Ultramicroelectrodes kinetics

Clearly, then, the chemical and physical properties of liquid interfaces represent a significant interdisciplinary research area for a broad range of investigators, such as those who have contributed to this book. The chapters are organized into three parts. The first deals with the chemical and physical structure of oil-water interfaces and membrane surfaces. Eighteen chapters present discussion of interfacial potentials, ion solvation, electrostatic instabilities in double layers, theory of adsorption, nonlinear optics, interfacial kinetics, microstructure effects, ultramicroelectrode techniques, catalysis, and extraction. 

MEMED meets all of the criteria listed in Section I, for the investigation of liquid-liquid interfacial kinetics, but is limited in the range of rate constants that can be determined. While SECM, discussed in Chapter 12, enhances the kinetic domain that can be measured with ultramicroelectrodes, there are many spontaneous reactions to which SECM cannot be applied. 

The kinetic investigation requires, as already stated in Section 5.1, page 252, a three-electrode system in order to programme the magnitude of the potential of the working electrode, which is of interest, or to record its changes caused by flow of controlled current (the ultramicroelectrode is an exception where a two-electrode system is sufficient). 

Quantitative investigations of the kinetics of these a-coupling steps suffered because rate constants were beyond the timescale of normal voltammetric experiments until ultramicroelectrodes and improved electrochemical equipment made possible a new transient method calledjhst scan voltammetry [27]. With this technique, cyclic voltammetric experiments up to scan rates of 1 MV s are possible, and species with lifetimes in the nanosecond scale can be observed. Using this technique, P. Hapiot et al. [28] were the first to obtain data on the lifetimes of the electrogenerated pyrrole radical cation and substituted derivatives. The resulting rate constants for the dimerization of such monomers lie in the order of 10 s . The same... 

The kinetics of AgGl dissolution in aqueous solutions without supporting electrolyte have been studied utilizing well-defined and high mass transport properties of the scanning electrochemical microscope [376]. An ultramicroelectrode probe positioned close to the AgGl surface was used to induce and monitor dissolution of the salt via reduction of Ag+ from the initially saturated solution. 

Fig. 7.34. An ensemble of ultramicroelectrodes. (Reprinted from E. Gileadi, Electrode Kinetics for Chemists, Chemical Engineers, and Materials Scientists, VCH Publishers, 1993, p. 450. Copyright 1993 John Wiley. Reprinted by permission of John Wiley Sons, inc.)...

Applications have been reported for photoelectrochemical experiments, for example, splitting of water [11], local generation of photoelectrodes by spatially selective laser excitation [12], and steady-state electrochemiluminescence at a band electrode array [13,14]. Band electrodes prepared from very thin films approaching molecular dimensions have been used to assess the limits of theory describing electrode kinetics at ultramicroelectrodes [9]. Spectroelectrochemical applications have been extensively reviewed [1], In an intriguing approach, thin, discontinuous metal films have been prepared on a transparent semiconductor substrate they are essentially transparent under conditions in which a continuous metal film containing the same quantity of metal would be expected to substantially absorb [15]. 

A recent development, termed by the inventors microelectrochemical measurements at expanding droplets (MEMED) [29], is a technique based on forming small droplets of a phase containing a reactant in a second immiscible liquid phase (Fig. 5.24). An ultramicroelectrode (UME, see Section 5.3.2.8 and Chapter 6) measures an electrochemical response as the droplet expands towards it, from which a concentration profile can be derived and, hence, the kinetics of related processes. Because the surface is continuously refreshed, it avoids... 

The time domain on a window accessed by a given experiment or technique, e.g., femtosecond, picosecond, microsecond, millisecond. The time scale (or domain) is often characterized by a set of physical parameters associated with a given experiment or technique, e.g., r2 ]/1) (for - ultramicroelectrode experiments) - thus if the electrode radius is 10-7 cm and the - diffusion coefficient D = 1 x 10-5 cm2/s-1 the time scale would be 10 9s. Closely related to the operative kinetic term, e.g., the time domain that must be accessed to measure a first-order -> rate constant k (s-1) will be l//ci the time domain that must be accessed to measure a given heterogeneous rate constant, k willbe /)/k2. In - cyclic voltammetry this time domain will be achieved when RT/F v = D/k2 with an ultramicroelectrode this time domain will be achieved (in a steady-state measurement when r /D = D/k2 or ro = D/k at a microelectrode [i-ii]. 

In the SECM measurements, an ultramicroelectrode tip is used as a probe for ET occurring at an O/W interface. The electrode potential is controlled by a three-electrode potentiostat, whereas the potential drop across the O/W interface is usually determined by adding a common ion to both phases (except for the recent study [23] using an externally polarized interface). This sets the SECM measurements free from the restriction of the potential window. It should also be noted that in ordinary SECM measurements, all electrodes are in a single phase, so that it is possible to avoid the problems of IR drop and charging current. These advantages of SECM have been realized in kinetic studies of ET at O/W interfaces. 

Baranski et al. [188] also applied ultramicroelectrodes in the study of the kinetics of oxidation of ferrocene in nine aprotic solvents and in three alcohols as a function of temperature. The data obtained in acetonitrile were concordant with those given by Wightman and coworkers [187]. 

The comparative and careful study of Lasia and coworkers [172] on electroreduction of A, A -bis(salicylidene)-ethylenediaminocobalt(II) (Co(salen)), with the use of ultramicroelectrodes has also shown that earlier kinetic results on Co(salen) in DMSO [147,171] are too low. 

The development of ultramicroelectrodes with characteristic physical dimensions below 25 pm has allowed the implementation of faster transients in recent years, as discussed in Section 2.4. For CA and DPSC this means that a smaller step time x can be employed, while there is no advantage to a larger t. Rather, steady-state currents are attained here, owing to the contribution from spherical diffusion for the small electrodes. However, by combination of the use of ultramicroelectrodes and microelectrodes, the useful time window of the techniques is widened considerably. Compared to scanning techniques such as linear sweep voltammetry and cyclic voltammetry, described in the following, the step techniques have the advantage that the responses are independent of heterogeneous kinetics if the potential is properly adjusted. The result is that fewer parameters need to be adjusted for the determination of rate constants. 

The peak-potential difference A p depends mainly on the kinetic parameter i/t, as illustrated in Table 2. By measurement of A p as a function of v for a given system, k° can be estimated. However, great care should be exerted to ensure that uncompensated resistance does not contribute to the value of A p, since this would hamper the procedure. Clearly, the use of ultramicroelectrodes can be recommended for this kind of measurements, as the ohmic drop is much smaller here compared to microelectrodes of normal size. This is particularly true when high sweep rates are required for determining large values of k° (see Section 2.4)... 

The RDE technique has found widespread use in analytical electrochemistry because of an excellent signal-to-noise ratio resulting from the enhanced mass transport. The RDE method has also been employed for monitoring concentrations in kinetic applications [59], as described for ultramicroelectrodes [60] and in the determination of the stoichiometry for electron-transfer reactions by means of redox titration [61]. The latter procedure will be described next. 

Besides the possibility of extracting cleavage rate constants kc for short-lived radical anions RX , the redox catalysis approach may also provide the standard potential of RX, rx5 from the measurements of A et- In practice, the rate constants kET are obtained for the reaction between a number of aromatic radical anions with different values of and a given substrate by means of CV, LSV, or a potentio-static technique employing an ultramicroelectrode or RDE. According to the theoretical treatment of the above kinetic scheme, the rate constant k j can be expressed as shown in Eq. 122 [125]. 

The efficiency of electron-transfer reduction of Cgo can be expressed by the selfexchange rates between Coo and the radical anion (Ceo ), which is the most fundamental property of electron-transfer reactions in solution. In fact, an electrochemical study on Ceo has indicated that the electron transfer of Ceo is fast, as one would expect for a large spherical reactant. This conclusion is based on the electroreduction kinetics of Ceo in a benzonitrile solution of tetrabutylammonium perchlorate at ultramicroelectrodes by applying the ac admittance technique [29]. The reported standard rate constant for the electroreduction of Ceo (0.3 cm s ) is comparable with that known for the ferricenium ion (0.2 cm s l) [22], whereas the self-exchange rate constant of ferrocene in acetonitrile is reported as 5.3 x 10 s , far smaller than the diffusion limit [30, 31]. 

Although the use of ultramicroelectrodes is not restricted to any specific measurement technique [125,143,178,181], only applications in the context of cyclic voltammetry at high sweep rates are considered here (see also Sec. IV). For the studies of reaction kinetics using ultramicroelectrodes under steady-state conditions the reader is referred to the original literature [182]. 

Ultramicroelectrodes are excellent tools also for the study of the kinetics of fast follow-up reactions, provided that the high sweep rates needed do not bring the electron transfer process into the quasi-reversible region. Thus, a reaction that during a study with con-... 

Scanning electrochemical microscopy (SECM) [196] is a member of the growing family of scanning probe techniques. In SECM the tip serves as an ultramicroelectrode at which, for instance, a radical ion may be generated at very short distances from the counterelectrode under steady-state conditions. The use of SECM for the study of the kinetics of chemical reactions following the electron transfer at an electrode [196] involves the SECM in the so-... 

The origins of SECM homogeneous kinetic measurements can be found in the earliest applications of ultramicroelectrodes (UMEs) to profile concentration gradients at macroscopic (millimeter-sized) electrodes (1,2). The held has since developed considerably, such that short-lived intermediates in electrode reactions can now readily be identified by SECM under steady-state conditions, which would be difficult to characterize by alternative transient UME methods, such as fast scan cyclic voltammetry (8). 

The versatility of this mode of operation has made it extremely powerful for fabrication of microstructures. In the feedback mode an ultramicroelectrode is held close above a substrate in a solution containing one form of electroactive species, either reduced or oxidized, that serves as a mediator (Fig. 1). The latter is usually used both as a means of controlling the distance between the UME and the surface and to drive the microelectrochemical process on the surface. This poses a number of requirements that must be taken into account when configuring the system. The basic limitation stems from the requirement that the electrochemical reaction be confined only to the surface. This means that the electroactive species generated at the UME will react with the surface or with other species attached to it. In addition, it is preferable in most cases that the redox couple used should exhibit chemical and electrochemical reversibility, so that it is effectively regenerated on the surface. The regeneration of the redox couple on the surface is required for controlling the UME-substrate distance. Finally, the thermodynamics and kinetics of the electrochemical process on the surface will dictate the choice of the redox couple introduced. 

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