Chronoamperometric experiment

FIGURE 3-1 Chronoamperometric experiment (a) potential-time waveform (b) change of concentration profiles with time (c) the resulting current—time response. 

Chronoamperometry is often used for measuring the diffusion coefficient of electroactive species or the surface area of the working electrode. Analytical applications of chronoamperometry (e.g., in-vivo bioanalysis) rely on pulsing of the potential of the working electrode repetitively at fixed tune intervals. Chronoamperometry can also be applied to the study of mechanisms of electrode processes. Particularly attractive for this task are reversal double-step chronoamperometric experiments (where the second step is used to probe the fate of a species generated in the first step). 

Figure 6.20 Extrapolated current density at t = 0 obtained from chronoamperometric experiments for Pt(l 11), Pt(lOO), and Pt(l 10) electrodes in 0.2 M HCOOH + 0.5 M H2SO4 on electrode. The straight lines show the regions where the Tafel behavior is observed. (Data taken from Herrero et al. [1994].)...

One must take into account that the maximum time for a chronoamperometric experiment is of about 10 s as mentioned in Chapter 1, Section 4.2.4, for longer times convection becomes dominant over diffusion. 

In the chronoamperometric experiment, we measure current (hence amp- ) as a function of time ( chrono- ). It is usual to commence with the solution around the electrode containing only one redox form of the analyte. 

In Chapter 3, we looked at the way the activity coefficients can be more or less equalized if there is a swamping electrolyte in solution (see Section 3.4.4, SAQ 3.9 and Figure 3.8). By the nature of the species studied in a chronoamperometric experiment, (a) a swamping electrolyte is added to the solution in order to minimize migration effects, and... 

Chronoamperometric experiments on zinc electroreduction on GC from acetate solutions showed that the nucleation density increases with increase of temperature [44]. M oreover, the nucleation rate constant is always very large, equal to 1.41 x 10 s . This indicates that the mechanism of zinc electrodeposition on the GC electrode follows a three-dimensional instantaneous nucleation and growth model within the controlled temperature range. 

Dendrimer 15 was the object of a very interesting theoretical work by Amatore et al.55 discussing the possibility to observe a stochastic behavior of electrochemical events. In the limiting case of only one dendrimer 15 adsorbed on the electrode surface, the current measured in a chronoamperometric experiment would show a random and discontinuous succession of single electron transfer events, while for an array of 7800 dendrimers, corresponding to complete coverage of an electrode of 500 nm in radius, the stochastic nature of the phenomenon is no longer clearly discernable. 

Chronoamperometry has proven useful for the measurement of diffusion coefficients of electroactive species. An average value of it1/2 over a range of time is determined at an electrode, the area of which is accurately known, and with a solution of known concentration. The diffusion coefficient can then be calculated from it1/2 by the Cottrell equation. Although the electrode area can be physically measured, a common practice is to measure it electrochemically by performing the chronoamperometric experiment on a redox species whose diffusion coefficient is known [6]. The value of A is then calculated from it1/2. Such an electrochemically measured surface area takes into account any unusual surface geometry that may be difficult to measure geometrically. 

All of the principles of semi-infinite potential-step experiments discussed so far apply to thin-layer work. Some modification in the quantitative response is necessary to account for the presence of a diffusion barrier. Figure 3.11 illustrates the diffusion phenomena occurring during a chronoamperometric experiment in a thin-layer cell of typical dimensions. It will be useful to compare this figure with the semi-infinite situation depicted in Figure 3.1. Notice that the supply of reactant in the bulk solution phase is effectively infinite in Fig-... 

Figure 3.31 illustrates the simplest example of LAPV at a stationary electrode. In this case we assume that the time delay td between pulses is such that the initial condition is restored. This implies that all concentration profiles are completely relaxed before the next pulse is applied and every pulse initiates a new chronoamperometric experiment. The length of td required to accomplish this will depend on the chemical reversibility of the system, being shorter for reversible reactions and longer for irreversible reactions. 

Figure 6.11 shows the result of such a chronoamperometric experiment, where a constant potential of 0.45V vs. AglAgCl is applied to the platinum electrode rotating at 6.67Hz in a pH of 12.5. The sodium dithionite concentration was increased in ten consecutive steps. It can be seen that the limiting-current corresponds to the values obtained with linear-sweep... 

Chronoamperometric experiment at F=0.45V vs. AglAgCI recorded at a rotating platinum-disc electrode in an alkaline solution (pH = 12.5) at a rotation rate of 400rpm for dithionite concentrations of (1) 6.0x10 4, (2) 1.2x10 3, (3) 1.8x10 3,... 

The above expression makes it easy to predict the shift of the voltammograms with the capillary size, experiment timescale, and diffusivities of the ion. Also, provided that the capillary diameter and diffusion coefficients of the ion are known (for example, by means of chronoamperometric experiments), the A swv.peak value allows the determination of the ion transfer formal potential and the consistency of the value obtained can be easily tested by varying the scan rate or the frequency [42, 44]. 

Figure 4.27 Normalized calculated concentration profiles for disk electrodes with different radii (r0), 1 s after start of a chronoamperometric experiment. (Reproduced with permission from Ref. 116.)...

Figure 1.13. Chronoamperometric experiment (a) potential-time waveform (b) change in concentration profiles as time progresses (c) the resulting current-time response [19]. (From Wang J. Analytical electrochemistry. 2006 Wiley-VCH. Reproduced with permission.)...

Figure 2. Chronoamperometric experiment for O + e R a) concentration profiles are shown after 1, 10, 100, and 1000 ms. The concentration gradient at the electrode surface (x = 0) is seen to decrease with time b) plot of the current i vs time t is shown in the interval 0-1000 ms. The current decreases as a function of...

Figure 25. (A) Vectorial electron transfer in the two-heme-reconstituted de novo protein molecules organized as a monolayer at an electrode surface. (B) Transient current recorded with the two-heme reconstituted de novo protein monolayer during the double-potential step chronoamperometric experiment. The potential steps from -0.2 to -0.5 V (vs. SCE) to reduce the hemes in the protein, and after 70 ms the potential steps back, from —0.5 to —0.2 V, to oxidize the reduced hemes. The experiment was performed in 0.1 M phosphate buffer, pH 7.0, under argon.

When conventional electrodes with diameters between 0.1 and 2 mm are used, the latter quantity has usually decayed to zero after 0.5 ms or less and may be neglected in experiments lasting 1 ms or more. This decay time is reduced to the microsecond time regime when ultramicroelectrodes are used [94,125,202]. According to Eq. (64), which for i = 0 is known as the Cottrell equation, the current approaches zero when the time approaches infinity. However, undisturbed linear diffusion can be maintained only over rather short time intervals unless special precautions are taken (see Sec. II.D.l), and the measurements of current-time curves, called chronoamperometry (CA), are often complicated by additional modes of transport. Therefore, the use of properly shielded electrodes [140] should be considered in chronoamperometric experiments exceeding approximately 1 s. The mathematical formalism for chronoamperometry has been developed also for the application of ultramicroelectrodes [203]. 

Full coupling of transport between the involved phases. This corresponds to a situation where there is a very fast cross-phase electron and/or mass transport exchange. Here, the apparent diffusion coefficient measured (e.g., via chronoamperometric experiments) should satisfy ... 

The UME response during the SECM chronoamperometric experiment is calculated by solving Eq. (1) with respect to the boundary conditions defining the processes occurring at the electrode and substrate. The initial conditions and boundary conditions are as previously stated in Eqs. (6)-(10), except C now corresponds to the normalized concentration of Cu2+. The substrate boundary condition, reflecting the dissolution rate law is ... 

The simulation of a chronoamperometric experiment for a reversible process at a planar electrode when the diffusion coefficients of species A and B are equal Dx = = D) can be tested by comparison with the... 

In order to improve the detection of short-lived intermediates, the potential step or chronoamperometric experiment can be replaced by a cyclic voltammet-ric experiment, which involves applying a triangular potential ramp. With a fast UVA is spectrometer, e.g. a diode array system, additional UVWis/NIR spectroscopic information as a function of the potential can be recorded simultaneously to the voltammetric data. However, recording cyclic voltammograms with the simple cell shown in Fig. II.6.4 is complicated by the presence of ohmic drop in the solution phase, which is amplified by poor cell design. In this kind of cell, the peak-to-peak separation in cyclic voltammograms of a reversible redox couple may increase by several hundreds of millivolts. Voltammetric data (and simultaneously recorded spectroscopic data) are therefore very difficult to interpret quantitatively. 

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