Thin-layer electrolysis system

Case II Reversible or Ouasi-Reversible Redox Species. If the tip-sample bias is sufficient to cause the electrolysis of solution species to occur, i.e., AEt > AEp, ev, the proximity of the STM tip to the substrate surface (d < 10 A) implies that the behavior of an insulated STM tip-substrate system may mimic that of a two-electrode thin-layer cell (TLC)(63). At the small interelectrode distances required for tunneling, a steady-state concentration gradient with respect to the oxidized (Ox) and and reduced (Red) electroactive species should be established between the tip and the substrate, and the resulting steady-state current will augment that present as a result of the convection of electroactive species from the bulk solution. In many cases, this steady state current is predicted to overwhelm the convective currents, so this situation is of concern when STM imaging under electrochemical conditions (64). 

Optically transparent electrode — (OTE), the electrode that is transparent to UV-visible light. Such an electrode is very useful to couple electrochemical and spectroscopic characterization of systems (- spectroelectro-chemistry). Usually the electrodes feature thin films of metals (Au, Pt) or semiconductors (In203, SnCb) deposited on transparent substrate (glass, quartz, plastic). Alternatively, they are in a form of fine wire mesh minigrids. OTE are usually used to obtain dependencies of spectra (or absorbance at given wavelengths) on applied potentials. When the -> diffusion layer is limited to a thin layer (i.e., by placing another, properly spaced, transparent substrate parallel to the OTE), bulk electrolysis can be completed in a few seconds and, for -> reversible or - quasireversible systems, equilibrium is reached for the whole solution with the electrode potential. Such OTEs are called optically transparent thin-layer electrodes or OTTLE s. 

Another popular mode for transmission experiments involves a thin-layer system (9, 10, 13, 18) like that shown in Figure 17.1.2. The working electrode is sealed into a chamber (e.g., between two microscope slides spaced perhaps 0.05-0.5 mm apart) containing the electroactive species in solution. The chamber is filled by capillarity, and the solution within it contacts additional solution in a larger container, which also holds the reference and counter electrodes. The electrolytic characteristics of the cell are naturally similar to those of the conventional thin-layer systems discussed in Section 11.7. One can do cyclic voltammetry, bulk electrolysis, and coulometry in the ordinary way, but there is also a facility for obtaining absorption spectra of species in the cell. 

The particular advantage of this optically transparent thin-layer electrode (OTTLE) is that bulk electrolysis is achieved in a few seconds, so that (for a chemically reversible system) the whole solution reaches an equilibrium with the electrode potential, and spectral data can be gathered on a static solution composition. 

To define a unique solution, we must specify the corresponding boundary and initial conditions. Normally electrolyte solutions are in contact with or bounded by electrodes. An electrode in its simplest form is a metal immersed in an electrolyte solution so that it makes contact with it. For example, copper in a solution of cupric sulfate is an example of an electrode. A system consisting of two electrodes forms an electrochemical cell. If the cell generates an emf by chemical reactions at the electrodes, it is termed a galvanic cell, whereas if an emf is imposed across the electrodes it is an electrolytic cell (Fig. 6.1.1). If a current is generated by the imposed emf, the electrochemical or electrolytic process that occurs is known as electrolysis. Now whether or not a current flows, the electrolyte can be considered to be neutral except at the solution-electrode interface. There a thin layer, termed a Debye sheath or electric double layer, forms that is composed predominately of ions of charge opposite to that of the metal electrode. We shall examine this double layer in Section 6.4, but for our purposes here it may be neglected. 

As an alternative to the use of a thin layer electrolysis method, systems based on the use of fiber optic cable for transmission of infrared radiation may be used in an in situ dip probe made for monitoring the course of conventional bulk electrolysis experiments. Unlike thin layer cell methods, this spectroelectrochemical technique requires no cell design compromises which diminish the accuracy of the voltammetric data that also may be obtained during the course of the electrolysis. 

In most cases the variable cell volume is of little practical use, since the minimum possible volume is usually required. The performance principles of detectors, where electrolysis takes place in the thin-layer of electrolyte, flowing parallel to the electrode surface, can be demonstrated on the commercially available thin-layer detector from Bioanalytical Systems (USA), schematically depicted in the Figure 12[36]. The detector works with a carbon paste or glassy carbon working electrode. The thickness of the layer is determined by the thickness of a Teflon film spacer ( 50 p) between two blocks of plastic. Since there is not enough space for location of the other two electrodes, they have to be placed outside of the thin-... 

ABTS + production has been described by using thin-layer spectroelectrochemistry [46]. Fifty microliters of ABTS in 0.1 M acetate buffer solution (pH 5) was oxidized in a quartz flat cell (0.1 cm width) containing an optical transparent thin-layer electrode (OTTLE system). The radical formation was measured with a potential scan from 0.65 to 0.70 V and returned to 0.65 V at the scan rate of 0.05 mV s . The reactions were monitored spectrophotometrically every 30 s. Figure 31.11 shows the 3D plots obtained for spectral changes during ABTS electrolysis at different intervals. At the beginning, with ABTS as the sole species, two peaks were observed (A = 214 nm, A.2 = 340 nm), but as the... 

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