Ultramicroelectrodes

The conventional voltammetric indicator electrodes are 0.5-5 mm in diameter. However, ultramicroelectrodes (UME) [8] that have dimensions of 1-20 pm, are also used as indicator electrodes. The tiny electrodes have some definite advantages over conventional ones  

12) If the radius of the disk is r and the inner and outer radii of the ring are r2 and r3, respectively, the calculated N value depends on the values of r2/n and r3/r2. For exam- 

Recently, arrayed electrodes, which consist of several ultramicroelectrodes of the same type or of different types are prepared and used in sophisticated ways [8h], 

If the surface of a metal or carbon electrode is covered with a layer of some functional material, the electrode often shows characteristics that are completely different from those of the bare electrode. Electrodes of this sort are generally called modified electrodes [9] and various types have been developed. Some have a mono-molecular layer that is prepared by chemical bonding (chemical modification). Some have a polymer coat that is prepared either by dipping the bare electrode in a solution of the polymer, by evaporating the solvent (ethanol, acetone, etc.) of the polymer solution placed on the electrode surface, or by electrolytic polymerization of the monomer in solution. The polymers of the polymer-modified electrodes are either conducting polymers, redox polymers, or ion-exchange polymers, and can perform various functions. The applications of modified electrodes are really limit- 

Fuel cell polymer battery photoelectric cell capacitor Storage element liquid crystal display device electrochromic display device electrochemiluminescence device photoelectric transducer Biosensor ion sensor detector in HPLC and FIA gas sensor voltam-metric indicator electrode reference electrode 

Ultramicroelectrode responses are independent of the diffusion layer thickness and of flow. They exhibit increased signal-to-noise ratio. 

Amperometric electrodes made on a microscale, on the order of 5 to 30 /rm diameter possess a number of advantages. The electrode is smaller than the diffusion layer thickness. This results in enhanced mass transport that is independent of flow, and an increased signal-to-noise ratio, and electrochemical measurements can be made in high-resistance media, such as nonaqueous solvents. An S-shaped sigmoid current-voltage curve is recorded in a quiet solution instead of a peak shaped curve because of the independence on the diffusion layer. The hmiting current, q, of such microelectrodes is given by 

Define half-wave potential, depolarizer, DME, residual current, and voltammetry. 

Give two reasons for using a supporting electrolyte in voltammetry. 

A solution contains about 10 M Fe and 10 M Pb. It is desired to analyze for the lead content polarographically. Fe is reduced to Fe at all potentials accessible with the DME up to —1.5 V versus SCE, and is reduced along with Fe- to the metal at potentials more negative than —1.5 V. Pb- is reduced at —0.4 V. Suggest a scheme for measuring the lead polarographically. 

Small electrodes have been in use since the 1980s in the field of electrochemistry [14]. An electrode with at least one of its dimensions below 50 pm will exhibit noticeably different properties than macroscopic electrodes made of the same material such 

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Figure Bl.19.12. Basic principles of SECM. (a) With ultramicroelectrode (UME) far from substrate, diflfiision leads to a steady-state current, ij, (b) UME near an insulating substrate. Flindered diflhision leads to < ij, 3D. (c) UME near a conductive substrate. Positive feedback leads to go. (Taken from [62],...

Wightman R M and Wipf D O 1989 Voltammetry at ultramicroelectrodes Electroanal. Chem. 15 267... 

Selzer Y and Manler D 2000 Scanning electrochemical microscopy. Theory of the feedback mode for hemispherical ultramicroelectrodes steady-state and transient behavior Anal. Chem. 72 2383... 

Aqueous diffusion coefficients are usually on the order of 5 x 10 cm /s. A second is typically a long time to an electrochemist, so 6 = 30 fim. The definition of far is then 30 ]lni. Short is less than a second, perhaps a few milliseconds. Microseconds are not uncommon. Small, referring to the diameter of the electrode, is about a millimeter for microelectrodes, or perhaps only a few micrometers for ultramicroelectrodes (13). 

The basic instrumentation required for controlled-potential experiments is relatively inexpensive and readily available commercially. The basic necessities include a cell (with a three-electrode system), a voltammetric analyzer (consisting of a potentiostatic circuitry and a voltage ramp generator), and an X-Y-t recorder (or plotter). Modem voltammetric analyzers are versatile enough to perform many modes of operation. Depending upon the specific experiment, other components may be required. For example, a faradaic cage is desired for work with ultramicroelectrodes. The system should be located in a room free from major electrical interferences, vibrations, and drastic fluctuations in temperature. 

Carbon-Fiber Electrodes The growing interest in ultramicroelectrodes (Section 4-5.4) has led to widespread use of carbon fibers in electroanalysis. Such materials are produced, mainly in connection with the preparation of high-strength composites, by high-temperature pyrolysis of polymer textiles or via... 

What are the advantages of using ultramicroelectrodes for electrochemical measurements ... 

Explain clearly why and how a change of the scan rate affects the shape of the cyclic voltammetric response of an ultramicroelectrode. 

The measurement of the electrode impedance has also been ealled Faradaie impedanee method. Since measurements are possible by applying either an electrode potential modulated by an AC voltage of discrete frequeney (which is varied subsequently) or by applying a mix of frequencies (pink noise, white noise) followed by Fourier transform analysis, the former method is sometimes called AC impedance method. The optimization of this method for the use with ultramicroelectrodes has been described [91Barl]. (Data obtained with these methods are labelled IP.)... 

Auother uauostructuriug techuique shares some commou features with that described above and is shown in Fig. 36.3. A polymer-coated Pt ultramicroelectrode is used as the tip of a STM, and graphite is used as substrate. Ag atoms are deposited on the tip at underpotentials, so that approximately one atomic monolayer is deposited on the tip. After this, a first bias pulse is applied to the tip, causing the formation of a shallow pit. Then a second bias pulse with a smaller amplitude is applied to cause the desorption of silver from the tip. The silver ions desorbed diffuse aud migrate across the tip-sample gap aud deposit ou the high coordiuatiou sites preseut... 

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. 

Fortunately the microinterfaces between two immiscible electrolytes seem to be a very useful experimental model of small liquid-liquid systems. The formation and investigation of the micro-ITIES is continuously perfected [74-76]. The smallest diameter so far achieved was 5 jiva. The main utilization of micro-ITIES is developed, in parallel with application of ultramicroelectrodes. 

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. 

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))... 

The ohmic potential difference in an electrolytic cell consisting of a spherical test electrode, termed, for a small radius r0, ultramicroelectrode, in the centre and another very distant concentrical counter-electrode is given by the equation... 

Obviously, the ohmic potential difference does not depend on the distance of the counterelectrode (if, of course, this is sufficiently apart) being situated mainly in the neighbourhood of the ultramicroelectrode. At constant current density it is proportional to its radius. Thus, with decreasing the radius of the electrode the ohmic potential decreases which is one of the main advantages of the ultramicroelectrode, as it makes possible its use in media of rather low conductivity, as, for example, in low permittivity solvents and at very low temperatures. This property is not restricted to spherical electrodes but also other electrodes with a small characteristic dimension like microdisk electrodes behave in the same way. 

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). 

The properties of the voltammetric ultramicroelectrode (UME) were discussed in Sections 2.5.1 and 5.5.1 (Fig. 5.19). The steady-state limiting diffusion current to a spherical UME is... 

Fig. 5.19 Electrodes used in voltammetry. A—dropping mercury electrode (DME). R denotes the reservoir filled with mercury and connected by a plastic tube to the glass capillary at the tip of which the mercury drop is formed. B—ultramicroelectrode (UME). The actual electrode is the microdisk at the tip of a Wollaston wire (a material often used for UME) sealed in the glass tube...

J. Min and A.J. Baeumner, Characterization and optimization of interdigitated ultramicroelectrode arrays as electrochemical biosensor transducers. Electroanalysis 16, 724—729 (2004). 

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