Normalized chronoamperometry

Note that D is now dependent on the square of the radius, so careful characterization of the UME s dimensions using a test compound is particularly important. An additional advantage of this normalization technique is that because the measurement of and i(l) can occur in the same experiment, calibration errors in the current amplifier are essentially eliminated. An alternative approach is to fit the normalized chronoamperometry data to the Shoup and Szabo equation (equation (19.7)), which only requires knowledge of r. ... 

In this chapter, the voltammetric study of local anesthetics (procaine and related compounds) [14—16], antihistamines (doxylamine and related compounds) [17,22], and uncouplers (2,4-dinitrophenol and related compounds) [18] at nitrobenzene (NB]Uwater (W) and 1,2-dichloroethane (DCE)-water (W) interfaces is discussed. Potential step voltammetry (chronoamperometry) or normal pulse voltammetry (NPV) and potential sweep voltammetry or cyclic voltammetry (CV) have been employed. Theoretical equations of the half-wave potential vs. pH diagram are derived and applied to interpret the midpoint potential or half-wave potential vs. pH plots to evaluate physicochemical properties, including the partition coefficients and dissociation constants of the drugs. Voltammetric study of the kinetics of protonation of base (procaine) in aqueous solution is also discussed. Finally, application to structure-activity relationship and mode of action study will be discussed briefly. 

Potential step voltammetry (chronoamperometry) or normal pulse voltammetry (NPV) and potential sweep or cyclic voltammetry (CV) were employed for investigating drugs at the NB/W or DCE/W interface. A thin O-layer cell [15,16,23] was used to realize the partition equilibrium of neutral species (that is, B) at the O/W interface initially at t = 0 within a reasonably short time. All measurements were carried out at 25°C. Experimental details should be consulted in the references cited. 

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. 

FIGURE 2.12. Double potential step chronoamperometry for an ECE (dashed line) and a DISP (solid line) mechanism. Variation of the normalized anodic-to-cathodic current ratio, RDps = [—ia(2tR)/ic(tR)]/(l — l/y/2), with the dimesionless kinetic parameter X — ktR. 

As with the other reaction schemes involving the coupling of electron transfer with a follow-up homogeneous reaction, the kinetics of electron transfer may interfere in the rate control of the overall process, similar to what was described earlier for the EC mechanism. Under these conditions a convenient way of obtaining the rate constant for the follow-up reaction with no interference from the electron transfer kinetics is to use double potential chronoamperometry in place of cyclic voltammetry. The variations of normalized anodic-to-cathodic current ratio with the dimensionless rate parameter are summarized in Figure 2.15 for all four electrodimerization mechanisms. 

Two electrochemical techniques are directly based on the expression for the faradaic current density jF, namely chronoamperometry and normal pulse polarography. A third technique, named chronocoulometry, deals with the integral of jF, giving the charge transferred per unit area via the faradaic process as a function of time. The general expression obtained... 

Figure 16.2 Double-potential-step chronoamperometry of 4.6 mM [Fe(CO)2(r)5-Cp)]2 in 0.1 M Bu4NPF6/propionitrile at -43°C. Step time Texp = 0.1 s. (A) Current transients for potential step from -0.8 to -1.9 V in blank electrolyte solution (thin line) and with added [Fe(CO)2(rj5-Cp)]2 (dark line). (B) Ratio of experimental currents -i(t + texp)/i(t) for 0 < t < Texp versus normalized time (t/texp) compared to theory (solid line) for kobs = 10.5 s 1. Electrode area = 0.0032 cm2. [Reprinted with permission from E.F. Dalton, S. Ching, and R.W. Murray, Inorg. Chem. 30 2642 (1991). Copyright 1991 American Chemical Society.]...

It is of interest at this point to compare the study of Multipulse Chronoamperometry and Staircase Voltammetry with those corresponding to Single Pulse Chronoamperometry and Normal Pulse Voltammetry (NPV) developed in Chaps. 2 and 3 in order to understand how the same perturbation (i.e., a staircase potential) leads to a sigmoidal or a peak-shaped current-potential response as the equilibrium between two consecutive potential pulses is restored, or not. This different behavior is due to the fact that in SCV the current corresponding to a given potential pulse depends on the previous potential pulses, i.e., its history. In contrast, in NPV, since the equilibrium is restored, for a reversible process the current-potential curve is similar to a stationary one, because in this last technique the current corresponding to any potential pulse is independent of its history [8]. 

A double-potential-step chronoamperometry (DPSC) experiment consists of two CA experiments. The potential of the second step is normally adjusted so that the R molecules formed upon reduction of O in the first step is reoxidized to O in a diffusion-controlled process, but it might also be adjusted to other values with the purpose of detecting other species formed [7]. In contrast to the CA technique, DPSC is a reversal technique, where the intermediates/products formed during the first step are probed directly in the second step. In this sense, it corresponds to a pump/probe experiment in photochemistry. While CA, in general, provides little if any information about follow-up chemistry, DPSC is a very strong tool for distinguishing between different mechanisms such as for example E, ECj, and DIMl. It is also a good tool for the determination of the relevant rate eonstants. 

Figure 13. Double potential step chronoamperometry of l-methyl-4-te/-t-butylpyridinium at a 5-pm ( , A) and a 17-pm ( ) diameter gold disk electrode. Variation of the normalized current ratio R = P ) with step time 9 = t). From C. P. Andrieux, P. Hapiot, and J.-M. Saveant, J. Phys. Chem. 92 5992 (1988) [24],...

Although multiple electrochemical techniques exist, those used in freely moving animals are chronoamperometry, differential normal-pulse voltammetry, and fast-scan cyclic voltammetry. Excellent comparisons between these can be found in literature, particularly Troyer et al.[5,7,30] and Robinson el al.[8] and therefore will not be diskussed here. 

When diffusion layers overlap by a large amount, an overall planar response will be expected, but with a characteristic area equivalent to the total array surface area rather than just the electroactive surface area. Hence, the Case 4 current will be (1/ ) times larger than the Case 1 current. This will occur when X(jiff d where d is the separation of the individual microdiscs. Therefore, Case 4 behaviour arises at t 0.1 s. This will therefore be the dominant behaviour for cyclic voltammetry at normal scan rates at this particular array. With chronoamperometry, short timescales are accessible and so Case 3 behaviour may also be observed. 

Normal pulse voltammetry in dual-plate electrode systems has also been studied for the hydroquinone redox system in aqueous phosphate buffer." When pulsing the generator electrode (in normal pulse voltammetry mode) and monitoring the collector electrode response (in chronoamperometry mode) the anticipated pulse response (see Fig. 16 B) is detected. The magnitude of the pulse response is maximised with a collector potential of — 0.2 V V5. SCE (see Fig. 16D). However, the using the first electrode as modulator (producing pulses of hydroxide), the pulse response is maximised with a sensor potential of -0.1 V vi. SCE (see Fig. 16E). In this case the... 

These cells are normally used for experiments (chronoamperometry and chronocoulometry) in which large-amplitude steps are applied in order to carry out an electrolysis in the diffusion region. For... 

FIGURE 12.6 (a) Typical amperograms recorded as a function of the potential at a single macrophage stimulated by a microcapillary, (b) Normalized steady-state voltammograms of some electroaetive species (working electrode=platinized carbon fiber UME, C = 1 mmol-L-, v = 20 mV s O that help to select the appropriate potential values in chronoamperometry. 

A rather general theory of double potential step chronoamperometry coupled with SECM (SECM/DPSC) developed for such processes in Refs. [73b,c] is applicable to both steady-state and transient conditions. The model accounts for reversibility of the transfer reaction and allows for diffusion limitations in both liquid phases. The possibility of different diffusion coefficients in two phases was also included. The steady-state situation was defined by three dimensionless parameters, that is, K =cjc (the ratio of bulk concentrations in organic and aqueous phases), y=DJD (the ratio of diffusion coefficients), and K=k a/D (normalized rate constant for the transfer from organic phase to water). The effects of these parameters on the shape of current-distance curves are shown in Figure 8.17. The tip current (at a given distance) increases strongly with both K and... 

Fig. 10.9 Chronoamperometry at a potential corresponding to Co(0,t) = 0 (a) perturbation signal (b) concentration profiles at increasing times (c) signal recorded i =f(t) according to Cottrell s equation normalization in (b) is performed by dividing Co(x,t) by Cq with respect to the bulk concentration, which is quite common practice...

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