Four-electrode potentiostat

Early studies of ET dynamics at externally biased interfaces were based on conventional cyclic voltammetry employing four-electrode potentiostats [62,67 70,79]. The formal pseudo-first-order electron-transfer rate constants [ket(cms )] were measured on the basis of the Nicholson method [99] and convolution potential sweep voltammetry [79,100] in the presence of an excess of one of the reactant species. The constant composition approximation allows expression of the ET rate constant with the same units as in heterogeneous reaction on solid electrodes. However, any comparison with the expression described in Section II.B requires the transformation to bimolecular units, i.e., M cms . Values of of the order of 1-2 x lO cms (0.05 to O.IM cms ) were reported for Fe(CN)g in the aqueous phase and the redox species Lu(PC)2, Sn(PC)2, TCNQ, and RuTPP(Py)2 in DCE [62,70]. Despite the fact that large potential perturbations across the interface introduce interferences in kinetic analysis [101], these early estimations allowed some preliminary comparisons to established ET models in heterogeneous media. 

The electrolyte dropping electrode [63] method, introduced in 1976, and subsequently used in conjunction with the four-electrode potentiostat [64], is a hydrodynamic technique, offering controlled convective transport. In essence, this approach is identical to the dropping mercury electrode [65] however, the drop consists of a flowing electrolyte liquid phase which forms a polarized ITIES with an immiscible continuous (receptor) phase. In... 

The study of processes at ITIES and in membrane electrochemistry requires elimination of two ohmic potential differences, achieved with a four-electrode potentiostat, voltage-clamp (Fig. 5.17). 

Fig. 5.17 A four-electrode potentiostatic circuit (voltage clamp). and R2 are reference electrodes with Luggin capillaries (arrows) attached as close as possible to the membrane or ITIES (dashed line), B, and B2 are auxiliary electrodes, P and P2 potentio-stats, G programmed voltage generator and Z recorder... 

Srinivasan et al. [17] have described a four-electrode potentiostat system which is suited to maintaining a constant voltage drop across a membrane in a two-chamber diffusion cell. This system was evaluated in connection with trans-dermal iontophoretic drug delivery of polypeptides. 

Figure 1 shows a schematic diagram of the basic SECM instrument employing an amperometric microprobe. An UME tip is attached to a three-dimensional (3D) piezo positioner controlled by a computer, which is also used for data acquisition. A bipotentiostat (i.e., a four-electrode potentiostat) controls the potentials of the tip and/or the substrate versus the reference electrode and... 

Electron-conductor separating oil-water (ECSOW) system — For studying the -> electron transfer (ET) at the -> oil/water interface, the ECSOW system was devised, in which the oil and water phases are separated by an electron conductor (EC), as shown in the Figure. Specifically, the oil and water phases are linked by two metal (e.g., Pt) electrodes that are connected by an electric wire. The ET across the EC phase can be observed voltammetrically in a similar manner to the oil/water interface, i.e., by controlling the potential difference between the two phases using a four-electrode potentiostat (see -> four-electrode system). Because ion transfer (IT) across the EC phase cannot take place, the ECSOW system is useful for discrimination between ET and IT occurring at the oil/water interface. 

Four-electrode system — Figure. Electronic circuit of a four-electrode potentiostat (X, potential input Y, current output RE1 and RE2, reference electrodes CE1 and CE2, counter electrodes PF, positive feedback circuit for IR drop compensation)... 

To operate an RRDE, one needs a four-electrode potentiostat, often referred to also as a "double" potentiostat. Now, we started this book with a simple two-electrode cell, then went on to the three-electrode configuration. How does the fourth electrode fit into this scheme The... 

Conventional electrochemical experiments at the ITIES are based on a four-electrode configuration with a reference and an auxiliary electrodes positioned in each liquid phase (16,17). A four-electrode potentiostat is used to apply a voltage between the reference electrodes and to measure the current flowing between auxiliary electrodes. The interfacial charge transfer is assumed to be rate limiting, and the whole potential drop occurs mostly within a thin interfacial layer. However, the ITIES in such an experiment is not microscopically probed directly, and the nature of CT reaction (i.e., ET vs. IT) typically remains uncertain. 

Figure 2 is a diagram of a typical cell used in the studies of immiscible electrolytes. The investigated interface is formed in the narrow part of the cell. To allow positioning of the interface within the desired location, a screw-driven plunger that varies the volume of the bottom part can be used. In voltammetric studies, care must be taken to eliminate voltage drop within the solutions. Because the resistance of the solvents should be eliminated, a four-electrode potentiostat with a pair of reference and counter electrodes should be used. The measured or controlled potential difference resides between the tips of the two reference electrodes thus, the potential across the liquid-liquid (LL) interface is monitored. 

Impedance Measurements. These experiments were carried out with a frequency analyzer (Solartron 1250) and a four electrode potentiostat (Solartron 1286). The cell, similar to that in Figure 2, was equipped with a... 

The cell in Figure 2 is a typical apparatus used in LL studies. However, recently small interfaces, called here microinterfaces, were shown to have some experimental advantage. The purpose of this modification was to use the same advantage that the ultramicroelectrodes have. Ultramicroelectrodes help to overcome solution resistance difficulties that originate from a potential shift due to an uncompensated iR drop. As the interfacial area becomes smaller, the diffusion geometry becomes a spherically symmetric process, which means that the ratio of charge transport current versus solution resistance increases and, ultimately, renders the iR drop minimal. In ITIES studies, restriction of the interfacial area and use of a current amplifier for voltammetric studies is a viable alternative to a four-electrode potentiostat. 

In the case where the ionic species in the aqueous electrolyte are fairly hydrophilic and the organic phase features hydrophobic ions, the liquid]liquid junction behaves similarly to an ideally polarizable metal electrode. Under this condition, the Galvani potential difference can be effectively controlled by a four-electrode potentiostat [4,5]. A schematic representation of a typical electrochemical cell is shown in Fig. 1 [6]. Cyclic voltammo-grams illustrating the potential window for the water] 1,2-dichloroethane (DCE) interface for various electrolytes are also shown in Fig. 1. In the presence of bis(triphenylpho-sphoranylidene)ammonium hexafluorophosphate (BTPPA PFe) the supporting electrolyte in DCE, the potential window is limited to less than 200 mV due to the hydrophilicity of the anion. Wider polarizable potential ranges are obtained on replacing... 

Interest in liquid-liquid interfaces or liquid-liquid redox systems has been fuelled by their importance in biology. The complexity in electrochemical measurements on such systems led to the development of cyclic voltammetry employing a four-electrode potentiostat, although use of a film [117] or deposit of droplets [118] on the electrode surface has been suggested as a method for undertaking cyclic voltammetric studies of reactions at liquid-liquid interfaces with only a three-electrode configuration. Reviews on the reactivity of the triple-phase boundary formed when droplets are in contact with the electrode surface and immersed in aqueous media have appeared [119]. 

Fig. 1 Cyclic voltammogram at the water l,2-dichloroethane interface in the presence of ferricyanide/ferrocyanide coupie and ferrocene (Fc) at 10 mV. This experiment was performed with a four-electrode potentiostat, while the electrochemical cell can be represented as...

The electrical contacts to the ring and disc electrodes are made so that the two electrodes may be controlled independently and, indeed, to obtain the maximum information it is also necessary to have a four electrode potentiostat which will allow the ring and disc electrodes to be potentiostatted independently of each other. The types of experiment that can usefully be performed with a RRDE are illustrated by three examples all using controlled potential operation ... 

For all experiments at a large planar liquid/liquid interface (mm- to cm-sized) a four-electrode system is usually adopted. A four-electrode potentiostat must be used in order to compensate the iR drop from both phases. The ceU design nsed is shown in Figure 17.3.8. The interface is not fully planar because of curvature due to surface tension effects. The position of the interface is adjusted close to the organic and aqueous reference Luggin capillaries in order to minimize the influence of iR drop. Two counter electrodes made of platinum... 

Unlike the situation at a solid/electrolyte interface where a three-electrode system is used, four- and two-electrode systems have been widely employed for large and small liquid/ liquid interfaces. Most of the four-electrode potentiostats are homemade and only a few instruments with such functions have been commercialized (98). This is probably one of the reasons why this field has not been very popular since most electrochemical laboratories are equipped with a three-electrode potentiostat. In 1998, Anson et al. reported that charge transfer reactions at a liquid/liquid interface could also be studied by a three-electrode system with a thin-layer cell (99,1(X)). Later, Scholz et al. reported a three-phase junction setup (101, 102). Shao et al. supported a small droplet of aqueous solution (pL) containing a certain concentration ratio of redox couples on a Pt surface and demonstrated that charge transfer could be studied by a three-electrode setup (103). Girault et al. extended this to a supported small droplet of aqueous (organic) phase on the surface of... 

The defined potential window can be used to investigate ion and electron transfer processes as discussed in Section 17.3 in Chapter 17 of this handbook where the potential of the ion/electron transfer process must fall within the available window. Experiments can be performed with a two-electrode configuration where currents are sufficiently small, but more generally a four-electrode potentiostat (two reference and two counter electrodes) is required. The experimental procedures required for this type of experiment are discussed in more detail in Section 17.3 in Chapter 17 of this handbook. 

Fig. 3. Block scheme of the four-electrode potentiostat with a positive feedback for the ohmic drop compensation [30]...

Using a drop time method for the determination of interfacial tension and a four-electrode potentiostat to polarize the interface, Kakiuchi and Senda measured electrocapillary curves for ideally polarized systems, in particular for the interface between an aqueous solution of lithium chloride and a solution in nitrobenzene of TBATPB. They showed that the surface charge density, Q, obtained by differentiation of the electrocapillary curve was equal to that calculated from the integration of the corresponding differential capacity versus potential curves. This demonstrated the validity of the Lippmann equation for the polarized ITIES ... 

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