Electrodes solution pool

The mercury-pool electrode is especially attractive because of its renewable surface and because it allows the use of a stirring bar to vigorously stir the electrode-solution interface. Its major limitation relative to a platinum electrode is its high mass and the awkwardness of rinsing and of weighing a liquid electrode relative to a solid electrode. 

Figure 11.2.3 Potential distribution in mV at surface of ring-shaped mercury pool electrode using an unsymmetrical auxiliary electrode. Solution 0.5 M H2SO4 total cell current 40 mA pool 1.5 in. o.d., 1.0 in. i.d. Small circle shows position of the auxiliary electrode s fritted-glass separator, situated 4 mm above the pool surface. [Reprinted with permission from G. L. Booman and W. B. Holbrook, Anal. Chem., 35, 1793 (1963). Copyright 1963, American Chemical Society.]...

The Calomel Electrode. A pool of mercury covered with a paste of calomel (mercurous chloride) and a solution of KCl. 

In 1989, a related technology of cleanup and barrier combined was described by Lageman, Pool, and Seffinga (Fig. 16.4). A line of electrodes placed across a plume of pollution was used to clean up the pollutant as it moved toward the electrokinetic fence. The electrodes are operated submerged in an electrode solution, which can be removed for later treatment. Full-scale trials report successful cleanup, approaching 80% (Lageman, 1993). Further details are given in Chapter 19. 

There is a practical limit to the number of samples which can be measured before the electrode solutions are replaced. The limit of detection in our hands is 2-3 pg of water, i.e., 0.02% in a 1 g sample. In theory, it is possible to pool multiple samples and perform analysis on the pool but we have not needed to resort to this in practice. Moisture determination by... 

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. 

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). 

A value close to 4.8 V has been obtained in four different laboratories using quite different approaches (solid metal/solution Ay, 44 emersed electrodes,40,47 work function changes48), and is apparently supported by indirect estimates of electronic energy levels. The consistency of results around 4.8 V suggests that the value of 4.44 V is probably due to the value of 0 not reflecting the actual state of an Hg jet or pool. According to some authors,44 the actual value of 0 for Hg in the stream should be 4.8 V in that the metal surface would be oxidized. 

In the classical version one uses a two-electrode cell with DME and a mercury AE (the pool) at the bottom of the cell (see Fig. 23.2). The latter, which has a large surface area, is practically not polarized. The current at the DME is low and causes no marked ohmic potential drop in the solution and no marked polarization of the AE. Hence, to change the DME potential, it will suffice to vary the external voltage applied to the cell. During the measurements, 7 vs. % rather than 7 vs. E curves are recorded. 

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... 

Tanaka et al.150 reported that in the presence of [Ru(bpy)2-C02](PF6)2, controlled-potential electrolysis of a C02-saturated H20/DMF (9/1 v/v) solution at -1.5 V versus SCE using an Hg pool electrode gave CO and H2 in acidic conditions, and formic acid and CO as well as H2 in alkaline conditions. 

The [MoOI(prP4)] precursor 3 was finally converted to the corresponding dinitrogen complex by electrochemical reduction with a Hg pool electrode in the presence of dinitrogen and phenol. The latter reagent was added as a weak acid to induce protonation of the oxo group and subsequent elimination as water. The blue solution of the Mo oxo complex thereby turned... 

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. 

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. 

Swartz and Stenzel (1984) proposed an approach to widen the applicability of the cathode initiation of the nucleophilic substitution, by using a catalyst to facilitate one-electron transfer. Thus, in the presence of PhCN, the cathode-initiated reaction between PhBr and Bu4NSPh leads to diphe-nydisulfide in such a manner that the yield increases from 10 to 70%. Benzonitrile captures an electron and diffuses into the pool where it meets bromobenzene. The latter is converted into the anion-radical. The next reaction consists of the generation of the phenyl radical, with the elimination of the bromide ion. Since generation of the phenyl radical takes place far from the electrode, this radical is attacked with the anion of thiophenol faster than it is reduced to the phenyl anion. As a result, instead of debromination, substitution develops in its chain variant. In other words, the problem is to choose a catalyst such that it would be reduced more easily than a substrate. Of course, the catalyst anion-radical should not decay spontaneously in a solution. 

Castner turned his interest to gold extraction, which required high-quality sodium hydroxide. Castner developed a three-chambered electrolytic cell. The two end chambers contained brine and graphite electrodes. The middle chamber held water. The cells were separated excepted for a small opening on the bottom, which contained a pool of mercury that served as the cell s cathode. When current flowed through the cell and the cell was rocked, sodium reduced from the brine came into contact with water in the middle cell to produce a sodium hydroxide solution. As Castner built his mercury cell, Kellner was working on a similar design. Rather than compete with each other, Castner and Kellner joined forces to establish the Castner-Kellner Alkali Company to produce sodium hydroxide, which competed with soda ash and potash as an industrial base, and chlorine, which was used primarily to make bleach. 

The most-used secondary standard is the calomel electrode, shown in Fig. 7.40. It consists basically of a pool of mercuiy on top of which is spread a thin layer of Hg2Cl2 (calomel), a substance only slightly soluble in water. A KC1 solution (either at the unit activity with respect to Cl- or saturated) is in contact with the calomel Hg system and a Pt wire connects the electrode to the rest of the circuit. 

It is easy to figure out why this is. The theory of ellipsometry assumes that the surface is atomically flat. It is possible to model roughness as a series of declivities in the surface. These are taken as being full of solution. Thus, the ellipsometer sees pools of solution where it assumes the electrode surface should be. Especially in the determination of submonolayers, the result can contain significant errors in n and K that have been calculated on the assumption of a completely smooth surface (Brusic and Cahan, 1969). 

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.

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.]...

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