Manufacture of Caustic Soda (NaOH) from brine

Manufacture of Caustic Soda (NaOH) from brine

Manufacture of Caustic Soda (NaOH) from brine

NaOH (caustic soda) is also formed in large scale by passing electricity through aqueous concentrated solution of NaCl called brine in Nelson’s cell.

Construction

Nelson’s cell is made up of steel tank. A u shaped perforated steel or iron cathode is present in its center along with asbestos diaphragm. It is filled with brine. A graphite anode is suspended in the center of brine. The function of this asbestos diaphragm is to separate the two electrodes that is anode and cathode. The steam is also blown in this cell during electrolysis to keep this solution warm and pores of the cathode clear.

Working

Brine consists of Na⁺¹_, Cl ^–1_, H⁺¹ and OH ^–1 ions. During electrolysis these ions move toward their respective electrodes and following reactions take place.

Ionization of brine

                          2NaCl (aq)   è    2Na⁺¹ + 2Cl^–1

Reaction at cathode

 As both Cl^–1 and OH^{– 1}move towards anode. The Cl^–1 will oxidize first by losing electrons because Cl ^{– 1} is more in concentration than OH^{– 1}

                        Cl ^{– 1} è     Cl + 1e (oxidation)

Reaction at anode

 As both H⁺¹ and Na^{+ 1} move towards cathode. The H⁺¹ will reduce first by gaining electrons because H⁺¹ is less reactive than Na⁺¹

                        H⁺¹ + 1e   è  H (reduction)

In this whole process only Na⁺¹ and OH ^{– 1} is left these will combine with each other and will formed caustic soda that NaOH.

                        Na+¹ + OH ^{– 1}  è NaOH

Over all reaction

                       2Nacl + 2H₂O     è  (electricity passed)   2NaOH + Cl₂ + H₂