Electrolysis aqueous

In the electrolysis of a molten salt, the possible half-reactions are usually limited to those involving ions from the salt. When you electrolyze an aqueous solution of an ionic compound, however, you must consider the possibility that water is involved at one or both electrodes. Let us look at the possible half-reactions involving water. 

Water can be reduced or oxidized in half-reactions, and you can easily obtain these half-reactions. To do this, first note the species likely to be involved. In addition to H2O, these are H2, O2, H, and OH . Only H2 and O2 involve a change of oxidation state. Hydrogen in H2 has a lower oxidation state (0) than in H2O (+1), whereas the oxygen in O2 has a higher oxidation state (0) than in H2O (—2). Thae-fore, you can reduce water to H2 or oxidize it to O2. 

Consider the reduction half-reaction. It must involve the reduction of H2O to H2. You need to balance the half-reaction by putting an oxygen-contaiiiing species on the right. The only species in which there are no changes in oxidation states from those in H2O is OH. The balanced half-reaction is 

You can obtain the oxidation half-reaction for water in a similar way. It involves the oxidation of H2O to O2. You need to put a hydrogen-containing species on the right side of the equation to balance it. The only species in which there is no change in oxidation state is H. The balanced half-reaction is 

Now let us consider the electrolysis of different aqueous solutions and try to decide what the half-reactions might be. Once you have the half-reactions, you can obtain the overall chemical change due to the electrolysis. 

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Aqueous electrolysis using matte as soluble anode... 

Aqueous electrolysis has been used by Margerum and co-workers to oxidize oligopeptide complexes of nickel(II).3047,3057-3059 They used a flow electrolysis system with a graphite powdered electrode packed in a porous glass column externally wrapped with a platinum wire electrode. 

Despite the superior chemical stability of the diamond layer, these electrodes face two challenges. The brittle nature of crystalline silicon limits the mechanical stability of the support, causing eventual loss of the BDD electrode by friction. Furthermore, any contact with the organic electrolyte has to be strictly eliminated, since silicon or niobium support is very prone to corrosion if non-aqueous electrolysis conditions are applied. In contrast to aqueous media, no passivation occurs by oxide... 

H. Kellogg, Anode Effect in Aqueous Electrolysis, Journal of the Electrochemical Society, vol 97 N°4 (1950)... 

H.H. Kellogg Anode effect in aqueous electrolysis. Journal of the Electrochemical Society 97 (1950), p. 133. 

Iodonium and sulfonium salts undergo irreversible one electron electrochemical or chemical reduction [51,52], Reduction of diaryliodonium salts in water exhibit two to four waves in the polarogram depending upon the concentration of iodonium salt, type of electrode, nature and concentration of the supporting electrolyte, and the maximum suppressor [51,53-56]. Reductive electrolysis of diphenyliodonium salts in water at mercury yields mixtures of diphenylmercury, iodobenzene, and benzene, depending upon the potential used during the electrolysis [51,53-55]. Reduction at platinum or glassy carbon electrodes occurs without appearance of the first wave (see below) [54,56,57]. The mechanism shown in Scheme 1 was proposed for aqueous electrolysis of diphenyliodonium salts [51a] ... 

Unique aqueous electrolysis media have been developed and highly efficient electrochemical oxidations of alcohols were accomplished using these systems. Tanaka s group developed the anodic oxidation of alcohols mediated by N-oxyl in an aqueous silica gel disperse system (Figure 12.6) [18]. The system could be successfully applied to the kinetic resolution of sec-alcohols and the enantioselective oxidation of meso-l,4-diols, affording optically active y-lactones (Scheme 12.3). 

What product forms at each electrode in the aqueous electrolysis of the following salts (a) LiF (b) SnS04 ... 

The common sources of indium are the minerals dark sphalerite, christophite, and marmatite. Indium is also found in small amounts in manganese, tungsten, zinc, and tin ores. Rarely found as a free element, indium is commonly associated with gallium in tin and zinc ores. The main commercial source for indium is from zinc smelter flue dusts (Smith etal. 1977). Enrichment of indium from zinc residues is performed by acid leaching followed by chemical separation processes. Aqueous electrolysis of indium salts yields a final metal of 99.9% purity. Canada has the greatest resources of indium with approximately 27% of the world s reserves (based on estimated indium content of zinc reserves) and the United States has about 12% of the world reserves (Brown 2000). In recent years, there have been major improvements in the recovery, refining and recycling of... 

The same principles apply to aU other aqueous electrolysis reactions. The two reactions involving the oxidation and reduction of water will always be competitors, and the oxidation or reduction reaction taking place will be that requiring the lowest potential. For example, consider the reduction of brine (concentrated salt solution) (Figure 9.24). The solution contains Na (aq) and Cl (aq). The two competing cathode reactions are ... 

This section discusses these routes in the order shown. Aqueous electrolysis of HCl is a commercial process that has been practiced widely. The anhydrous route is developmental but has the potential for large reductions in operating cost. The incentive for indirect electrolysis is the reduction in cathode voltage when reducing certain metals rather than hydrogen ions. The last-named process depends on the added value for its commercial success. The chief product is a metal, with chlorine as a useful byproduct. Sodium and magnesium are the examples covered in the text. 

PROCESSES BASED ON AQUEOUS ELECTROLYSIS Chloride systems... 

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