CHEMISTRY OF SODIUM HYDROXIDE
 
Commercial name : caustic soda
   On industrial scale sodium hydroxide can be prepared by the following methods.
   1) Castner - Kellner Process.
   2) Gibb's Method.
   3) Nelson's Method.
Castner - Kellener Process
 
Principle
 
   In castner-kellner method NaOH is prepared by the electrolysis of aqueous solution of NaCl(Brine).
Concentration of brine
 
   25 % mass/mass
   i.e. 25 gm of NaCl is dissolved in 75 gm of water.
Castner-kellner cell
 
   It is a rectangular tank of steel.
   Inside of tank is lined with "ebonite".
   Anode is made of titanium.
   Flowing layer of mercury (Hg) at the bottom of tank serves as cathode.
Details of process
   Ionization of NaCl
                                                                         2NaCl
è 2Na+ + 2Cl-
   When electric current is passed through brine, +ve and -ve ions migrate towards their respective    electrodes. Na+ions are discharged at mercury cathode. The sodium deposited at mercury forms SODIUM    AMALGAM. Chlorine produced at the anode is removed from the top of the cell.
Reaction at cathode
2Na+ +2 e- è 2Na
   Na forms amalgam.
Na + Hg è Na/Hg
   Na+ ions are discharged in preference to H+ ions due to high over voltage.
   Na+/Na: E.P. = -2.71 volt
   H+/H : E.P. = 0.00 volt
Reaction at anode
2Cl- è Cl2 + 2e-
Formation of NaOH
   Amalgam moves to another chamber called "denuder", where it is treated with water to produce NaOH    which is in liquid state. Solid NaOH is obtained by the evaporation of this solution.
2Na/Hg + 2H2O è 2NaOH + H2 + 2Hg
Advantages of castner's process
   NaOH obtained is highly pure.
   The process is very efficient.
   Possible reaction between NaOH and Cl2 is avoided as NaOH is obtained in a separated chamber.
Disadvantages
   High electricity consumption.
   Environmental pollution due to escape of Hg vapours.
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