Mercury Pool Electrode

The other group comprises silver-silver halide electrodes, mercury pools, metal-metal-ion electrodes, and others normally prepared In the solvent used for the compound being studied (and often, indeed, employed as internal "reference" electrodes). For such an electrode, the abbreviation alone signifies that the solvent was the same throughout the cell, while the symbol "(w)" for ("water") following the abbreviation signifies that the reference electrode was prepared with water and used as an external reference electrode. 

The mercury-pool electrode. Mercury pools of sufficient diameter to approach a planar configuration obey the equations derived for linear diffusion to a planar electrode. This has certain theoretical advantages because of the large number of equations that have been derived for the planar electrode geometry, especially in terms of constant-current chronopotentiometry and linear-potential sweep chronoamperometry. 

TABLE 7. Potentiostatic electrolyses of some p-toluenesulphonamides in DMF/Bu4NI 0.15 M. Working electrode mercury pool of 10 cm2 area. Reference electrode Ag/Agl/I 0.1 M2 9... 

Besides these usual reference electrodes, mercury pool electrodes and pseudo- or quasi-reference electrodes of platinum or silver have often been used in nonaqueous solutions. Though these electrodes can have potentials approximately constant under appropriate conditions, the potentials are usually not defined. Thus, in this chapter, the use of these electrodes in nonaqueous solutions is not dealt with in a separate section. But see Chap. 14 for the problem of pseudo-reference electrodes. 

Dropping mercury electrode Standard calomel electrode Rotating platinum electrode Mercury pool electrode Rotating pyrolytic graphite electrode Hanging mercury drop Millimoles... 

The electroreductive cyclization of the furanone 118 (R = -(CH2)4CH=CH— COOMe Scheme 36) using a mercury pool cathode, a platinum anode, a saturated calomel reference electrode, and a degassed solution of dry CH3CN containing -Bu4NBr as the electrolyte, gave the spirocyclic lactones 119 and 120 in a ratio 1.0 1.1 (Scheme 37)(91T383). 

Before considering the role of the electrode material in detail, there is one further factor which should be pointed out. The product of an electrode process may be dependent on the timescale of the contact between the electroactive species and the electrode surface, particularly when a chemical reaction is sandwiched between two electron transfers in the overall process. This was first realized when it was found that ir E curves and reaction products at a dropping mercury electrode were not always the same as those at a mercury pool electrode (Zuman, 1967a). For example, the reduction of p-diacetylbenzene at a mercury pool was found to be a four-electron process, giving rise to the dialcohol, while at a dropping mercury electrode the product was formed by a two-electron process where only one keto group was reduced (Kargin et al., 1966). These facts were interpreted in terms of the mechanism... 

Most electrodes are stationary, such as a mercury pool electrode (SMDE) or the HMDE others are non-stationary, such as the DME, MFE, MTFE, RDE and RRDE. 

In many instances electrogravimetry must be preceded by a separation between metals suitably this can be an electroseparation by means of constant-current electrolysis as previously described, but more attractively an electroseparation by means of controlled-potential electrolysis at a mercury pool or sometimes at an amalgamated Pt or brass gauze electrode. In this way one can either concentrate the metal of interest on the Hg or remove other metals from the solution alternatively, it can be a rougher separation, i.e., the concentration of a group of metals such as Fe, Ni, Co, Cu, Zn and Cd on the Hg whilst other metals such as alkali and alkaline earth metals, Be, Al, Ti and Zr remain in solution151. In all these procedures specific separation effects can be... 

Potentiometric EDTA titrations are best carried out with a mercury pool electrode (Figure 5.6) or a gold amalgam electrode. When this electrode dips into a solution containing the analyte together with a small amount of added Hg-EDTA complex, three interdependent reactions occur. For example, at pH = 8 the half cell reaction (a) which determines the electrode potential is related to the solution equilibrium by (b) and (c). 

The magnesium will be liberated quantitatively and may then be titrated with a standard EDTA solution. Where mixtures of metal ions are analysed, the masking procedures already discussed can be utilized or the pH effect exploited. A mixture containing bismuth, cadmium and calcium might be analysed by first titrating the bismuth at pH = 1-2 followed by the titration of cadmium at an adjusted pH = 4 and finally calcium at pH = 8. Titrations of this complexity would be most conveniently carried out potentiometrically using the mercury pool electrode. 

Analyte solution Dropping-mercury working electrode Mercury drop Pool of used mercury... 

A counter electrode of constant potential is obtained making use of a half-cell system in which the components are present in concentrations so high as to be appreciably unaffected by a flow of current through it. The saturated calomel electrode (SCE) is the most common example of such an electrode. As shown in Figure 7, it is comprised of a mercury pool in contact with solid mercury(I) chloride and potassium chloride that lie at the bottom of the KC1 saturated solution. The aqueous solution is thus saturated with Hgl+, K+ and Cl- ions, the concentrations of which are governed by the solubility of the respective salts. 

The working electrode generally consists of a cylindrical platinum gauze or a mercury pool thereby offering the largest surface area possible to the redox process. 

The auxiliary electrode, which is normally a mercury pool, must be positioned in a compartment separate from the working electrode. Such a separation compromises the desired symmetric disposition of the electrodes. Normally, the compartments of a macroelectrolysis cell are separated by sintered glass frits, such that the catholyte and the anolyte are not mixed. In fact, if the working electrode is involved, for example, in a cathodic process, the auxiliary electrode will act as an anode. This implies that the auxiliary electrode will produce oxidized material (by anodic decomposition of the solvent itself, of the supporting electrolyte, of mercury-contaminated products or of electroactive residues diffused at the auxiliary electrode) that may subsequently be reduced at the working electrode, contaminating and falsifying the primary process. 

Figure 7 shows a typical electrolysis cell with a platinum gauze working electrode. Eventually the platinum gauze can be replaced by a mercury pool working electrode. The volume of solution contained in such a cell is about 30-50 ml. 

Analogously, l,2-dicyano-l,2-diphenylethylene, which is free from steric strains, and its strained isomer l,l-dicyano-2,2-diphenylethylene are reduced at practically the same potentials (Ioffe et al. 1971, Todres and Bespalov 1972). In DMF, with the support of Et4NI, the reversible two-step one-electron reductions are characterized by the following potentials (mercury pool as a reference electrode) -0.48 and -0.98 V for 1,2-dicyanoethylene and -0.50 and -1.07 V for 1,1-dicyano isomer. Thus, electrochemical reduction does not fix the difference in isomer structures. 

Voltammetric peaks, especially those of UDP and UTP, were sharper than those of the third compound, as they were adsorbed at the electrode surface. Coulometric studies (at — 1.0 V, electroreduction) at mercury pool electrode (also glassy carbon) and analysis of the obtained products have shown that for all three compounds, the two-electron reductive cleavage of the carbon-mercury bond occurs. The electrode... 

Voltammetry is a term used to include all the methods that measure current-potential curves (voltammograms) at small indicator electrodes other than the DME [6], There are various types of voltammetric indicator electrodes, but disk electrodes, as in Fig. 5.17, are popular. The materials used for disk electrodes are platinum, gold, graphite, glassy carbon (GC), boron-doped diamond8, carbon paste, etc. and they can be modified in various ways. For electrode materials other than mercury, the potential windows are much wider on the positive side than for mercury. However, electrodes of stationary mercury-drop, mercury-film, and mercury-pool are also... 

Figure 9.3 Stationary solution voltammetry cells, (a) Platinum wire loop auxiliary electrode, (b) reference electrode or reference electrode probe tip, (c) carbon paste working electrode, (d) graphite auxiliary electrode, (e) dropping mercury electrode, (0 platinum wire contact to mercury pool working electrode, (g) nitrogen gas inlet tube, (h) magnetic stirrer, (i) mercury pool working electrode, (j) glass frit isolation barrier.

The auxiliary electrode shown in Figure 9.3A and C is a platinum wire loop concentric with the working electrode. A platinum wire or foil, a mercury pool, and carbon can also be used for this purpose. The graphite rods used for emission spectroscopy are useful as auxiliary electrodes. For optimum performance at short times, the position of the reference probe might have to be changed from that shown when using different auxiliary electrodes. 

Figure 9.5 Controlled-potential coulometry cell with a mercury pool working electrode. a, Platinum wire contact to mercury pool working electrode b, mercury pool working electrode c, reference electrode d, auxiliary electrode e, porous Vycor f, sample solution g, inert gas inlet h, stirrer i, reference electrode salt bridge j, clean mercury k, waste. [From Ref. 11, adapted with permission.]...

Figure 9.11 Spectroelectrochemical cells using a bifurcated optical fiber probe (a) mercury pool working electrode (b) reflective metal working electrode. [From Ref. 67.)...

Various types of mercury pool electrodes have been used [56-63]. These electrodes are prepared by placing a suitable amount of mercury in containers... 

An assay for the determination of vitamin K3 (2-methyl-1,4-naphthoquinone) by a combination of both constant-potential and constant-current cou-lometry has been reported [13]. The assay requires the two-electron reduction of the compound to the corresponding hydroquinone at a mercury pool electrode (E = -0.60 V vs. SCE) in acetate buffer, pH 5.9, followed by the coulometric titration of the reduction product with electrogenerated Ce(VI). This method is preferable to the standard method requiring preliminary reduction to the hydroquinone by zinc dust in acid medium, followed by titration with standard Ce(IV) solution. It is capable of low-level determination (1-2 mg) of this vitamin in pharmaceuticals, biological fluids, and foods. 

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