, and (c) 0.1 V. (b) Plot of overpotential versus for the transients illustrated in Fig. 1.53 (1). These plots are in good agreement with Eqn. 277. (Adapted from Ref. 163.)... 

A chronopotentiometric experiment must be performed in quiescent solutions. Thus, while the concentration at the surface declines to zero, the bulk concentration is unchanged, as prescribed by the boundary conditions (Eq. (14.22)). The meaning of the transition time, x, may be clarified by considering Figure 14.9, which displays a transient obtained for a reversible reaction. The corresponding equation describing the variation of potential with time is... 

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