Microelectrodes, voltammetric

Baranski and Lu [209] have carried out, applying microelectrodes, voltammetric studies on ammonium amalgam in propylene carbonate solutions at room temperatures. The sweep rates up to 80 V s were appropriate for the analysis of the formation kinetics of this compound. Experimental and numerical simulation results have shown that ammonium amalgam was formed via fast charge-transfer process and its first-order decomposition was characterized by the rate constant of about 0.6 s . Diffusion coefficient of NH4 radical in mercury was estimated to be about 1.8 X 10 cm s k The formal potential of NH4+ (aq)/NH4(Hg) couple was determined as—1.723 V (SHE). 

Figure 4.17 (a) Schematic of the ME graphene monolayer electrode preparation, (b) Microelectrode voltammetric response obtained on mono-, bi-, and multilayer graphene flakes, normalized to the flake radius. (Reprinted from Ref. [120].)... 

In the presence of excess inert electrolyte, the voltammetric responses of Pt disk electrodes of 5-50 nm in radii do deviate from the predication of the conventional voltammetric theory as a result of the enhanced EDL effects at nanoscale electrochemical interfaces, but the deviations are quantitatively small (e.g., within 20% even at electrodes of a few nanometers), and in most cases might be hardly distinguished with the experimental uncertainties. In the absence of the excess of the supporting eleetrolyte, the voltammetric responses for disk electrodes larger than 200 nm in radii show reasonable agreements with the predications of the eonventional microelectrode voltammetric theory. However, for electrodes smaller than 200 nm, the voltammetric responses predicated by the present theory exhibit significant deviation from the microelectrode theory. The deviations are mainly resulted from the overlap between the diffuse double layer and the CDL at nanoscale electrochemical interfaces in weakly... 

Evans D FI 1991 Review of voltammetric methods for the study of electrode reactions Microelectrodes Theory and Applications (Nate ASI Series E vol 197) ed M I Montenegro, M A Queiros and J L Daschbach (Dordrecht Kluwer)... 

Scale of Operation Voltammetry is routinely used to analyze samples at the parts-per-million level and, in some cases, can be used to detect analytes at the parts-per-billion or parts-per-trillion level. Most analyses are carried out in conventional electrochemical cells using macro samples however, microcells are available that require as little as 50 pL of sample. Microelectrodes, with diameters as small as 2 pm, allow voltammetric measurements to be made on even smaller samples. For example, the concentration of glucose in 200-pm pond snail neurons has been successfully monitored using a 2-pm amperometric glucose electrode. ... 

FIGURE 1. Voltammetric curves in DMF in the presence of Bu4NBF4 (0.1m), reference electrode Ag/Agl/I (0.1m), mercury stationary microelectrode (A) PhS02Me (10 3m), sweep rate 500mVs". (B) PhS02Ph (103m), sweep rate 500mVs 1. 

FIGURE 3. Voltammetric curves at a stationary mercury microelectrode for anthracene in the presence of 17b (a) anthracene alone, 6.5 x 10 3m (b, c, d) previous solution with 2.3, 5.1 and 7.6 x 10"3m of 17b (e) disulphone 17b without anthracene. Medium, DMF-Bu4NC104 0.14m sweep rate, 10mVs 1 (after Reference 25). 

FIGURE 6. Voltammetric curves in DMF-TBAP 0.1 m, stationary mercury microelectrode, sweep rate lOmVs" (1) without phenol, (2)10 m phenol added (a) PhSOjCHjPh, (b) PhSOjC(Et)(Me)Ph. 

Jiang J, Kucernak A. 2005. Sohd pol3nner electrolyte membrane composite microelectrode investigations of fuel cell reactions. B Voltammetric study of methanol oxidation at the nanostructured platinum microelectrode Nafion membrane interface. J Electroanal Chem 576 223-236. 

S.L Vilakazi and T. Nyokong, Voltammetric determination of nitric oxide on cobalt phthalocyanine modified microelectrodes. J. Electroanal. Chem. 512, 56-63 (2001). 

S.B. Hocevar, J. Wang, R.P. Deo, M. Musameh, and B. Ogorevc, Carbon nanotube modified microelectrode for enhanced voltammetric detection of dopamine in the presence of ascorbate. Electroanalysis 17, 417-422 (2005). 

Bren del and Luther [356] have described a solid-state voltammetric gold analyser microelectrode for the measurement of iron and magnesium in pore waters. 

Galceran, J., Puy, J., Salvador, J., Cecilia, J. and van Leeuwen, H. P. (2001). Voltammetric lability of metal complexes at spherical microelectrodes with various radii, J. Electroanal. Chem., 505, 85-94. 

Wang et al. have reported a catheter microelectrode assembly for in vivo and in vitro voltammetric analyses of chlorpromazine in biological fluids [159]. 

Voltammetry has been adapted to HPLC (when the mobile phase is conducting) and capillary electrophoresis (CE) as a detection technique for electroactive compounds. In this usage, the voltammetric cell has to be miniaturised (to about 1 pi) in order not to dilute the analytes after separation. A metal or carbon microelectrode has a defined potential (vs the reference electrode) depending on the substances to be detected (ions or molecules) and the mobile phase flows through the detection cell (Fig. 19.5). This method of amperometric detection in the pulsed mode is very... 

What are the advantages of using a microelectrode for voltammetric measurements ... 

Cyclic voltammetric and related techniques are particularly valuable for determining Ea values in cases where one member of the redox couple is unstable. At a microelectrode in electrolytes of low resistivity, cyclic voltammograms can be recorded at scan rates up to about 100 V s-1 and at low temperatures. This allows the detection of reversibility when the unstable partner has a life-time of the order of a millisecond or so. Ultra-microelectrode techniques promise to lower this limit even further.1-3... 

An optically transparent, porous platinum film has been produced by photoelectrodeposition on an InP semiconductor substrate [15], Polyester sheet covered with a thin film of sputtered gold has also proved suitable as an OTE [71]. When overcoated with a layer of Ti02, these electrodes exhibited electrochemical behavior consistent with a microelectrode array, including cyclic voltammetric current plateaus instead of clearly defined peaks, although this feature was not recognized at the time [71]. 

The small area of a microelectrode, with its proportionately low capacitance, allows its use at very short time scales compared to the time scale used with a classical voltammetric electrode. As we have seen earlier in this chapter, when microelectrodes are used at short time scales, the current follows the behavior expected for diffusion in one dimension. Thus, the development of high-speed voltammetric methods with microelectrodes was a logical step, and has greatly expanded the scope and capabilities of electrochemical techniques [41]. Rapid electrochemical methods allow evaluation of the larger rate constants of rapid heterogeneous and/or homogeneous reactions. For example, theories of hetero-... 

The magnitude of the ohmic drop at a microelectrode can be evaluated quite readily for case 1 from a knowledge of the specific solution resistance (obtained from conductivity measurements such as in Table 12.1) and the expressions for the voltammetric current for the specific microelectrode employed. Case 2 is also straightforward if the free concentration of ions exceeds that of the electroactive species. However, the situation is somewhat more complicated for the third class. In this case, and in case 2 for fully associated electrolyte, migration as well as diffusion can affect the observed voltammetric signals. In all three cases, the situation may be further complicated by a change in structure of the double layer. However, this is ignored for now, and is considered in the section on very small electrodes. 

When using microelectrodes to obviate resistance problems, it is convenient to develop a procedure to determine what conditions are required to reduce the error to an acceptable level. The results of such a procedure applied to disk electrodes are shown in Figure 16.6 [45]. In this and the remaining discussion, the technique of cyclic voltammetry is considered, as it is one of the most widely used voltammetric methods. The region of practical working conditions of electrode radius and scan rate is defined by the area set off by lines A, B, and C. 

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