Electrodes thin layer

FIGURE 3-26 Dual-electrode thin-layer detector configurations for operation in the series (a) and parallel (b) amperometric modes. 

Fenn, R. J., Siggia, S., and Curran, D. J., Liquid chromatography detector based on single and twin electrode thin-layer electrochemistry application to the determination of catecholamines in blood plasma, Anal. Client., 50, 1067,1978. 

Bergstrom et al. [63] used HPLC for determination of penicillamine in body fluids. Proteins were precipitated from plasma and hemolyzed blood with trichloroacetic acid and metaphosphoric acid, respectively, and, after centrifugation, the supernatant solution was injected into the HPLC system via a 20-pL loop valve. Urine samples were directly injected after dilution with 0.4 M citric acid. Two columns (5 cm x 0.41 cm and 30 cm x 0.41 cm) packed with Zipax SCX (30 pm) were used as the guard and analytical columns, respectively. The mobile phase (2.5 mL/min) was deoxygenated 0.03 M citric acid-0.01 M Na2HP04 buffer, and use was made of an electrochemical detector equipped with a three-electrode thin-layer cell. The method was selective and sensitive for mercapto-compounds. Recoveries of penicillamine averaged 101% from plasma and 107% from urine, with coefficients of variation equal to 3.68 and 4.25%, respectively. The limits of detection for penicillamine were 0.5 pm and 3 pm in plasma and in urine, respectively. This method is selective and sensitive for sulfhydryl compounds. 

In 1985, Feldman et al. reported in situ conductivity measurements on polypyrrole. These were obtained using a twin electrode, thin-layer cell... 

Figure 3.75 Schematic illustration of a twin electrode thin layer cell used for static conductivity measurements. From Feldman et ai (1985). Copyright 1985, American Chemical Society.

Reconditioning the surface of a working electrode (thin-layer and wall-jet cells) can be done ... 

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). 

If the total current can be assumed to be limited by diffusion to the STM tip, Case III is similar to diffusion to a microdisk electrode (one electrode) thin-layer cell (63). Murray and coworkers (66) have shown that for long electrolysis times, diffusion to a planar microdisk electrode TLC can be treated as purely cylindrical diffusion, provided that the layer thickness is much smaller than the disk diameter (66). In contrast to the reversible case discussed above (Case I), the currents in this scenario should decrease gradually with time at a rate that is dependent on the tip radius and the thickness of the interelectrode gap. Thus, for sufficiently narrow tip/sample spacings, diffusion may be constrained sufficiently (ip decayed) at long electrolysis times to permit the imaging of surfaces with STM. 

A twin electrode thin layer Kissinger cell was designed in which the channel volume could be varied through the use of PTFE spacers [174]. The working and counter electrodes were carbon paste (3.14 m ) and the reference electrode was Ag/AgCl. The performance of the cell was tested on 50 pL portions of chlorpromazine solutions in 0.01 M HCl, and the three cited methods were compared. Linear sweep voltammetry was found to be the simplest to apply and showed moderate sensitivity. 

The detection limit for TLV has been improved substantially by using differential pulse and square-wave voltammetry (Chap. 5). For example, detection limits in the 10 8 M range and below have been demonstrated in thin-layer cells requiring less than 100 /xL of sample [61,62]. One practical application of twin-electrode thin-layer cells is in the automatic electrochromic rearview mirror for automobiles. A cell with optically transparent electrodes is placed in front of a mirrored surface. At night, electrolysis in the cell to generate colored material can rapidly reduce glare from following vehicles. 

Figure 6. Comparison of simulated SECM transients with transients corresponding to different electrode geometries (all processes are diffusion controlled). (A) the SECM transient for a conductive substrate (B) two-electrode thin-layer cell (C) microdisk (D) planar electrode (E) SECM with an insulating substrate (F) one-electrode thin-layer cell. Curves A, B, E, and F were computed with L = d/a = 0.1. Adapted with permission from Ref. [62], Copyright 1991, American Chemical Society.

Electrochemical polymerization is preferred to chemical polymerization, especially if the polymeric product is intended to be used as a polymer film electrode, thin layer sensor, in microtechnology etc., because the potential control is a precondition of the production of good-quality material and the polymer film is formed at the desirable spot that serves as an anode during the synthesis. 

As examples, experimental F(E) isotherms at = constant for Pb and T1 UPD in acidic perchlorate electrolyte on AgQikl) single crystal faces with Qtkl) = (111) and (100) are shown in Figs. 3.9 and 3.10, respectively [3.97, 3.105]. These isotherms were measured stepwise, waiting for equilibrium conditions according to polarization routines illustrated in Fig. 3.11 using the twin-electrode thin-layer technique" (TTL) for Pb UPD and the flow-through thin-layer technique" (FTTL) for T1 UPD. 

These relative surface excess parameters can be determined experimentally using different methods /)(w) can be determined by radiotracer studies, o-measurements, electroanalytical techniques (twin-electrode thin-layer, flow-through thin layer, rotating ring-disk experiments) etc., whereas q can be determined by charging curves, capacitance measurements etc. Isotherm conversion q-Fiiyf) is obtained by the corresponding Maxwell relations ... 

In evaporation-intercalation devices solar energy conversion would, at least in the more efficient case of a thermal system, not be converted by exciting electrons and rapidly separating them from holes, but by transferring atoms or molecules across a phase boundary by evaporation which is usually a very efficient process. It is, consequently, neither necessary to use materials which are well crystallized like those developed for photovoltaic cells nor is it necessary to prepare sophisticated junctions. A compacted polycrystalline sheet of a two-dimensional material which is on one side placed in contact with an electrolyte, sandwiched between the layer-type electrode and a porous counter electrode, as it is used in fuel cells, would constitute the central energy conversion unit. Some care would have to be taken to choose an electrolyte which is suitable for intercalation reactions and which is not easily evaporated through leaks in the electrodes. Thin layers of polymeric or solid electrolytes would seem to be promising. 

Schematic diagram of a single-electrode thin-layer cell, (b) Micrometer, twin-electrode thin-layer cell with adjustable solution layer thickness.

Consider the twin-working-electrode, thin-layer cell (Figure 11.7.1 ), with the potential stepped from a value where no current flows, to 2, where the reaction O ne R is virtually complete and the concentration of O at the electrode surface is essentially zero. To obtain the current-time behavior and the concentration profile one must solve the diffusion equation... 

Concentration of O during reduction of O in twin-electrode thin-layer cell. 

A number of electrochemical cell designs have been described but the most popular configurations are the three-electrode thin-layer cell and the wall-jet cell. Figure 5.21[20,102,166-171,189]. The eolumn eluent is introduced either parallel to... 

Elbicki etal. 984) reviewed the optimum configurations for each of the above electrodes (thin-layer, tubular, and wall-jet) based on a mathematical treatment of the diffusive and convective phenomena in force. Boundary conditions on such physical restraints as electrode area, cell dimensions, and inlet configuration were established. Some confusion in the past has resulted from misinterpreting these equations (Weber, 1983) they are derived for cells in which the boundary layer may freely grow unencumbered. In certain cells (e.g., low-volume wall-jet or long-channel electrodes), walls, nozzles, etc. may impede the growth of the diffusion layer and bias the output current expected. Under these conditions, the wall-jet electrode behaves virtually as a thin-layer cell (if the nozzle spacing is small and the nozzle acts as a point source). Both detectors were concluded to yield output currents of... 

The Kissinger type twin electrode thin layer cell is a widely used tool in the everyday analytical practice especially in the field of flow injection techniques and as an amperometric detector in liquid chromatography. Less attention is paid to the possibilities offered by this cell 2LS a microanalytical tool when it is filled with a quiescent solution sample. In this way the determination of electroactive components in a volume of about 50-100 pi can be carried out by applying a proper excitation potential program. 

Fig.l. The cross section of the home built Kissinger type twin electrode thin layer cell. R Ag/AgCl wire, in a closed salt bridge compartment. 

DPV was first tried with twin electrode thin layer cell. From the point of optimization its behaviour is quite similar to the cell used by Jarbawi, Heinemann and Patriarche. In Table 1 the effect of clock time is shown for a 10" M CPZ solution. The pulse amplitude was 20 mV and the dc rate of polarization 2 mV/s. Two different spacer thicknesses, 40 and 80 /xm, were used. 

Figure 4.30 Determination of the adsorbed mass of lead with the twin electrode thin-layer method descrihed in Figure 4.15. Pb UPD on (a) Ag(lll) and (b) Ag(lOO). Concentrations as given in Figures 4.27 and 4.28. (Reproduced with permission from Ref. [60], 1978, Elsevier.)...

Isaksson and Kissinger (1987) detected physostigmine in plasma with a different system. Physostigmine was eluted on a C-8 column using an acidic eluent with an ion-pairing reagent. This detection system was based on the redox mode, where a dual-electrode thin-layer cell was used in a series configuration, i.e., the reduction current from the second electrode was measured. 

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