Sodium chloride, electrolysis

Sodium hydroxide is manufactured by electrolysis of concentrated aqueous sodium chloride the other product of the electrolysis, chlorine, is equally important and hence separation of anode and cathode products is necessary. This is achieved either by a diaphragm (for example in the Hooker electrolytic cell) or by using a mercury cathode which takes up the sodium formed at the cathode as an amalgam (the Kellner-Solvay ceW). The amalgam, after removal from the electrolyte cell, is treated with water to give sodium hydroxide and mercury. The mercury cell is more costly to operate but gives a purer product. 

By the electrolysis of concentrated sodium chloride solution this process was initially used primarily for the production of sodium hydroxide but the demand for chlorine is now so great that the chlorine is a primary and not a by-product. 

By the electrolysis of fused magnesium chloride or fused sodium chloride. 

The aqueous solution of sodium chlorate(I) is an important liquid bleach and disinfectant. It is produced commercially by the electrolysis of cold aqueous sodium chloride, the anode and cathode products being mixed. The sodium chloride remaining in the solution does not usually matter. There is evidence to suggest that iodic(I) acid has some basic character... 

It is now obtained commercially by the electrolysis of absolutely dry fused sodium chloride. This method is much cheaper than that of electrolyzing sodium hydroxide, as was used several years ago. 

Chlorine and caustic soda are coproducts of electrolysis of aqueous solutions of sodium chloride [7647-14-5] NaCl, (commonly called brine) following the overall chemical reaction... 

Other Metals. AH the sodium metal produced comes from electrolysis of sodium chloride melts in Downs ceUs. The ceU consists of a cylindrical steel cathode separated from the graphite anode by a perforated steel diaphragm. Lithium is also produced by electrolysis of the chloride in a process similar to that used for sodium. The other alkaH and alkaHne-earth metals can be electrowon from molten chlorides, but thermochemical reduction is preferred commercially. The rare earths can also be electrowon but only the mixture known as mischmetal is prepared in tonnage quantity by electrochemical means. In addition, beryIHum and boron are produced by electrolysis on a commercial scale in the order of a few hundred t/yr. Processes have been developed for electrowinning titanium, tantalum, and niobium from molten salts. These metals, however, are obtained as a powdery deposit which is not easily separated from the electrolyte so that further purification is required. 

Sir Humphry Davy first isolated metallic sodium ia 1807 by the electrolytic decomposition of sodium hydroxide. Later, the metal was produced experimentally by thermal reduction of the hydroxide with iron. In 1855, commercial production was started usiag the DeviUe process, ia which sodium carbonate was reduced with carbon at 1100°C. In 1886 a process for the thermal reduction of sodium hydroxide with carbon was developed. Later sodium was made on a commercial scale by the electrolysis of sodium hydroxide (1,2). The process for the electrolytic decomposition of fused sodium chloride, patented ia 1924 (2,3), has been the preferred process siace iastallation of the first electrolysis cells at Niagara Falls ia 1925. Sodium chloride decomposition is widely used throughout the world (see Sodium compounds). 

The cell bath in early Downs cells (8,14) consisted of approximately 58 wt % calcium chloride and 42 wt % sodium chloride. This composition is a compromise between melting point and sodium content. Additional calcium chloride would further lower the melting point at the expense of depletion of sodium in the electrolysis 2one, with the resulting compHcations. With the above composition, the cells operate at 580—600°C, well below the temperature of highest sodium solubiUty in the salt bath. Calcium chloride causes problems because of the following equiUbrium reaction (56) ... 

Other commercial cells designed for the electrolysis of fused sodium chloride iaclude the Danneel-Lon2a cell and the Seward cell, both used before World War I. The former had no diaphragm and the sodium was confined to the cathode 2one by salt curtains (ceramic walls) the latter utili2ed the contact-electrode principle, where the cathode was immersed only a few millimeters ia the electrolyte. The Ciba cell was used over a longer period of time. 

It was an adaptation of the Castner cell to sodium chloride for fused caustic electrolysis. A mixture of sodium chloride and other chlorides, molten at 620°C, was electroly2ed ia rectangular or oval cells heated only by the current. Several cells have been patented for the electrolysis of fused salt ia cells with molten lead cathodes (65). However, it is difficult to separate the lead from the sodium (see Electrochemical processing). 

Manufacture. Most chlorate is manufactured by the electrolysis of sodium chloride solution in electrochemical cells without diaphragms. Potassium chloride can be electroly2ed for the direct production of potassium chlorate (35,36), but because sodium chlorate is so much more soluble (see Fig. 2), the production of the sodium salt is generally preferred. Potassium chlorate may be obtained from the sodium chlorate by a metathesis reaction with potassium chloride (37). 

Brine Preparation. Rock salt and solar salt (see Chemicals frombrine) can be used for preparing sodium chloride solution for electrolysis. These salts contain Ca, Mg, and other impurities that must be removed prior to electrolysis. Otherwise these impurities are deposited on electrodes and increase the energy requirements. The raw brine can be treated by addition of sodium carbonate and hydroxide to reduce calcium and magnesium levels to below 10 ppm. If further reduction in hardness is required, an ion-exchange resin can be used. A typical brine specification for the Huron chlorate ceU design is given in Table 6. 

Chlora.tes. Sodium chlorate is produced by the electrolysis of sodium chloride at pH 6.5—7.5 in a one-compartment cell. DSA anodes and steel cathodes are generally used in chlorate cells. The electrolysis products, hypochlorous acid, and hypochlorite ions, react chemically to produce chlorate (eq. 

Electrolysis Reactions. The electrodes in electrophoresis equipment are typically constmcted from platinum wire, and sodium chloride generally carries the bulk of the current in any electrophoretic medium. This results in the reactions... 

These facts would suggest that die electrolysis of molten alkali metal salts could lead to the inuoduction of mobile elecU ons which can diffuse rapidly through a melt, and any chemical reduction process resulting from a high chemical potential of the alkali metal could occur in the body of the melt, rather than being conhned to the cathode volume. This probably explains the failure of attempts to produce tire refractoty elements, such as titanium, by elecU olysis of a molten sodium chloride-titanium chloride melt, in which a metal dust is formed in the bulk of the elecU olyte. 

Later, Du Pont in America developed its own ionically conducting membrane, mainly for large-scale electrolysis of sodium chloride to manufacture chlorine, Nafion , (the US Navy also used it on board submarines to generate oxygen by electrolysis of water), while Dow Chemical, also in America, developed its own even more efficient version in the 1980s, while another version will be described below in connection with fuel cells. Meanwhile, Fenton et al. (1973) discovered the first of a... 

Other industrial applications of electrolysis include extraction/purification of metals from ores, electroplating, and the manufacture of certain chemicals such as sodium hydroxide. In the latter, sodium chloride solution when electrolysed is converted to sodium hydroxide to produce chlorine at the anode and hydrogen at the cathode. Both of these gaseous by-products are collected for industrial use chlorine is used in the production of bleach and PVC hydrogen is used as a fuel, to saturate fats, and to make ammonia. 

The development of electrical power made possible the electrochemical industry. Electrolysis of sodium chloride produces chlorine and either sodium hydroxide (from NaCl in solution) or metallic sodium (from NaCl fused). Sodium hydroxide has applications similar to sodium carbonate. The ad vantage of the electrolytic process is the production of chlorine which has many uses such as production of polyvinyl chloride. PVC, for plumbing, is produced in the largest quantity of any plastic. 

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. 

Beryllium is extracted from the main source mineral, the alumino-silicate beryl, by conversion to the hydroxide and then through either the fluoride or the chloride to the final metal. If the fluoride is used, it is reduced to beryllium by magnesium by a Kroll-type reaction. The raw metal takes the form of pebble and contains much residual halides and magnesium. With the chloride on the other hand, the pure metal is extracted by electrolysis of a mixture of fused beryllium chloride and sodium chloride. The raw beryllium is now dendritic in character, but still contains residual chloride. 

Titanium is the only one of the more common structural metals which is not attacked by wet chlorine gas and it is thus widely used as a heat exchange material for cooling the gas after the electrolysis stage. Preheating of sodium chloride brine is carried out in titanium plate heat exchangers, while titanium butterfly valves, demisters, and precipitators handle the chlorine gas produced in the cell. The most important use of titanium in chlorine production is as anodes in place of graphite in the electrolytic process. This is covered in more detail later. 

From a commercial standpoint, the most important electrolysis carried out in water solution is that of sodium chloride (Figure 18.10, p. 500). At the anode, Cl- ions are oxidized to chlorine gas ... 

Sodium metal is obtained by the electrolysis of molten sodium chloride (Figure 20.3, p. 537). The electrode reactions are quite simple ... 

Write a balanced equation to represent the electrolysis of molten sodium chloride. What volume of Cl2 at STP is formed at the anode when... 

The fluoride and chloride ions are very difficult to oxidize (Elx F = —2.889 V E°x Cl- = —1.360 V). Hence the elements fluorine and chlorine are ordinarily prepared by electrolytic oxidation, using a high voltage. As pointed out in Chapter 18, chlorine is prepared by the electrolysis of aqueous sodium chloride ... 

Sodium hypochlorite is produced by the electrolysis of cold sodium chloride solution. How long must a cell operate to produce 1.500 X 103 L of 5.00% NaCIO by mass if the cell current is 2.00 X 103 A Assume that the density of the solution is 1.00 g/cm3. 

Sodium bicarbonate, 112-113 Sodium carbonate, 61 Sodium chloride 44q common ion effect and, 439 electrolysis, 499 formation, 3 structure, 36 Sodium chlorine, 4 Sodium hydroxide, 61,84,441 Sodium hypochlorite, 369-370 Sodium stearate, 595 Sodium vapor lamps, 135 Solids... 

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