Microelectrochemical measurements

The ET reaction between aqueous Fe(CN)g and the neutral species, TCNQ, has been investigated extensively with SECM, in parallel with microelectrochemical measurements at expanding droplets (MEMED) [84], which are discussed in Chapter 13. In the SECM studies, a Pt UME in the aqueous phase generated Fe(CN)g by reduction of Fe(CN)g. TCNQ was selected as the organic electron acceptor, because the half-wave potential for TCNQ ion transfer from DCE to water is 0.2 V more positive than that for ET from Fe(CN)g to TCNQ [85]. This meant that the measured kinetics were not compromised by TCNQ transfer from DCE to the aqueous phase within the potential window of these experiments. 

A recent development, termed by the inventors microelectrochemical measurements at expanding droplets (MEMED) [29], is a technique based on forming small droplets of a phase containing a reactant in a second immiscible liquid phase (Fig. 5.24). An ultramicroelectrode (UME, see Section 5.3.2.8 and Chapter 6) measures an electrochemical response as the droplet expands towards it, from which a concentration profile can be derived and, hence, the kinetics of related processes. Because the surface is continuously refreshed, it avoids... 

Liquid/liquid interfaces have been discussed in Chapter 8. A related approach involves using an expanding droplet of a nonaqueous solvent positioned above a stationary microelectrode (microelectrochemical measurements at expanding droplets, or MEMED) (18,19). More recent work has probed bilayer lipid membranes (BLMs). A BLM is produced by placing a small amount of a lipid, such as lecithin, on a small orifice ( 600 /xin... 

Fig. 2. Schematic of microelectrochemical measurements on inhomogeneous substrates using various capillary sizes.

The samples were mechanically ground with SiC emery paper up to 4000-grit and finally polished with 1 pm diamond paste. They were rinsed with distilled water and ultrasonically cleaned in ethanol. Prior to microelectrochemical measurements all substrates were cathodically prepolarized at -500 mV vs. SCE (saturated calomel electrode)... 

A new microelectrochemical technique with a current detection limit of < 10 fA and the use of an electrochemical microcell allow reduction of the exposed surfece area. Such microelectrochemical measurements have several advantages ... 

There is a new project with die aim of developing a method which combines the advantages of microelectrochemistiy and of SPM. It should be possible to perform microelectrochemical measurements and SPM investigations at the same local points on a sample. This technique will allow observations of corrosion attacks and their surroundings before and after exposure to corrosive conditions. 

E.G. Webb, T. Suter, R.C. Alkire, Microelectrochemical measurements of the dissolution of single MnS inclusions and the prediction of the critical conditions for pit initiation on stainless steel, J. Electrochem. Soc. 148 (2001) B186-B195. 

I. Muto, Y. Izumiyama, N. Hara, Microelectrochemical measurements of dksolution of MnS inclusions and morphological observation of metastable and stable pitting on stainless steel, J. Electrochem. Soc. 154 (2007) C439-C444. 

Figure 6.2.1.2 illustrates the range of electrochemical cells that have been developed for microelectrochemical measurements. Significant features include the cell volume, distance of the electrodes, electrolyte flow, and optical transparency. For example, scanning electrochemical microscopy (SECM, see Chapter 12) is performed in a macroscopic cell, but the... 

Zhang, J., Unwin, P. R. Microelectrochemical measurements of electron transfer rates at the interface between two immiscible electrolyte solutions Potential dependence of the ferro/ferricyanide-7,7,8,8-tetracyanoquinodimethane (TCNQ/TCNQ ) system. Phys. Chem. Chem. Phys. 2002,4, 3820. 

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