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2. s-Block Elements eLearn.Punjab

The Fig. 2.6 (b) shows a simplified version of the cell in order to
understand the purpose of diaphragm. When the electrolysis takes place,
chlorine is given off at the anode according to the following reaction.

2Cl− (aq) → Cl2 (g) + 2e−

At the cathode hydrogen is discharged by the reduction of water.

2H2O + 2e− → 2OH− (aq) + H2 (g) (Cathode)

The overall result of the above reactions is that the brine loses its chloride
ions and the solution turns increasingly alkaline in cathode compartment.

We can face two major problems during the working of the cell.
1. Chlorine produced can react with hydroxide ions in cold giving
hypochlorite ions.

Cl2 (g) + 2OH− (aq) → OCl− (aq) + Cl− (aq) + H2O

2. Hydroxide ions may be attracted towards anode, where
they can be discharged releasing oxygen gas. This oxygen
gas may contaminate the chlorine and renders it impure.

The first problem is solved by using asbestos diaphragm. This keeps
the two solutions separate while allowing sodium ions
to move towards the cathode. This movement of ions
keep the current following through the external current.

The second problem is solved keeping the level of brine in anode
compartment slightly higher, this keeps the direction of flow of liquid
toward the cathode and thus preventing the
possibility of hydroxides ions to reach the anode.

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