Ultramicroelectrodes steady-state conditions

Electrode processes are often studied under steady-state conditions, for example at a rotating disk electrode or at a ultramicroelectrode. Polarog-raphy with dropping electrode where average currents during the droptime are often measured shows similar features as steady-state methods. The distribution of the concentrations of the oxidized and reduced forms at the surface of the electrode under steady-state conditions is shown in Fig. 5.12. For the current density we have (cf. Eq. (2.7.13))... 

At spherical and disc ultramicroelectrodes under steady-state conditions the peak potential coincides with the reversible half-wave potential ( j lcro),... 

It is evident that the square wave charge-potential curves corresponding to surface-bound molecules behave in a similar way to the normalized current-potential ones observed for a soluble solution reversible redox process in SWV when an ultramicroelectrode is used (i.e., when steady-state conditions are attained), providing the analogous role played by 2sw (surface-bound species) and (soluble solution species), and also 2f (Eq- (7.93)) and the steady-state diffusion-limited current (7 css), see Sect. 2.7. This analogy can be made because the normalized converted charge in a surface reversible electrode process is proportional to the difference between the initial surface concentration (I ) and that... 

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]. 

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). 

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]. 

These methods constitute the frame on which any particular method can be elaborated. Yet in practice, the experimental difficulty is that with standard apparatus, 5 /D cannot be varied over an extremely wide range. For example, with the rotating disk electrode (RDE), which is the most convenient steady-state method (with the exception of ultramicroelectrodes [109]), 8 depends on the rotation frequency w of the electrode (see Chapter 2). Yet to maintain correct hydrodynamic conditions w cannot be varied, with... 

Usual conditions for LSV or CV experiments require a quiet solution in order to allow undisturbed development of the diffusion layer at the electrode. Some groups, however, have purposely used the interplay between diffusion and convection in electrolytes flowing in a channel or similar devices [23]. In these experiments (see also Chapter 2.4), mass transport to the electrode surface is dramatically enhanced. A steady state develops [54] with a diffusion layer of constant thickness. Thus, such conditions are in some way similar to the use of ultramicroelectrodes. Hydro-dynamic voltammetry is advantageous in studying processes (heterogeneous electron transfer, homogeneous kinetics) that are faster than mass transport under usual CV or LSV conditions. A recent review provides several examples [22]. 

ABSTRACT. Several aspects of electrochemistry at ultramicroelectrodes are presented and discussed in relevance to their application to the analysis of chemical reactivity. The limits of fast scan cyclic voltammetry are examined, and the method shown to allow kinetic investigations in the nanosecond time scale. On the other hand, the dual nature of steady state at ultramicroelectrodes is explained, and it is shown how steady state currents may be used, in combination with transient chronoamperometry, for the determination of absolute electron stoichiometries in voltammetric methods. Finally the interest of electrochemistry in highly resistive conditions for discussion and investigation of chemical reactivity is presented. 

Figure 5. Steady state at ultramicroelectrodes. (a) Normalized theoretical concentration profiles for ferrocene chronoamperometric oxidation at a hemispherical electrode (r = 1 fim) under conditions where a steady state current is observed. From top to bottom, t = 0, 10, 10", 0.1 and 1 s (b) Theoretical simulation of ECL at a double band assembly, showing the current and ECL intensities (i jj and ECL jj are the limits at infinite time).

The considerable decrease of ohmic drop at ultramicroelectrodes has allowed significant electrochemical data to be obtained from voltammetry in highly resistive media. Thus voltammetry can be performed in usual electrochemical solvents, but without purposefully added supporting electrolyte." Also, steady state voltanunograms can be obtained in solvents with very low constants,such as alcanes or arenes, if a small concentration of an inert electrolyte is added. In the following we want to present two examples of application of voltammetry under these conditions, to the unravelling of organometallic reactivity. 

Che G, Dong S (1992) Application of ultramicroelectrodes in studies of homogeneous catalytic reactions- Part 111. The condition for quasi-first and second order homogeneous catalytic reactions at ultramicrodisk electrodes in the steady state. Electrochim Acta 37 2701-2705... 

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