Electrolysis of aqueous salt solutions

Fig. 1.7-8. Schematic representation of the electrolysis of aqueous salt solutions by the diaphragm process.

Electrolysis of Aqueous Salt Solutions Overvoltage and the Chlor-Alkali Process... 

Understand the basis of an electrolytic cell describe the Downs cell for the production of Na, the chlor-alkali process and the importance of overvoltage for the production of CF, the electrorefining of Cu, and the use of cryolite in the production of Al know how water influences the products at the electrodes during electrolysis of aqueous salt solutions ( 21.7) (SP 21.8) (EPs 21.63-21.75,21.82)... 

Electrolysis of Aqueous Salt Solutions Overvoltage and the Chlor-Alkali Process Aqueous salt solutions are mixtures of ions and water, so we have to compare the various electrode potentials to predict the electrode products. 

Since chloramine itself is prepared by chlorination of ammonia, hydrazine is generally prepared by treatment of excess ammonia with chlorine. The similarity to H2O2 is exemplified by the detection of hydrazine at the anode in electrolysis of ammonium salts, just as H2Q2 or its derivatives can result from electrolysis of aqueous acid solutions. 

Electrolysis of Water and Nonstandard Half-Cell Potentials Before we can analyze the electrolysis products of aqueous salt solutions, we must examine the electrolysis of water itself. Extremely pure water is difficult to electrolyze because very few ions are present to conduct a current. If we add a small amount of a salt that cannot be electrolyzed in water (such as Na2S04), however, electrolysis proceeds rapidly. A glass electrolytic cell with separated gas compartments is used to keep the H2 and O2 gases from mixing (Figure 21.25). At the anode, water is oxidized as the O.N. of O changes from —2 to 0 ... 

Variety of methods have been reported in literature for producing nickel and cobalt powders from different starting materials these include spray pyrolysis [5], ultrasonic spray pyrolysis under reduced atmosphere [6-10], wet chemical (reduction) methods [3,11], electrolysis [12,13] and controlled hydrothermal reduction of aqueous salt solutions of nickel and cobalt [14,15. Among these reported laboratory scale methods, aqueous processing routes such as hydrothermal, wet chemical and electrolysis processes seem economically and technically attractive because of their low temperature operation and control over particle characteristics through easy manipulation of process parameters including use of additives and/or surfactants [16,17]. 

Predicting the Electrolysis Products of Aqueous Salt Solutions... 

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. 

Because of its high reactivity, production of barium by such processes as electrolysis of barium compound solution or high temperature carbon reduction is impossible. Electrolysis of an aqueous barium solution yields Ba(OH)2, whereas carbon reduction of an ore such as BaO produces barium carbide [50813-65-5] BaC2, which is analogous to calcium carbide (see Carbides). Attempts to produce barium by electrolysis of molten barium salts, usually BaCl25 met with only limited success (14), perhaps because of the solubiUty of Ba in BaCl2 (1 )-... 

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. 

Although the electrolysis of molten salts does not in principle differ from that of aqueous solutions, additional complications are encountered here owing to the problems related to the higher temperatures of operation, the resultant high reactivities of the components, the thermoelectric forces, and the stability of the deposited metals in the molten electrolyte. As a result of this, processes taking place in the melts and at the electrodes cannot be controlled to the same extent as in aqueous or other types of solutions. Considerations pertaining to Faraday s laws have indicated that it would be difficult to prove their applicability to the electrolysis of molten salts, since the current efficiencies obtained are generally too small in such cases. 

The electrolysis of aqueous solutions may not yield the desired products. Sir Humphry Davy (1778-1829) discovered the elements sodium and potassium by electrolyzing their molten salts. Before this discovery, Davy had electrolyzed aqueous solutions of sodium and potassium salts. He had not succeeded in reducing the metal ions to the pure metals at the cathode. Instead, his first experiments had produced hydrogen gas. Where did the hydrogen gas come from ... 

Predict the products of the electrolysis of molten salts and aqueous solutions. 

The main reason for avoiding water as a solvent is the fact that the electrolysis of aqueous solutions of alkali and alkaline-earth metal salts commences at 1.7-2.0 volts (depending on the electrode material) and results in the evolution of O2 and H2. If the cell itself has a higher voltage, internal electrolysis can, but not always does occur, accompanied by the evolution of H2 and O2 and by self-discharge (117). However, this fact does not preclude attempts to create moist primary batteries with Li, Na or Ca, if the activity of H2O is kept sufficiently low. 

Cesium hydroxide is prepared hy electrolysis of cesium salts to obtain cesium metal, which then reacts with water to yield hydroxide. It also is prepared hy the action of harium hydroxide with an aqueous solution of cesium sulfate. 

Since chemical reduction means gain of electrons, electrolysis is the most direct way of recovering a metal from its ores, as long as these can be handled in a fluid state.6 Consideration of E° values for reactive metal halfcells such as Na+(aq)/Na(s), Mg2+(aq)/Mg(s), and Al3+(aq)/Al(s) (-2.71, -2.36, and -1.67 V, respectively) shows that these metals can never be obtained by electrolysis of aqueous solutions of their salts, as H2 would be produced instead, but they can often be obtained by electrolysis of suitable molten salts such as NaCl and MgCl2 ... 

When an aqueous salt solution is electrolyzed, the electrode reactions may differ from those for electrolysis of the molten salt because water may be involved. In the electrolysis of aqueous sodium chloride, for example, the cathode half-reaction might be either the reduction of Na+ to sodium metal, as in the case of molten sodium chloride, or the reduction of water to hydrogen gas ... 

Potash is used as a collective name for mined or produced salts that contain water-soluble potassium (for a large part as sylvite, potassium chloride mineral), with a total worldwide production of 35 Mt in 2007 [1]. Potassium (bi)carbonate production methods involve the carbonation of potassium hydroxide (KOH), which in turn is produced by electrolysis of aqueous potassium chloride solutions. 

Contact above 0°C of excess chlorine or a chlorinating agent with aqueous ammonia, ammonium salts or a compound containing a hydrolysable amino-derivative, or electrolysis of ammonium chloride solution produces the highly endothermic (AHf (g) +230.1 kJ/mol, 1.91 kJ/g) and explosive nitrogen trichloride as a water-insoluble yellow oil [1,2,3]. Detonation at constant volume generates 5,500 Bar maximum pressure and 2,100°C maximum temperature. As a vapour it decomposes explosively at pressures as low as 1 mBar and may sensitise flammable gas mixtures even as a... 

On the basis of the preceding generalizations, it is possible to predict the products of electrolysis of aqueous solutions of simple salts. It is not... 

Similar predictions may be made readily with regard to the electrolysis of aqueous solutions of other salts. 

The following subsections discuss a few chemical processes that involve the electrolysis of aqueous solutions or fused salts. It is not intended to provide here an exhaustive treatment of the subject but rather to select a few typical cases that serve to acquaint the student with the nature, the scope, and the importance of these industries. 

---