Basics Steps For The PREPARATION OF SODIUM BY DOWN’S CELL 2022

Most of the Na is prepared by the electrolysis of fused NaCl in Down’s cell.

CONSTRUCTION

Down cell was designed by J.C Down.

It consists of a circular furnace. There is a large block of graphite in the center which acts as an anode. Above it is a dome for the collection of chlorine.

A circular bar of copper surrounds the anode. This bar acts as a cathode. It is separated from the anode by an iron screen which ends in a gauze.

The cell is so designed, that electric current easily passes through it. But Na and Cl, produced make no contact with each other.

WORKING

•   NaCl is fed into the cell. Some CaCl₂ is also added to lower the m.p of NaCl from 801 °C to 600 °C. The cell is operated at this temperature.

• When electricity is passed, Na rises in a special compartment from where it is withdrawn from time to time.

Dry Cl, gas is collected above the anode.

The cell produces dry chlorine gas and 99.9 % pure sodium.

DOWN'S CELL
PREPARATION OF SODIUM HYDROXIDE BY THE DIAPHRAGM CELL

ADVANTAGES:

The Down’s cell is operated at 600 °C which has the following advantages

The metallic fog is not produced

Liquid sodium can be easily collected at 600 °C.

Products formed do not react with the material of the cell.  

Reaction

                             NaCl         Na⁺ +   Cl⁻

At Anode (Oxidation)

                             Cl⁻             ½Cl₂   +  le⁻

At Cathode (reduction)

                             Na⁺    +   e⁻        Na

Net Reaction

                             NaCl         Na⁺   +   ½Cl₂  

COMMERCIAL PREPARATION OF SODIUM HYDROXIDE BY THE DIAPHRAGM CELL

A concentrated solution of NaCl is called brine

In Industry NaOH is prepared by electrolysis of brine on a diaphragm cell called Nelson’s cell.

CONSTRUCTION

There is a shaped diaphragm in it that holds the salt solution.

Graphite anode project into the salt solutions.

A current of steam is passed through the cell to keep the salt solution war be perforation open.

The cathode is made up of oblong perforated steel internally lined with a diaphragm.

There is a special device, which keeps a constant level of salt solution in the cell. an oblong steel tank.

WORKING

Brine is introduced into the cell and its constant level is maintained during the process.

When electricity is passed Cla rises into the dome at the top of the anode while He is liberated at the cathode and escapes through a pipe, where they are collected.

NaOH solution slowly moves downward through the diaphragm and is collected in a catch basin below the diaphragm

REACTIONS

                             NaCl        2Na⁺ +   2Cl⁻

At Anode (Oxidation)

                             2Cl⁻            Cl₂   +  le⁻

At Cathode (reduction)

                               2H₂O    +  2 e⁻       2OH⁻   +   H₂

Net Reaction

                    2NaCl   +  2H₂O        2Na⁺    +  2OH⁻ +  Cl₂  +  H₂         

The overall result of these reactions is that brine loses its CI ions and the solution in the cathode compartment becomes increasingly alkaline.

PROBLEMS

Two problems may occur during the process.

First Problem

Cl₂ produced can react with OH⁻ ions to give OCl⁻ (hypochlorite) ions.

      Cl₂    +   2OH⁻        OCl⁻   +   Cl⁻   +   H₂O    

This problem can be removed by using an asbestos diaphragm which keeps the two solutions separate and allows sodium ions to move to the cathode. The movement of ions keeps the current flowing through the external circuit.

Second Problem

OH⁻ may be attracted to the anode where it can discharge to clue O₂ gas which makes the liberated Cl₂, impure.

                       40H⁻       2H₂O    +     O₂ +     4e⁻

It is removed by keeping the level of brine in the anode compartment slightly higher.

This keeps the direction of the flow of liquid toward the cathode. Thus it prevents the movement of OH towards the anode.

COLLECTION OF NaOH

The solution obtained from the cathode contains 11 % NaOH and 16 % NaCl. This solution is evaporated and less soluble NaCl crystallizes out which is filtered off. The remaining liquid contains 50 % NaOH and only 1 % NaCl which is not harmful for commercial use.

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