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Sodium Hydroxide

Sodium hydroxide 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 to dihydrogen gas which is liberated at the cathode and collected as an important by-product.

NaCl  Na+ Cl-
2H2O + 2e 2OH- + H2 (at cathode)
Na+ + OH NaOH (at cathode)
2Cl- - 2e Cl2 (at anode)
The products formed from the electrolysis of sodium chloride are sodium hydroxide, hydrogen and chlorine. All of these products have a large number of uses.

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