Electrolytic cells undivided

Different results may also be obtained depending on whether the electrolytic cell is divided (by a diaphragm separating the cathode and anode spaces) or undivided [729]. 

Cathodic reduction is also used to purify some metals, such as copper. Slabs of impure copper serve as the anode, while a pure copper sheet serves as the cathode in an undivided electrolytic cell. The electrolytic bath is copper(II) sulfate. During electrolysis, Cu2+ ions leave the anode and plate on the cathode. Impurity metals more reactive than copper are oxidized and stay in solution. Less reactive metals collect at the bottom of the cell. After about a month, the enlarged copper cathodes are removed (Ebbing and Gammon, 2005). Metals can also be oxidized electrolytically at the anode (anodized). It is even possible to further oxidize some metals in a low oxidation state to a higher oxidation state. 

Fig. 19.5 Scheme of an undivided electrolytic cell fed with oxygen to electrogenerate H202 for the treatment of wastewaters [adapted from Panizza et al. (2001)]... 

In an undivided electrolytic cell, the electro-Fenton process leads to the destruction of organics contained in wastewaters by simultaneous oxidation with OH formed at the anode surface from reaction (19.9) and in the medium from Fenton s reaction (19.12). Parallel slower reaction of pollutants with weaker oxidants such as H202, H02, S20g2-, and O3 formed from reactions (19.1), (19.4), (19.10), and (19.11), respectively, is also possible. In addition, final carboxylic acids can form complexes with iron ions that are difficult to be oxidized by OH. We will see the notable influence of the anode material (Pt or BDD) on the degradation of these compounds in further sections. 

Since oxygen is one of the products of electrolysis, if the electrolytic cell is undivided, reoxidation of U " ion takes place which hampers the production of uranyl with higher concentration. Cation-selective membrane spacers are effective in improving the production of uranyl nitrate solution. 

In an undivided electrolytic cell these two products will react quickly to form NaOCl. However, if an ion-permeable barrier is positioned between the electro-... 

Electrolytic cells are also used in a variety of devices - and appliances " to produce dilute solutions of electrolyzed water for cleaning and sanitization. Miniature, battery-powered cells are used to generate hypochlorite in handheld sprayers and small portable water disinfection devices. " " The brine may be saturated or have 1-5 g L of salt. An acidic solution of chlorine, hypochlorous acid, and possibly chlorine dioxide is obtained from the anode of a divided cell. It typically has 10-100 mg L of available chlorine and pH values of 2-4. Its stability is poor, and the volatile oxidants are rapidly lost from open solutions. A neutral solution of hypochlorous acid and sodium hypochlorite is dispensed from undivided cells or by combining effluents from the anode and cathode. It typically has 80-100 mg L of available chlorine and pH 5-8. Devices to make 500-1000 mg L" of available chlorine are also available. Neutral solutions made using larger versions of these devices are bottled and sold in some regions. 

Electroassisted Barbier Reactions 1985 and 1986. More recently a new and efficient method was reported for the electrocarboxylation of organic halides, using an undivided electrolytic cell and a sacrificial magnesium electrode [101]. 

Several configurations have been used in order to enhance the oxidation ability of the EF technology, with promising results for two- and three-electrode divided and undivided electrolytic cells. The efficiency of the process is always a function of temperature, pH, O2 feeding, stirring or liquid flow rate, electrolyte composition, applied potential or current, and catalysts and pollutant concentrations. 

Two-Dimensional Electrode Flow Cells. The simplest and least expensive cell design is the undivided parallel plate cell with electrolyte flow by some form of manifold. Electrical power is monopolar to the cell pack (72). An exploded view of the Foreman and Veatch cell is shown in Figure 7. Note that electrolyte flow is in series and that it is not easily adapted for divided cell operation. 

Asahi also reports an undivided cell process employing a lead alloy cathode, a nickel—steel anode, and an electrolyte composed of an emulsion of 20 wt % of an oil phase and 80 wt % of an aqueous phase (125). The aqueous phase is 10 wt % K HPO, 3 wt % K B O, and 2 wt % (C2H (C4H )2N)2HP04. The oil phase is about 28 wt % acrylonitrile and 50 wt % adiponitrile. The balance of the oil phase consists of by-products and water. The cell operates at a current density of 20 A/dm at 50°C. Circulated across the cathode surface at a superficial velocity of 1.5 m/s is the electrolyte. A 91% selectivity to adiponitrile is claimed at a current efficiency of 90%. The respective anode and cathode corrosion rates are about mg/(Ah). Asahi s improved EHD process is reported to have been commercialized in 1987. 

There have been a number of cell designs tested for this reaction. Undivided cells using sodium bromide electrolyte have been tried (see, for example. Ref. 29). These have had electrode shapes for in-ceU propylene absorption into the electrolyte. The chief advantages of the electrochemical route to propylene oxide are elimination of the need for chlorine and lime, as well as avoidance of calcium chloride disposal (see Calcium compounds, calcium CHLORIDE Lime and limestone). An indirect electrochemical approach meeting these same objectives employs the chlorine produced at the anode of a membrane cell for preparing the propylene chlorohydrin external to the electrolysis system. The caustic made at the cathode is used to convert the chlorohydrin to propylene oxide, reforming a NaCl solution which is recycled. Attractive economics are claimed for this combined chlor-alkali electrolysis and propylene oxide manufacture (135). 

When chlor-alkali electrolysis is conducted in an undivided cell with mild-steel cathode, the chlorine generated anodically will react with the alkali produced cathodically, and a solution of sodium hypochlorite NaClO is formed. Hypochlorite ions are readily oxidized at the anode to chlorate ions this is the basis for electrolytic chlorate production. Perchlorates can also be obtained electrochemically. 

Because of the low-cost construction and simple operation, an undivided cell is always desired but it cannot be realized in all cases. A precondition for electrolysis in an undivided cell is that disadvantageous reactions and reaction products at the counter electrode can be avoided, for example, by selection of the electrode material and/or of the electrolyte composition. 

For cases directly comparable to the cyclization originating from (27) above, the yields of the product were not as high. However, a related reaction used in the synthesis of an 11-substituted dibenzo[a,d]-cycloheptenimine derivative was very successful as shown in Scheme 11 (Eq. 2) [32]. In this reaction, a controlled potential electrolysis of (33) led to the formation of the tetracyclic (34) in an 85% isolated yield. The reaction was performed on a 1 g scale using an undivided cell, a graphite felt anode, a stainless steel cathode, a saturated calomel reference electrode, and a 1% NaBF4 in 70 30 THF/water electrolyte solution. The electrolysis was scaled up further with the use of a flow cell. In this experiment, 200 g of (33) were oxidized in order to afford a 75% isolated yield of (34). 

Finally, the oxidation of nitrate anions leads to nitrate radicals (NOs ) that add to olefins (Scheme 14) [37]. These oxidations were carried out at a platinum anode using constant current conditions, an undivided cell, a mixed MeCN H2O Et20 solvent system, and LiN03 as the electrolyte. The initial oxidation led to a nitrate product that was not stable and hence... 

A bromide was introduced in the reaction instead of a fluoride in performing the anodic oxidation of a-stannyl ethers in dibromomethane solvent with tetrabutyl-ammonium perchlorate as the electrolyte (Scheme 19) [28]. The bromide ion was generated by the reduction of the solvent at the cathode of an undivided cell. 

The significance of the supporting electrolyte cation depends crucially on whether a divided or an undivided cell is used. In a divided cell, the choice of cation is of minor importance but in an undivided cell the cathode process should not lead to formation of base and thereby to buffering of the solution. Metal cations such as Li+, Na+ or Mg + are often the choice since in aprotic solvents the metal cation may be the most easily reduced component. This has been observed as deposits of metal on the surface of the cathode arising from... 

The phenyl u-anion is strongly basic but may be obtained at relatively modest potentials by reductive cleavage of haloben-zenes when the right cathode material is chosen [63]. Typically, the reactions are carried out at constant current in undivided cells using Cd-coated Ni-cathodes, A1 or Mg sacrificial anodes, DMF (or MeCN for cyanomethylation) as the solvent and BU4NBF4 as the electrolyte. Under these conditions, iodobenzene is reduced at —1.6 V versus SCE and bromobenzene at —1.9 V, which may allow their use in situ [63, 91-93]. 

Steckhan and coworkers found that the indirect anodic oxidation of N-pro-tected dipeptide esters 56, in which the C-terminal amino acid is a-branched, can afford methyl imidazolidin-4-one-2-carboxylate 57 in 45-84% yields [86], This reaction can be performed at a Pt-anode by using Et4NCl as an electrolyte in the presence of 5% methanol in an undivided cell (Scheme 30). 

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