Voltammetric ultramicroelectrodes

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. 

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

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... 

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 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 ... 

On the contrary, the radical cation of anthracene is unstable. Under normal volt-ammetric conditions, the radical cation, AH +, formed at the potential of the first oxidation step, undergoes a series of reactions (chemical -> electrochemical -> chemical -> ) to form polymerized species. This occurs because the dimer, tri-mer, etc., formed from AH +, are easier to oxidize than AH. As a result, the first oxidation wave of anthracene is irreversible and its voltammetric peak current corresponds to that of a process of several electrons (Fig. 8.20(a)). However, if fast-scan cyclic voltammetry (FSCV) at an ultramicroelectrode (UME) is used, the effect of the follow-up reactions is removed and a reversible one-electron CV curve can be obtained (Fig. 8.20(b)) [64], By this method, the half-life of the radical cat-... 

The introduction of ultramicroelectrodes in the field of voltammetric analysis offers access to cyclic voltammetry experiments that are impossible with conventionally sized macroelectrodes. In addition to analyses in small volumes or at microscopic locations, microelectrodes allow measurements in resistive media and make it possible to perform high scan rate voltammetry [9,10]. 

Unfortunately, it is far from trivial to obtain oxidation potentials for commonly encountered 17-electron metalloradicals M, because many such radicals dimerize at rates approaching diffusion-control, rendering it nearly impossible to observe such species by cyclic voltammetry. The use of ultramicroelectrodes was shown [41] to give a reversible signal for the oxidation of Mn(CO)5 at scan rates of ca 5000 V s , but the fmther oxidation of this radical to the 16-electron cation was not reported. There are, however, certain frequently encountered systems for which such radicals are stable at least on the time-scale of normal voltammetric measurements. Figure 4 shows an example, the oxidation of CpCr(CO)3 in acetonitrile. 

E. Computer-based methods for analysis of voltammetric data Ultramicroelectrodes and Scanning Electrochemical Microscopy... 

Electrodes of different materials have been miniaturised in many geometric shapes with the common characteristic that the electrode is significantly smaller than the diffusion layer at the electrode surface for ordinary voltammetric time scales (e.g. 1-10 s)[36]. According to Koichi[37], if the characteristic length of a small electrode, such as an ultramicroelectrode, is made infinitesimally small, it tends to adopt the geometry of either a point, a line, or a plane. On this basis, ultramicroelectrodes can generally be classified into a point electrode, a line electrode, and a plane electrode. 

The cell in Figure 2 is a typical apparatus used in LL studies. However, recently small interfaces, called here microinterfaces, were shown to have some experimental advantage. The purpose of this modification was to use the same advantage that the ultramicroelectrodes have. Ultramicroelectrodes help to overcome solution resistance difficulties that originate from a potential shift due to an uncompensated iR drop. As the interfacial area becomes smaller, the diffusion geometry becomes a spherically symmetric process, which means that the ratio of charge transport current versus solution resistance increases and, ultimately, renders the iR drop minimal. In ITIES studies, restriction of the interfacial area and use of a current amplifier for voltammetric studies is a viable alternative to a four-electrode potentiostat. 

Enhanced Response to Catechols. An important problem in bio-electroanalytical chemistry is the determination of catechol compounds in the presence of ascorbic acid (vitamin C) ( ). The development of carbon fiber ultramicroelectrodes has allowed for the monitoring of these compounds iji vivo ( ). Because the oxidation potential of ascorbate is very similar to those of catechol compounds, because the concentration of ascorbate is at least an order of magnitude higher than that of the catechol species present in the brain (27), and because ascorbate may become involved in an electrocatalytic reaction sequence with oxidized catechol species leading to loss in voltammetric resolution (29,30), the ability to detect catechols in the presence of ascorbate is a non-trivial problem. 

When a voltammetric scan is initiated at a bare carbon electrode placed in a solution of 10 mM ascorbate/1 mM DOPAC, the peaks due to the oxidation of the two compounds are poorly resolved (24-26). If, however, a PNVP-coated electrode is placed in the same solution, the two peaks are partially resolved, as illustrated in the upper square wave voltammogram shown in Figure 5. Because the concentrations of the two species are more nearly equal close to the electrode surface, the oxidative signals are partially resolved. If the electrode potential is held sufficiently positive to oxidize both species for several minutes, and another scan is acquired, the lower voltammogram shown in Figure 5 is obtained. In this case, ascorbate was oxidized with subsequent rapid hydrolysis to an electroinactive form whereas DOPAC was oxidized to its ortho-quinone form. When the potential was stepped back to a more negative value, the DOPAC quinone was reduced back to the dihydroxy- form. Thus the lower scan shown exhibits a peak due primarily to oxidation of DOPAC since the ascorbic acid present in the film was oxidatively depleted. We have recently reported similar results obtained at gamma-irradiated, PNVP-coated, carbon fiber ultramicroelectrodes (24,25). 

In liquid SO2 and CsAsFg as supporting electrolyte the oxidation of saturated hydrocarbons could be studied up to potentials of 6.0 V vs see by using platinum ultramicroelectrodes. Irreversible voltammetric waves were found for the compounds methane to n-octane that were studied The anodic peak currents suggest n-values around 2 a bulk electrolysis consumed 2 faradays per mole of alkane. The primary products in dilute solutions (1-10 mM) were shorter chain hydrocarbons. Electrolysis of more concentrated solutions led, via reaction of the oxidation products with starting material, to longer-chain hydrocarbons. 

Ultramicroelectrodes (see Chapter 2.5) have attracted considerable attention for voltammetric experiments recently [39,... 

The decreased iR drop in voltammetric experiments at ultramicroelectrodes has been exploited to perform electrochemistry under conditions in which no or only a small concentration of supporting electrolyte is added and allows measurements in low-polarity solvents (e.g. hydrocarbons), without the presence of excess ions, or even in the gas phase [51]. This topic is discussed further in Chapter 2.5 (Sect. 2.5.5.6). In these cases, the transport of charge in the electrolyte is realized by small amounts of impurities [48], by ions of the substrate material itself [52], or those generated in the electrode reaction [39]. Thus, migration has to be considered as an additional mode of transport, in particular for multiply charged species [52]. A recent modeling study [53] has provided evidence that LSV should be best suited to deal with situations of high uncompensated resistance as compared to chronopotentiometry and chronoamperometry. 

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