Chronoamperometric response

FIGURE 4-29 Cottrell plot of the chronoamperometric response for 1 x 1(T3M Ru(NH3)63 + at a Kel-F/gold composite electrode. Points are experimental data, the solid line is the least-squares fit to theory. Dashed lines are theoretical Cottrell plots for a macroelectrode with active area equal to the active area of the composite (curve a) and to the geometric area of the composite (curve b). (Reproduced with permission from reference 87.)... 

Figure 22. Chronoamperometric responses obtained when a bilayer was submitted to step potentials from 200 mV to different anodic potentials in the 600 to 2000-mV range in 0.1 M LiC104 aqueous solution. (Reprinted from T. F. Otero and J. Rodriguez, in Intrinsically Conducting Polymers An Emerging Technology, M. Aldissi, ed., pp. 179-190, Figs. 1, 2. Copyright 1993. Reprinted with kind permission of Kluwer Academic Publishers.)...

Figure 38. Evolution of the proposed surface aspect of a polypyrrole film during an oxidation reaction initiated from high cathodic potentials (E < -800 mV vs. SCE). The chronoamperometric response is shown at the bottom. Experimental confirmation can be seen in the pictures in Ref. 177. (Reprinted from T. F. Otero and E. Angulo, Oxidation-reduction of polypyrrole films. Kinetics, structural model, and applications. Solid State Ionics 63-64, 803, 1993, Figs. 1-3. Copyright 1993. Reprinted with kind permission of Elsevier Science-NL, Sara Burgerhartstraat 25, 1055, KV Amsterdam, The Netherlands.)...

Otero and co-workers208,212 have visually observed nuclei of oxidized polymer in thin polypyrrole films on electrodes. They attribute these to sites of counter-ion and solvent ingress. A nucleation model based on the growth of ionically conductive zones provides good agreement with experimental chronoamperometric responses. 

CO Stripping Chronoamperometiy Before discussing experimental results, let us examine what the LH mechanism predicts for the chronoamperometric response of an experiment where we start at a potential at which the CO adlayer is stable and we step to a final potential E where the CO adlayer will be oxidized. We will also assume that the so-called mean field approximation applies, i.e., CO and OH are well mixed on the surface and the reaction rate can be expressed in terms of their average coverages dco and qh- The differential equation for the rate of change of dco with time is... 

This is a case where another electrochemical technique, double potential step chronoamperometry, is more convenient than cyclic voltammetry in the sense that conditions may be defined in which the anodic response is only a function of the rate of the follow-up reaction, with no interference from the electron transfer step. The procedure to be followed is summarized in Figure 2.7. The inversion potential is chosen (Figure 2.7a) well beyond the cyclic voltammetric reduction peak so as to ensure that the condition (Ca) c=0 = 0 is fulfilled whatever the slowness of the electron transfer step. Similarly, the final potential (which is the same as the initial potential) is selected so as to ensure that Cb)x=0 = 0 at the end of the second potential step whatever the rate of electron transfer. The chronoamperometric response is recorded (Figure 2.7b). Figure 2.7c shows the variation of the ratio of the anodic-to-cathodic current for 2tR and tR, recast as Rdps, with the dimensionless parameter, 2, measuring the competition between diffusion and follow-up reaction (see Section 6.2.3) ... 

FIGURE 2.7. Double potential step chronoamperometry for an EC mechanism with an irreversible follow-up reaction, a Potential program with a cyclic voltammogram showing the location of the starting and inversion potentials to avoid interference of the charge transfer kinetics, b Example of chronoamperometric response, c Variation of the normalized anodic-to-cathodic current ratio, R, with the dimensionless kinetic parameter X. 

As mentioned in the introduction to controlled potential electrolysis (Section 2.3), there are various indirect methods to calculate the number of electrons transferred in a redox process. One method which can be rapidly carried out, but can only be used for electrochemically reversible processes (or for processes not complicated by chemical reactions), compares the cyclic voltammetric response exhibited by a species with its chronoamperometric response obtained under the same experimental conditions.23 This is based on the fact that in cyclic voltammetry the peak current is given by the Randles-Sevcik equation ... 

The dynamics of upd reactions have also been examined by STM. The formation of the ordered copper/sulfate layer [354] and copper chloride layer [355] on Au(lll) was examined in a dilute solution of Cu where the reaction was under diffusion control so that growth proceeded on a time scale compatible with STM measurements [354]. In another study, the importance of step density on nucleation was examined and the voltammetric and chronoamperometric response for Cu upd on vicinal Au(lll) was shown to be a sensitive function of the crystal miscut, as... 

The conclusions drawn from analysis of the chronoamperometric response of sphere and disk electrodes apply equally to other electrochemical techniques, such as cyclic voltammetry. The characteristic time, tc, of a cyclic voltammetry experiment can be conveniently expressed by the reciprocal of the scan rate RT/nFv. When rc (Dtc),/ , the voltammogram will appear as predicted for a macroplanar electrode (Chap. 3), and when rc (Dtc) A, the voltammogram will take on a sigmoidal shape given by ... 

Fig. 2.25 Chronoamperometric responses of a thin layer cell calculated from Eq. (2.197) for different values of / (in pm) 150 (solid line) 100 (dashed line) 75 (dashed-dotted line) and 50 (dotted line). The current calculated with Cottrell Eq. (2.28) is also plotted (black dots). D = 10 5 cm2 s 1, A = 0.01 cm2, c Q = 10 6 mol cm 3...

Figure 3.15 Chronoamperometry and chronocoulometry (a) excitation potential step (b) chronoamperometric response (c) chionocoulometric response.

X < 380 nm) restores the slow current response, as shown by, curve (2). Thus, the chronoamperometric responses of the functionalized monolayer upon irradiation agree well with the shuttling of the Fc- -CD within the molecular network. In the presence of the trans-azobenzene unit, the Vc-f-CD is located close to the electrode surface, and a rapid interfacial electron transfer is observed. 

The latest contribution to the theory of the EC processes in SECM was the modeling of the SG/TC situation by Martin and Unwin [86]. Both the tip and substrate chronoamperometric responses to the potential step applied to the substrate were calculated. From the tip current transient one can extract the value of the first-order homogeneous rate constant and (if necessary) determine the tip-substrate distance. However, according to the authors, this technique is unlikely to match the TG/SC mode with its high collection efficiency under steady-state conditions. 

Acetylcholineesterase and choline oxidase A glassy carbon electrode (GCE) modified by electrodepositing sub pm Pt-black particles on the surface. ACh and Ch. micro biosensor arrays were fabricated based on immobilization of AChE—ChO or ChO by cross linking with gentar-aldehyde on Pt-black GCE. Significant enhancement in the performance of these biosensors was achieved. The chronoamperometric response of 1 p biosensor array was linear from 29 to 1200 pM with detection limit of 8.7 pM acetylcholine. [109]... 

Fig. 10.15. Experimental (—) and theoretical (—) chronoamperometric response for the diffusion-limited oxidation of 2 x 10-3 mol dm-3 Fe(CN)S in 0.1 mol dm-3 KC1 at a rectangular electrode, 2.5 mm long and 6.25 mm wide, in a 0.5 mm high channel flow cell under channel stopped flow conditions. The initial volume flow rate of the solution was 0.197 cm3 s-1, which gave a limiting current at the channel electrode, defined as / . At time, f.top, solution flow was retarded (Evans et al., in preparation). The theoretical data has been simulated assuming Df (cn)2 = 6.5 x 10-6 cm2 s l.

Use of the chronoamperometric response following a step change in rotation rate, as a general method for determination of the diffusion... 

If the chronoamperometric response of a - polymer-modified electrode is measured alone - in contact with inert - supporting electrolyte - Cottrell-type response can be obtained usually for thick films only, because at short times (f < 0.1-1 ms) the potential is not established while, at longer times (t > 10-100 ms), the finite diffusion conditions will prevail and / exponentially decreases with time. Another complication that may arise is the dependence of D on the potential in the case of - conducting polymer films [vi]. 

An attempt was made to obtain the electron self-exchange rate constant for the [Fe(CN)2-(phen)2]" couple by an electrochemical method. Real space Laplace analysis was used for the chronoamperometric response of 7.05 x 10 4 M (M = mol dm 3) solutions of the iron(III) complex with 0.1 M tetra(/t-butyl)ammonium perchlorate as supporting electrolyte. A glassy carbon working electrode with 3 mm diameter was used for the measurements. The Butler-Volmer plot gave an excellent straight line, and the electrochemical self-exchange rate constant was obained to be 1.2 X lO cm s l from the In k value at the zero over-potential. From the... 

FiCr. 7.38 (A) The assembly and photnisomerization of a switchabie rotaxane on aAu-eleomode. (B) The chronoamperometric response of the monolayer in (a) the trans-estate, and (b) the ds-state. (C) The electron transfer rate constant between the ele[Pg.259]

Fig. 20.13. Chronoamperometric responses of the glucose oxidase (Gox)-Au-Silicate thin film electrode at 0.6 V, for successive additions of 5 mM glucose in 0.1 KH2PO4 solution pH 5.6. (Adapted from Langmuir, 2001, 77, 2602).

Figure 13. (A) Chronoamperometric response of Mb/Au/ITO electrode at -0.4 V in pH 7.0 phosphate buffer while successively injecting 10 pM H202. (B) Plot of catalytic current vs. H202 concentration. Inset Linear calibration curve of 1/i vs. 1/Ch2o2. Reproduced from [44], copyright 2005, with permission from Elsevier.

A discussion of the chronoamperometric feedback response is valuable, as potential step measurements have been used to obtain homogeneous kinetic parameters (3). Calculations of the chronoamperometric response also provide information required for the design of experiments to be made under steady-state conditions. Theoretical results have demonstrated that values of r in excess of 100 may be necessary to achieve steady-state conditions (within a few percent) (3). For typical tip microdisk radii of 12.5 and 2.5 /xm and a typical diffusion coefficient of 10 5 cm2 s, this corresponds to real times that are greater than 15 and 0.6 s, respectively. For the larger disk size, the time is longer than required for many other steady-state measurements with SECM (25). 

Figure 21 shows a typical example of the chronoamperometric response observed when dissolution is initiated in a region towards the edge of the crystal face. In stark contrast to the behavior described above (Fig. 17), SECM-induced dissolution, in the absence of dislocations, occurs via an oscillatory mechanism (4). After an initial period of approximately one unit...

Fig. 17. Theoretical chronoamperometric response for a simple electron transfer process at a channel electrode to a potential step, from a region in which no current flows to one at which...

The key to the characterization of a particular analyte is thus the extent to which it modifies the chronoamperometric response of the platinum electrode to an anodic potential step. 

This defines the pure kinetic region (KP). The chronoamperometric response can be employed to determine A (or k C ) from a suitable working curve based on (12.3.32) (Figure 12.3.21). 

The CyD resides preferentially on the trans-azobenzene component and photoisomerization to the cis-azobenzene state causes translocation of the Fc-y -CyD to the alkyl chain component of the assembly. This light-driven translocation is reversible and proceeds by isomerization of azobenzene between the trans and cis states. The chronoamperometric response of the redox-active ferrocene group associated with a CyD unit reflects a change of position on the molecular array. 

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