Analytical Chemistry
By SameenFatima
Roll No.908529
Caustic Soda

Submitted to Dr Tahir Mehmood
Submitted by Sameen Fatima
Roll No. 908529
Semester 6th
Course Code CHM-508
Course Title Analytical Chemistry II
Department Chemistry
Topic: Raw material, Flow sheet diagrams, Unit operations and unit
processes of caustic soda.

Raw material, Flow sheet diagrams, Unit operations and unit processes of
caustic soda.
Raw materials of Caustic Soda:
Sodium Chloride is the raw material that is used to produce caustic soda. Sodium chloride NaCl
is also called Common table salt. On the other-hand water is used to form brine solution my
mixing with this table salt.
Processes offorming caustic soda:
Sodium hydroxide that is also called caustic soda is prepared by following three processes.
 Castner-Kellner process.
 Nelson Diaphragm cell.
 Member Cell.
DiscussionofCastner-Kellnerprocess:
Principle: The brine solution used as an electrolyte in this
process.
Cell: It consists of a rectangular shape tank; ebonite material is
lined inside the tank. The mercury which is resided at the
bottom acts as cathode and the anode is made up of graphite.
The apparatus shown is divided into two types of cells
separated by slate walls. The first type, shown on the right and
left of the diagram, uses an electrolyte of sodium chloride solution, a graphite anode (A), and
a mercury cathode (M). The other type of cell, shown in the center of the diagram, uses an
electrolyte of sodium hydroxide solution, a mercury anode (M), and an iron cathode (D). The
mercury electrode is common between the two cells.
This is achieved by having the walls separating the cells dip below the level of the electrolytes
but still allow the mercury to flow beneath them.
Process: First ionization of brine solution takes place and the reaction is:
Ebonite isthe hard rubberwhichis
formedbyvulcanizingnatural rubber.
Andmay produce bySulphurand
linseedoil.

This ionization happens when electric current is passed through the brine solution. The positive
and negative ions then move toward electrodes. The sodium ions deposit at mercury cathode and
there, Na+ ions form amalgam by combining with mercury and negative ions deposit at positive
anode.
At cathode:
At anode:
The sodium metal formed by this reaction dissolves in the mercury to form an amalgam. The
mercury conducts the current from the outside cells to the center cell. In addition, a rocking
mechanism (Bshownby fulcrumonthe leftand rotatingeccentriconthe right) agitatesthe mercuryto
transportthe dissolvedsodiummetal fromthe outside cellstothe centercell.
The anode reactioninthe centercell takesplace atthe interface betweenthe mercuryandthe sodium
hydroxide solution.
At iron cathode which is on point D in the diagram:
Now the concentration of NaOH increases in the center cell and concentration of NaCl decreases
in the outside cells.

NelsonDiaphragm cell.
Member Cell.

Flow Sheetdiagrams:
According to Castner kellner cell:

According to Diaphragm cell:

According to Member cell:

What Are Unit operations:
Such operations which bring physical change in the process and no chemical changes occur. E: g
distillation, crystallization, mixing, blending, crushing, filtration, boiling, freezing etc.
For example: if we are processing the juice then its unit operations are:
Unit Operation Result
Mass transfer Fruit delivered, dry cleaned
Extraction Washed
Separation Sized, graded
Separation Peeled, cored and deseed
Size reduction Crushed, comminuted
Pressure application Juice extracted
Separation Solids screened
De-aeration Oxygen removed
Centrifugation Solids separated
Filtration Clarification
Fluid flow Juice transferred, pumped
Heat transfer Enzymes inactivated, juice pasteurized and cooled
Concentration/evaporation Volume reduction, stability
Mass transfer Packaging, shipping
Unit operations that occur in CastnerKellner process:
 Preparation of brine solution: Mixing of table salt in water.
 Filtration of brine solution after it’s precipitation.
 Removal of residue.
 Mixing of purified brine.
 Crystallization of NaCl in the solution after adding of HCl.
 Removal of chlorine gas.
 Cooling of Cl2
 Drying of Cl2
 Compression of Cl2

 Liquefaction of chlorine gas: changing of Cl2(g) into liquid.
 Evaporation of liquefied gas.
 Cooling of caustic solution.
 Removal of mercury from it.
 Storage of NaOH.
 Cooling of H2 after decomposition of amalgam.
 After cooling removal of mercury from it, then we get pure hydrogen (gas).
What Are Unit Processes:
Such processes in which chemical changes take place are called unit processes.
E: g reduction, oxidation, halogenation, sulphonation, nitration, electrolysis etc.
For example: if methane (CH4) is reacted with water (or hydrolyzed) in the presence of heat then
carbon monoxide (CO) and 3H2 is formed.
Unit Processes in Castnerkellnerprocess:
 Precipitation of brine solution.
 Electrolysis: Applying current from battery to anode and cathode.
 DE chlorination of anolyte: removing residual chlorine.
 Decomposition of amalgam.
 Formation of NaOH by adding H2O to Na+ ion.
The unit operations and unit processes of the Diaphragm cell are almost the same as in
the Castner Kellner cell.
And in the member cell, the unit operations are the same as in the above two but the
only unit process is different that is an ion-exchange method (that is used for
purification).