Electrolysis in aqueous solutions

Electrolysis in aqueous solution (principally copper and zinc)... 

Controlled potential electrolysis in aqueous solution was carried out with a carbon-cloth working electrode and in the presence of a Ag/AgCl double junction reference electrode, with constant nitrogen purge, as described by us previously." The electrolysis was stopped when the current dropped to less than 1% of the original value. 

A black phosphorus electrode [7, 30] has a low hydrogen overvoltage, and electrolysis in aqueous solution is accompanied by reduction of the cathode material with formation of phosphane. The largest PH3 yield of 27.5% (calculated from the loss of the electrode weight) at 12.5% current efficiency was obtained in 1M K2HPO4 (pH 7.5) at a current density of 0.07 to 0.09 A/cm2 and at 20 C. Altering the pH to either the acidic or basic side reduces the PH3 yield. An increase in catholyte temperature from 20 to 55 0 lowers the PH3 yield by a factor of 5 to 6. Decreasing the current density also lowers the current efficiency [30]. 

Copper and zinc are the principal metals extracted by electrolysis in aqueous solution the total world production of both approaches 10 ton yr although the electrolytic route accounts for only 10% of the copper and 50% of the zinc produced. Moreover the large electrolytic plants are limited to sites in Africa, Australia and Canada where hydroelectric power is available close to the mines. Cobalt, nickel, chromium, manganese, cadmium, gallium, thallium, indium, silver and gold have also been reported to have been extracted by a hydrometallurgical process but, since these metals are only produced in a low tonnage, the electrolytic processes are on a small scale. 

In contrast to the zeroth order removal kinetics in ethanol, more con lex kinetic behavior was observed for triclosan electrolysis in aqueous solutions. Figure 6 conqiares triclosan removal rates at current densities of 5 and 15... 

Many of the commercial uses of electrolysis involve aqueous solutions. We will look at electrolysis in aqueous solution in the next section. 

Colourless crystals m.p. I25°C, soluble in water and alcohol. In aqueous solution forms equilibrium with its lactones. Gluconic acid is made by the oxidation of glucose by halogens, by electrolysis, by various moulds or by bacteria of the Acetobacter groups. 

The reactions with water are summarised in Table 6.3. Since the metals are powerful reducing agents (p. 98) they cannot be prepared in aqueous solution electrolysis of the fused anhydrous halides is usually employed using a graphite anode. 

Electrolysis of Aqueous Solutions. The electrolytic process for manganese metal, pioneered by the U.S. Bureau of Mines, is used in the Repubhc of South Africa, the United States, Japan, and beginning in 1989, Bra2il, in decreasing order of production capacity. Electrolytic manganese metal is also produced in China and Georgia. 

Ttinitroparaffins can be prepared from 1,1-dinitroparaffins by electrolytic nitration, ie, electrolysis in aqueous caustic sodium nitrate solution (57). Secondary nitroparaffins dimerize on electrolytic oxidation (58) for example, 2-nitropropane yields 2,3-dimethyl-2,3-dinitrobutane, as well as some 2,2-dinitropropane. Addition of sodium nitrate to the anolyte favors formation of the former. The oxidation of salts of i7k-2-nitropropane with either cationic or anionic oxidants generally gives both 2,2-dinitropropane and acetone (59) with ammonium peroxysulfate, for example, these products are formed in 53 and 14% yields, respectively. Ozone oxidation of nitroso groups gives nitro compounds 2-nitroso-2-nitropropane [5275-46-7] (propylpseudonitrole), for example, yields 2,2-dinitropropane (60). 

Electrolysis. Electrowinning of zirconium has long been considered as an alternative to the KroU process, and at one time zirconium was produced electrolyticaHy in a prototype production cell (70). Electrolysis of an aH-chloride molten-salt system is inefficient because of the stabiUty of lower chlorides in these melts. The presence of fluoride salts in the melt increases the stabiUty of in solution, decreasing the concentration of lower valence zirconium ions, and results in much higher current efficiencies. The chloride—electrolyte systems and electrolysis approaches are reviewed in References 71 and 72. The recovery of zirconium metal by electrolysis of aqueous solutions in not thermodynamically feasible, although efforts in this direction persist. 

Magnesium is reduced from a mixture of magnesium, calcium, and sodium chlorides. Electrolysis from aqueous solution is also possible zinc, copper, and manganese dissolved as sulfates in water can be reduced electrolytically from aqueous solution. 

Pulsed current experiments of aqueous acetate solutions indicate that at least in aqueous solution a platinum oxide layer seems to be prerequisite for the da arboxy-lation to occur. Only at longer pulse durations (> 10 s) is ethane produced [73,74]. These are times known to be necessary for the formation of an oxide film. At a shorter pulse length (<10"" s) acetate is completely oxidized to carbon dioxide and water possibly at a bare platinum surface [75]. The potent dynamic response in the electrolysis of potassium acetate in aqueous solution also points to an oxide layer, whose... 

The electrolysis of carboxylic acids in aqueous solution can lead to alcohols and esters as major reaction products [5,241]. When electrolyses are performed in methanol or acetic acid methyl ethers or acetates can be found as side or major products. These observations led Walling and others [242] to suggest that in these cases the inter-... 

Ions not solvated are unstable in solutions between them and the polar solvent molecules, electrostatic ion-dipole forces, sometimes chemical forces of interaction also arise which produce solvation. That it occurs can be felt from a number of effects the evolution of heat upon dilution of concentrated solutions of certain electrolytes (e.g., sulfuric acid), the precipitation of crystal hydrates upon evaporation of solutions of many salts, the transfer of water during the electrolysis of aqueous solutions), and others. Solvation gives rise to larger effective radii of the ions and thus influences their mobilities. 

In aqueous solutions, approximately one atom of deuterium, D, is present for every 7000 atoms of the ordinary hydrogen isotope (protium, H). In the evolntion of heavy hydrogen, HD, the polarization is approximately 0.1 V higher than in the evolution of ordinary hydrogen, H2. Hence during electrolysis the gas will be richer in protium, and the residual solution will be richer in deuterium. The relative degree of enrichment is called the separation factor (S) of the hydrogen isotopes,... 

In aqueous solutions, concentrations are sometimes expressed in terms of normality (gram equivalents per liter), so that if C is concentration, then V = 103/C and a = 103 K/C. To calculate C, it is necessary to know the formula of the solute in solution. For example, a one molar solution of Fe2(S04)3 would contain 6 1CT3 equivalents cm-3. It is now clear as to why A is preferred. The derivation provided herein clearly brings out the fact that A is the measure of the electrolytic conductance of the ions which make up 1 g-equiv. of electrolyte of a particular concentration - thereby setting conductance measurements on a common basis. Sometimes the molar conductance am is preferred to the equivalent conductance this is the conductance of that volume of the electrolyte which contains one gram molecule (mole) of the ions taking part in the electrolysis and which is held between parallel electrodes 1 cm apart. 

The first catalysts reported for the electroreduction of C02 were metallophthalocyanines (M-Pc).126 In aqueous solutions of tetraalkylammonium salts, current-potential curves at a cobalt phthalocyanine (Co-Pc)-coated graphite electrode showed a reduction current peak whose height was proportional to the C02 concentration and to the square root of the potential sweep rate at a given C02 concentration. On electrolysis, oxalic acid and glycolic acid were detected, but formic acid was not. Mn and Pd phthalocyanines were inactive, while Cu and Fe phthalocyanines were slightly active. At the potentials used for C02 reduction, M-Pc catalysts would be in their dinegative state, and the occupied dz2 orbital of the metal ion in the metallophthalocyanine was suggested to play an important role in the catalytic activity. 

Hiratsuka et al102 used water-soluble tetrasulfonated Co and Ni phthalocyanines (M-TSP) as homogeneous catalysts for C02 reduction to formic acid at an amalgamated platinum electrode. The current-potential and capacitance-potential curves showed that the reduction potential of C02 was reduced by ca. 0.2 to 0.4 V at 1 mA/cm2 in Clark-Lubs buffer solutions in the presence of catalysts compared to catalyst-free solutions. The authors suggested that a two-step mechanism for C02 reduction in which a C02-M-TSP complex was formed at ca. —0.8 V versus SCE, the first reduction wave of M-TSP, and then the reduction of C02-M-TSP took place at ca. -1.2 V versus SCE, the second reduction wave. Recently, metal phthalocyanines deposited on carbon electrodes have been used127 for electroreduction of C02 in aqueous solutions. The catalytic activity of the catalysts depended on the central metal ions and the relative order Co2+ > Ni2+ Fe2+ = Cu2+ > Cr3+, Sn2+ was obtained. On electrolysis at a potential between -1.2 and -1.4V (versus SCE), formic acid was the product with a current efficiency of ca. 60% in solutions of pH greater than 5, while at lower pH... 

The same group, in a previous work, reported on the realization of a hybrid anode electrode [197]. An appreciable improvement in methanol oxidation activity was observed at the anode in direct methanol fuel cells containing Pt-Ru and Ti02 particles. Such an improvement was ascribed to a synergic effect of the two components (photocatalyst and metal catalyst). A similar behavior was also reported for a Pt-Ti02-based electrode [198]. Another recent study involved the electrolysis of aqueous solutions of alcohols performed on a Ti02 nanotube-based anode under solar irradiation [199]. 

Although the lower alcohol homologues such as MeOH and EtOH can be oxidized electrochemically in aqueous solution, the mechanism is complex and barely investigated [6]. Electrolysis of the neat liquid is one way to achieve the direct oxidation of MeOH and EtOH [7]. 

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