Sacrificial electrode

Fenoprofen, Mg is used as a sacrificial electrode and additionally a phase transfer catalyst is used (Anderson, 2000)... 

Group 14 metals can serve as sacrificial electrodes. Both anodic and cathodic reactions can be considered. Pb and Sn alkyls can be prepared by their use as a sacrificial metallic anode in a reaction with carbanions, for example in a Grignarcl reagent ... 

Electrochemical synthesis was utilized to prepare labeled compounds. Tetramethyllead labeled with 14C was prepared in a double compartment cell in DMF with NaClC>4, by electrolyzing 14CH3l on lead electrodes. The method is reported as superior to transmet-allation with methylmagnesium halide. It is also possible to incorporate lead isotopes. 2i°Pb2+ ions were deposited on a Cu foil and the latter was used as a sacrificial electrode in solutions of CH3I. The yield of labeled tetramethyllead was 85%65. Synthesis of 210Pb-labeled chlorotrimethylplumbane was also described66. 

Synthesis with sacrificial electrodes is employed as a direct method in several other preparations of organometallic compounds and complexes. 3-Hydroxy-2-methyl-4-pyrone derivatives of Sn 1 (and of Zn, Cu, In and Cd as well) were prepared using the metal as an anode. The low oxidation state Sn(II) compound is obtained by direct electrolysis134. 

This direct electrochemical synthesis has proved efficient in the preparation of several other complexes, among these the tin derivatives of 3-hydroxy-2-phenylflavone (2) and 2-ethoxyphenol (3), respectively. The use of sacrificial electrodes proved very efficient the produced complex precipitates during electrolysis and is easy to isolate135. 

Although it does not deal with the direct redox processes of sulfur organic compounds, the use of sacrificial electrodes, both cathodes and anodes producing... 

One of the most common sources of contamination is the electrolyte since impurities in it would diffuse to the electrode and adhere to it during the course of the experiment. Impurities in the electrolyte can be reduced substantially by careful purification of solvent and solute. Distillation or ultrafiltration purifies water, the most common solvent. Usually solute materials can be bought in a very high purity, and whenever this is not the case, they can be cleaned by standard procedures such as recrystallization or calcination. Electrolysis of the electrolyte is also a common practice. Here, two sacrificial electrodes are immersed in the electrolyte and a potential is applied between them for about 36 hr in such a way that impurities are oxidized or reduced on their surfaces—the electrodes act as a garbage disposal thus the name of sacrificial electrodes. 

Osasa K, Kawanami H, Tukamoto E, Sakai K. Performance of continuous electroflotation of an oil-in-water (O/W) emulsion using a packed-bed-type sacrificial electrode as an anode. Kagaku Kogaku Ronbunshu 1996 22 1450-1456. 

Osasa K, Miyazaki T, Sakai K. Performance test of a rectangular electroflotation cell with packed-bed-type sacrificial electrodes. Kagaku Kogaku Ronbunshu 1997 23 594-596. 

The good electron acceptor property of carbon dioxide can be used for the hydrodimerization to oxalic acid 4. Electrolysis is performed in an undivided cell using zinc as sacrificial electrode material. The reported electrolysis data are 6 mA/cm2 with 12 mM NBu4+BF4 in DMF. The current efficiency for the formation of oxalic acid is stated to be 60%. Unfortunately, no further details are currently available about this particular process (Lehmann and Dunach, 2009, personal communication). 

The characteristics of the electrooxidation of fluorosulfate anions in the electrolysis of a potassium fluorosulfate solution in fluorosulfonic acid have been investigated. The formation of oxide layers on platinum and the modification of glassy carbon with fluorosulfate groups during anodic polarization in fluorosulfonic acid are studied. The reactions of fiuoroolefin fluorosulfation are considered and a mechanism is suggested223. Trifluoromethylation of carbonyl compounds can be achieved using bromo-trifluoromethane and a sacrificial electrode in solvents such as DMF/pyridine, and DMF/TMEDA, as seen in equation 126224. 

An improvement in this technique of sacrificial electrodes (Se and Mg) for the formation of the Z2-2 ions was achieved by using an undivided cell and by addition of fluoride ions to avoid the precipitation of the Se2Mg304. Thus, the reaction of 2-chloroquinoline with Se2-2 gave 79% yield of 248a. Following this methodology the diselenides 250 were prepared by reaction with aryl halides 249 (equation 162)303. 

In a bipolar arrangement, the sacrificial electrodes are placed between the two parallel electrodes without any electrical connection. The two monopolar electrodes are connected to the electric power source with no interconnections between the sacrificial electrodes. This cell arrangement provides a simple setup, which facilitates easy maintenance. When an electric current is passed through the two electrodes, the neutral sides of the conductive plate will be transformed to charged sides, which have opposite charge compared with the parallel side beside it. The sacrificial electrodes are known as bipolar electrodes. It has been reported that EC cell with monopolar electrodes in series connection was more effective where aluminum electrodes were used as sacrificial and iron was used as anode and cathode. And, electrocoagulation with Fe/Al (anode/cathode) was more effective for the treatment process than Fe/Fe electrode pair (Modirshahla et al. 2007). 

The disadvantages of EC are as follows. (1) The sacrificial electrodes are dissolved into wastewater as a result of oxidation, and need to be regularly replaced. (2) The passivation of the electrodes over time has limited its implementation. (3) The use of electricity may be expensive in many places. (4) High conductivity of the wastewater suspension is required (Yildiz et al. 2007). 

Magnesiiun Dolomite ([Mg,Ca]C03) Electrolysis of fused MgCb (obtained from brines) Lightweight structures (alloys) Sacrificial electrodes Flares, pyrotechnics, incendiary bombs... 

A sacrificial electrode has a finite operational lifetime, or capacity, defined as the amount of charge that can be passed before the reactants are effectively depleted. The capacity of an electrode can be empirically determined, or can be calculated from the following equation ... 

In nonaqueous media, a sacrificial electrode or hydrogen gas diffusion electrode may be advantageous (discussed earlier). 

However, sulfur is poorly soluble in common solvents at room temperature. Consequently, Berge [282] introduced sulfur in an electrode together with powdered graphite to assure sufficient electric conductivity (1 part carbon for 4 parts sulfur). Later on [283], improvement of the building of such a sacrificial electrode was proposed A mixture of carbon - - sulfur (1/2, w/w) was introduced in a test tube and heated at 130 C in an oven. After melting, a carbon rod or stainless-steel bar is introduced in the melt to assure electric contact. After return to room temperature, the test tube is broken to release the compact electrode. Similarly, by using this procedure, small-area electrodes could be built up by using Pasteur pipette for voltammetric analysis. 

SNPE, France (electrolyses with sacrificial electrodes)... 

Sacrificial anodes (Al,7,8 Mg9,10) have been proposed for electro-organic processes as they allow one to use simple undivided electrochemical cells. In coordination electrosynthesis, they may provide a means to obtain complexes containing the metal of the sacrificial electrode (see Section 1.44.3.2). 

The cylindrical cell is a more convenient cell because the potential or current distribution is nearly perfect. It can be used undivided or divided with a cylindrical diaphragm. This cell type is used with a sacrificial electrode cell, generally with the undivided configuration. 

Electrolyses at sacrificial electrodes allow direct synthesis of metal complexes from bare metal electrodes. Systematic studies have been reported and periodically reviewed.22-24 The sacrificial electrode may either be a cathode (Pb, Sn, Hg), or an anode (metals) which is the most usual configuration. The experiments are generally carried out under galvanostatic conditions (10-30 mA cm-2) and so a reference electrode and a potentiostat are not required. Different types of products have been obtained from the simplest complexes to clusters. 

Electrochemical processes are often touted as being green chemistry because electricity is considered inexpensive, and toxic metal reagents are usually avoided. Electrochemical processes have produced tons of bulk chemicals [37], the best-known of which may be adiponitrile from reductive dimerization of acrylonitrile (Figure 13.17) [38]. An electrochemical synthesis to manufacture fenoprofen is shown in Figure 13.18, with the magnesium provided as a sacrificial electrode [39], Flow cell technology has been used for these operations on a commercial basis. 

Insonation of an electrosynthetic reaction can produce altered product ratios, greater efficiencies, lessened cell power requirements, and a diminution of detrimental electrode fouling. In electrodeposition, ultrasound alters the properties of the product coating, be it a metal deposited, a semiconductor, a polymer, or some other electrogenerated material. Sonication also affects corrosion and electrode dissolution, and is useful, for example, in systems employing sacrificial electrodes. 

Sacrificial anodes for galvanic cathodic protection are commonly made from magnesium, which from Table 7-4 one can see is the least noble (most anodic) of all the metals listed. Zinc is also utilized, but it has a lower oxidation potential. Good electrical connections (by soldering or brazing) must be made between the sacrificial electrode and the structure to be protected. 

Alternatively, iron-rich sacrificial electrodes, which dissolve under acidic conditions generated at the anode by the application of electric field, may be used. The dissolved iron, in cationic form, migrates toward the cathode and then precipitates as iron-rich mineral phases (ferric iron oxyhydroxides, hematite, goethite, magnetite, and ZVl) near the cathode due to high-pH conditions. Contaminants such as Cr(Vl) can react with this iron and reduce into Cr(III). Cr(VI) transport may be limited by high sorption under low-pH conditions therefore, alkaline solution may be injected from the anode to increase the soil pH, and thereby reduce sorption and increase transport of Cr(Vl) to react with iron. 

Faulkner, Hopkinson, and Cundy, 2005). Because of the adverse effect of OH on soil remediation, due to the immobilization of many metal ions by precipitation in alkalinized soils, and the reduced efficiency of electrokinetic remediation when sacrificial iron-rich electrodes are employed (e.g. Leinz, Hoover, and Meier, 1998), noncorrosive electrodes and techniques to minimize soil alkalinization are generally employed for electrokinetic remediation (e.g. Rohrs, Ludwig, and Rahner, 2002 Virkutyte, Sillanpaa, and Latostenmaa, 2002). However, low adsorption of Cr(VI) in soils occurs in alkaline conditions, whereas high adsorption of Cr(VI) is favored in acidic conditions (Reddy et al, 1997). Furthermore, the reduction of Cr(VI) to Cr(III) by the delivery of iron (Fe°, Fe " ) is fairly well documented (Rai, Sass, and Moore, 1987 Eary and Rai, 1991 Haran et aL, 1995 Powell et aL, 1995 Pamukcu, Weeks, and Wittle, 1997 Batchelor et al., 1998 Reddy et /., 2003). Accordingly,under an applied direct current (DC) electric field, stabilization of Cr(VI)-contaminated soils may potentially be achieved where oxidative dissolution of iron-rich anodic electrodes provides Fe(j,q) to react with the anode-bound migration of Cr(VI). Hence, the use of iron-rich sacrificial electrodes and soil alkalinization may find application in the electrokinetic stabilization of Cr(VI)-contaminated soils. This concept is explained in this chapter based on the results of laboratory stabilization experiments on three Cr(VI)-impacted soils taken from three sites within the UK. 

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