Electrolysis of sodium hydroxide

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). 

Industrially, it is manufactured either by fractional distillation of air, or by electrolysis of sodium hydroxide and it is distributed as a non-liquefied gas in pressurized black cylinders at ca 2200 psig at 21°C. Since it is non-corrosive no special materials of construction are required. 

The first commercial process for sodium production used the electrolysis of sodium hydroxide, but it had the disadvantage that the current efficiency was less than 50%. At present this process is the only way of producing metallic sodium. The cell reaction is... 

Hydrochloric acid is an aqueous solution of hydrogen chloride gas produced by a number of methods including the reaction of sodium chloride and sulfuric acid the constituent elements as a by-product from the electrolysis of sodium hydroxide and as a by-product during the chlorination of hydrocarbons. 

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. 

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. 

The term chlor-alkali refers to those products obtained from the commercial electrolysis of aqueous sodium chloride. These are chlorine, sodium hydroxide, and sodium carbonate. The first two are produced simultaneously during the electrolysis while the latter is included because it is also produced in small quantities and shares many of the end uses of sodium hydroxide. Perfluorinated ionomer membranes are permeable to sodium ions but not the chloride ions, and hence they are useful for these electrolytic cells. The arrangement of a typical membrane cell is shown in Figure 10.2. 

Among electrolytic processes used to produce materials, we customarily distinguish those in which electrodes are reacting that is, where the metal or other electrode material is involved in the reaction (Chapter 16) from those with nonconsumable electrodes (Chapter 15). A very important industrial process with nonconsumable electrodes is the electrolysis of sodium chloride solution (brine) producing chlorine at the anode and sodium hydroxide NaOH (caustic soda) in the catholyte via the overall reaction... 

Sodium chloride is found in salt beds, salt brines, and sea water throughout the world, and it is also mined is some locations. Consequently, sodium chloride is the source of numerous other sodium compounds. A large portion of the sodium chloride utilized is consumed in the production of sodium hydroxide (Eq. (11.23)). The production of sodium metal involves the electrolysis of the molten chloride, usually in the form of a eutectic mixture with calcium chloride. Sodium carbonate is an important material that is used in many ways such as making glass. It was formerly produced from NaCl by means of the Solvay process, in which the overall reaction is... 

Sodium was first isolated by Sir Humphry Davy in 1807 by electrolysis of caustic soda. In the following year. Gay Lussac and Thenard obtained metallic sodium by chemical reduction of caustic soda with iron at elevated temperatures. Deville, in 1854, prepared the metal by reduction of sodium carbonate and lime with charcoal at a temperature above the boiling point of sodium. Castner, in 1886, improved the chemical reduction process preparing the metal by heating sodium hydroxide with iron carbide at high temperature. Five years later he patented a process based on electrolytic reduction of sodium hydroxide. The first major commercial plant was set up in 1921 with the introduction of Downs cell. 

Sodium hydroxide is manufactured together with chlorine by electrolysis of sodium chloride solution. Various types of electrolytic cells are used commercially. They include the mercury cell, the diaphragm cell, and the membrane cell. 

Although the theoretical studies predict solvent medium breakdown before the onset of actinium electrodeposition, there have been reports of Ac(0) electrodeposition from aqueous solutions utilizing several different methods [8, 9]. One set of studies [8] describes the electrodeposition of actinium from nitric acid solutions, with varying pH values (1.0-4.0) being set to the appropriate level by the addition of sodium hydroxide. The anode and cathode in these studies were platinum metal, and the current density was varied from 50 to 200 mA cm . The authors found that quantitative electrodeposition of actinium could be achieved under various conditions, with the shortest electrolysis time of 1 h being obtained with a current density of200 mA cm and a pH of 2.0. A second study employed a saturated aqueous solution of urea oxalate (ca 6.6% at 30 °C) as an electrolyte for the electrodeposition of Ac onto a nickel foil cathode [9]. The authors of this study found that the yield of electrodeposited Ac increased with time and reached a near quantitative maximum yield of 97% at a current density of 53 mAcm after 2 h. The Ac electrodeposits were suitable for further study using nuclear spectroscopy. 

Chlorine gas is produced by the electrolysis of brine (sodium chloride) in Castner-Kellner cells (first operated in 1886), the main purpose of which is the production of sodium hydroxide solution. Williams (1972, p98) reports that the extent to which the Castner-Kellner process was worked in Britain depended on the ability to dispose of the chlorine. In this light the production of chlorinated organic compounds can be seen as a response to the need to use chlorine. The argument that we need to produce chlorinated organic compounds to use up chlorine was indeed put to me by manufacturers of polyvinyl chloride (PVC) at a DETR-organized seminar on the lifecycle assessment of PVC in July 2001 PVC is the only product made in sufficient quantities to use up all the chlorine produced by other processes. This suggests that if a particular use of a chemical is stopped, because there is a better (less hazardous) way of achieving that purpose, it will have knock-on effects on the availability of chemicals that are co-produced... 

We are told by H. Davy that, in 1790, M. Tondi attempted to obtain the constituent elements of these earths by heating a mixture of the oxide with carbon. In some cases he obtained metallic reguli. H. Davy also pointed out that G. E. Stahl approached very nearly to the discovery of the pure alkalies, for he exposed a mixture of solid caustic potash and iron filings to a long-continued heat, and stated that in this way an intensely caustic alkali—valde causticum—is produced. The oxides were not definitely resolved into their elements until 1807, when H. Davy isolated potassium and sodium by the electrolysis of their hydroxides. 

G. von Hevesy obtained greater yields of potassium by the electrolysis of its hydroxide than of sodium with its hydroxide at the same temp. For example, between 320° and 340°, a 27 per cent, yield of sodium, and a 55 per cent, yield of potassium, were obtained. This was traced to the increased rate of diffusion of sodium from cathode to anode where the metal is lost by secondary reactions and this the more, the higher the temp. F. C. Wickel and W. Lobel obtained potassium from its hydroxide and metallic sodium, by melting the two together in the absence of air sodium oxide is formed and potassium distils from the mixture hydrogen gas also escapes. 

The temp. coeS. of the eq. conductivity of sodium carbonate soln. for the mean temp. 22° is 00265 and for potassium carbonate, 0-0249. H. C. Jones and A. P. West, and C. Deguisne have also studied the temp, coeff. of the conductivity of these salts. M. H. van Laar studied the formation of sodium hydroxide by the electrolysis of soln. of sodium carbonate with and without the addition of an oxy-salt. W. Bien calculates the transport number for the anion in 0 052V-soln. at 23° to be 0 590, but as in the case of lithium carbonate hydrolysis interferes with the... 

Only a small number of compounds are produced directly by electrolysis. To illustrate this type of process, the electrolytic production of sodium hydroxide is described in detail. Then it is shown how this process may be modified to permit the formation of two other valuable commercial chemicals. 

Another somewhat more complicated cell for the production of chlorine and sodium hydroxide by the electrolysis of sodium chloride solution is the Castner-Kellner cell, which employs a liquid mercury cathode. 

Sodium hypochlorite. If, in the electrolysis of sodium chloride solution, provision is made for the intimate mixing of the chlorine and sodium hydroxide at low temperatures, the following reaction occurs ... 

Sodium carbonate. When sodium hydroxide is produced by the electrolysis of sodium chloride solution saturated with carbon dioxide, sodium carbonate is produced as follows ... 

The production of sodium hydroxide by electrolysis has been described in sufficient detail, and this procedure serves equally well for the production of potassium hydroxide. 

In 1892, the electrolysis of brine was discovered as a method for making both sodium hydroxide and chlorine, and since the 1960s it has been the only method of manufacture of sodium hydroxide (Fig. 1). 

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