Extraction of Chlorine

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Extraction of Chlorine

The chlor-alkali electrolysis process is used in the manufacture of chlorine, hydrogen and sodium hydroxide solution. Of these three, the primary product is chlorine.

In each one, a salt solution is electrolyzed by action of direct electric current that converts chloride ions to elemental chlorine [1].

 Anode and cathode reactions in the membrane cell

 

A membrane allows positive ions such as Na+ (aq), to pass through it, but not negative ions, like Cl- (aq), or water molecules.

On one side circulating brine is in contact with a titanium anode and chloride ions are discharged producing chlorine gas [2]. (Figure 1) 

Figure 1: The membrane cell [3]

Redox reactions involve electron transfer. Redox reactions can be split into two half-reactions, one producing electrons, and one accepting them [4]. If the ions in the electrolyte solution are positively charged, they will flow towards the cathode and be reduced. If the ions are negatively charged, they will flow to the anode and be oxidised. The extraction of chlorine is an example of such reaction:

2NaCl + 2H2O        Cl2 + H2 + 2NaOH

Anode: 2Cl- (aq)                             Cl2 (g)  + 2e-

Cathode: 2 H+ (aq) + 2e             H2 (g)

Redox reactions in the membrane cell

If chlorine is produced in the presence of sodium hydroxide solution it will combine with it to produce ClO-  and Cl- ions, which will result in production of sodium chlorate ( | ), NaClO. To overcome this problem, chlorine and sodium hydroxide in a  mercury cathode cell are produced in two different containers. Titanium anodes, coated with an oxide are in the top of the container where chlorine ions are discharged and Cl2 is formed.

2Cl- (aq)                       Cl2 (g) + 2e-

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A layer of mercury flowing along the base of the container is the cathode.The bottom of this container is called the decomposer. There, sodium ions are reduced to sodium and sodium and the sodium dissolves in the mercury.

Whereas in a membrane cell, the products are kept apart because a membrane allows positive ions like Na+ (aq), to pass through it, but not negative ions, like Cl- (aq), or water molecules [2]. Therefore the products cannot react again.  

Uses of chlorine, sodium hydroxide and hydrogen in present-day industry

Chlorine (Figure 2), sodium hydroxide ...

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