Ultramicroelectrodes highly resistive conditions

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. 

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. 

The second category of electrode response for an ultramicroelectrode occurs at high V. Under these conditions, linear diffusion is operative and the response does not differ from that of conventional electrodes with surface diameters in the 0.1 to 2 mm range. However, the effects of double-layer charging (see Chapter 1) and solution resistance are considerably reduced owing to the small size of the electrode. The transition from low to high V is shown in Fig. 22 for an electrode with r= 1pm at r>=0.03, 10, 500, and... 

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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