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Preparation of Sodium Hydroxide

Also known as caustic soda, it is usually manufactured by the electrolysis of sodium chloride solution in a diaphragm cell or in the Castner-Kellner cell .

Preparation of Sodium Hydroxide in Diaphragm Cell
The diaphragm cell consists of a U-shaped perforated steel cathode tube which is lined inside with asbestos. Sodium chloride is kept in this cathode tube and a graphite anode dips in the solution. On passing the current, chlorine is evolved at the anode and is led away through a pipe. Sodium ions penetrate through the asbestos and react with OH- ions (from steam) to form sodium hydroxide which is collected at the bottom of the cell. Hydrogen ions (H+) obtained from steam are converted into dihydrogen gas which is liberated at the cathode and collected as an important by-product.

Fig. Diaphragm cell for Production of Caustic Soda

NaCl Na+ + Cl-
2H2O + 2e- 2OH- + H2 (at cathode)
Na+ + OH- NaOH (at cathode)
2Cl- - 2e- Cl2 (at anode)

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