Mercury cells Solvay cell

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. 

Mercury is extensively used in various pieces of scientific apparatus, such as thermometers, barometers, high vacuum pumps, mercury lamps, standard cells (for example the Weston cell), and so on. The metal is used as the cathode in the Kellner-Solvay cell (p. 130). 

Early demand for clilorine centered on textile bleaching, and clilorine generated through the electrolytic decomposition of salt (NaQ) sufficed. Sodium hydroxide was produced by the lime —soda reaction, using sodium carbonate readily available from the Solvay process. Increased demand for clilorine for PVC manufacture led to the production of clilorine and sodium hydroxide as coproducts. Solution mining of salt and the availability of asbestos resulted in the dominance of the diaphragm process in North America, whereas solid salt and mercury availability led to the dominance of the mercury process in Europe. Japan imported its salt in solid form and, until the development of the membrane process, also favored the mercury cell for production. 

A second method uses the Kellner-Solvay cell. Saturated sodium chloride solution is electrolyzed between a carbon anode and a flowing mercury cathode. In this case the sodium is produced at the cathode rather than the hydrogen because of the readiness of sodium to dissolve in the mercury. The sodium-mercury amalgam is then exposed to water and a sodium hydroxide solution is produced. 

The main mercury cathode electrolytic cells used in the industry were developed chiefly by De Nora and Uhde, and to a lesser extent by IC1, Krebs Kureha, Olin and Solvay. 

Historically, the Japanese chlor-alkali industry started in 1881, when the LeBlanc process was used to produce caustic soda. Osaka Soda and Hodogaya Chemical commercialized the mercury- and diaphragm-cell technologies in 1915. Asahi Glass started the Solvay process soon after. By 1973, 95% of the chlorine was produced by the mercury-cell process and 5% by diaphragm cells. In 1973, mercury pollution issues... 

The second class of metallic solutes is represented by the less electropositive metals. Here, the situation is the reverse of that discussed above. Sodium amalgam is widely used in industry and in the laboratory and is a good example of this class. Upon addition of mercury to liquid sodium, the reactivity of the sodium toward aqueous solutions is vastly reduced, and reaction with hydrogen is slower by an order of magnitude than that for pure sodium. This fact is important in the operation of the Solvay cell for the industrial production of sodium hydroxide by electrolysis of brine, in which sodium amalgam forms one of the electrodes. In such amalgams, valency electrons from the conduction band of liquid sodium, which would normally be responsible for its chemical reactivity, are partially localized on the mercury atoms, thus inhibiting the reactivity of sodium. 

Parallel to this development in the United States, Brichaux and W. Wilsing of Solvay built the first long cell, in Germany, in 1898. This cell had the modem characteristics of forced mercury circulation. 

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