SODIUM HYDROXIDE OR CAUSTIC SODA (NaOH) || CAUSTIC SODA || SODIUM HYDROXIDE || NaOH ||

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SODIUM HYDROXIDE OR CAUSTIC SODA (NaOH):

PREPARATION: It is the most easily  manufactured by one of the following process:

(a) Methods involving sodium carbonate(Na2CO3) as a starting material :

» Gossage process (causticising process) :

          Na2 CO3+ Ca(OH)2 → Ca CO3 + Na OH

The most suitable concentration of sodium carbonate(Na2 CO3) taken in this process is about 15% – 20%. The caustic soda(Na OH) produced by this process is not pure and contains some amount of calcium carbonate(Ca CO3) , sodium carbonate and calcium hydroxide(Ca(OH)2 )as impurities.

» Lowig’s process :

         Na2 CO3+  Fe2 O3 → 2 Na Fe O2 + CO2

The sodium ferrite(Na Fe O2) is cooled and thrown into hot water, the hydrolysis of sodium ferrite(Na Fe O2) occurs forming Na OH.

        2 Na Fe O2 + H2O → 2 Na OH + Fe2 O3

» Methods involving sodium chloride as starting material :

Electrolysis(treatment with electricity) of sodium chloride(Na Cl) solution occurs according to following principle.

Na Cl →Na+ + Cl¯       ;    H2O→  H+  + OH¯

On passing electricity,  Na+ and H+ ions move towards cathode and Cl¯ and OH¯ ions move towards anode.

The discharge potential of H+ ions is less than Na+ ions, thus hydrogen ions get discharged easily and hydrogen gas is liberated. Similarly, at anode Cl ions are easily discharged as their discharge potential is lesser than that of OH¯ ions. Cl2 gas is, therefore, liberated at anode.

It is necessary that chlorine(Cl) should not come in contact with sodium hydroxide(Na OH) during electrolysis, otherwise following reaction takes place.

       2Na OH + Cl2 → Na Cl + Na ClO + H2O

To solve this problem, the anode is separated from the cathode in the electrolytic cell either by using a porous(hole-like) diaphragm or by using a mercury(H g) cathode. 

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