Preparation of Sodium:
We know that sodium (Preparation of sodium)is a chemical element and have symbol NA and atomic Number 11. Its belong to group Number 1 because it has a single electron in outermost shell. It can readily donate electron from outer shell. It is highly reactive , soft and silvery white alkali metal.
Commercial Preparation of Sodium by Down’s cell :
In down’s cell sodium is prepared by electrolysis of molten (fused) sodium chloride.
Some CaCl2 is add to lower the melting point of Nacl from 801 ℃ to 600 ℃ . The cell consists of an iron vessel. A large block of graphite acts as an anode. Above the anode there is an iron dome for the collection of chlorine. The cathode is made of iron or copper. It is seperate from anode by iron gauze screen. When electric current passes through molten Nacl, following reaction take place.
Nacl ⮂ Na+ + Cl–
Na++ e– → Na (at cathode)
2Cl– → Cl2+ 2e– (at anode)
The sodium collects over cathode and chlorine over anode. They are take out from different holes.
Advantages of Down’s cell Process:
- The metallic fog is not produce.
- Liquid Sodium is collect at 600c.
- It gives 99.9% Pure Sodium.
- Materials of the cell is not attack by the products form during electrolysis.
Commercial Preparation of Sodium Hydroxide by Nelson Cell or Diaphragm cell:
Sodium hydroxide is prepare on large scale by electrolysis of aqueous solution by sodium chloride. It is done in a cell known as Nelson or diaphragm cell.
The cell consists of a steel tank provided with a constant level device. This device keeps the brine level in inner part at a specific height (1m). An oblong perforate steel vessel is line inside with porous asbestos diaphragm.
It acts as cathode. A graphite rod is suspend into U-Shaped diaphragm containing brine (salt solutio). It acts as anode.
The purpose of diaphragm becomes clear from following figure.
When electric current passes through solution of Nacl , then Cl-, OH-, Na+ and H+ ions are produce.
2Cl– → Cl2 + 2e– (at anode)
2H2O + 2e– → 2OH–+ H2
The Na+ ions migrate to the cathode through the asbestos diaphragm. The Cl2 and H2 gasses are take out from different outlets. The aqueous Solution of NaoH remains in cathode compartment.
Two Problems and Their Solution :
During the working of cell we can face two major problems.
(01) In anode compartment the Cl2 can react with OH- ions.
Cl2 + 2OH– → Cl– + OCl– + H20
(02) The OH- ions may collect over anode. Their oxidation gives oxygen which contaminates the Cl2.
4 OH– → 2 H2O + O2 + 4 e–
The first problem is solve by using asbestos diaphragm. Its keeps the two solutions separated and migrate Na+ ions towards the chloride.
The second problem is solve by keeping the brine level slightly higher in anode compartment . So, flow of liquid continues towards the cathode and OH- ions can not react the anode.
The Solution which flows out of the cathode collects in catch basin. It contains 11% NaOH and 16% NaCl. By evaporation, NaCl forms crystals and NaOH remains in Solution . The solid NaCl is filter off and mother liquor contains 50% NaOH and 1% NaCl as an impurity .
(1) Down’s cell is use to prepare.
A) Sodium carbonate B) Sodium Bicarbonate
C)Sodium Metal D) Sodium hydroxide
Answer: (c) sodium metal