Castner Kellner’s Process

Introduction

Caustic Soda or Sodium Hydroxide is one of the most important chemicals of industrial use. Caustic Soda must be handled carefully because it is caustic to touch and causes painful burns.

Castner Kellner’s Process

Caustic Soda is manufacture by an electrolytic process. This process is carried out in an electrolytic cell known as Castner-Kellner’s cell and the process is called Castner-Kellner’ Process.

Construction Of Castner-Kellner’s Cell

The electrolytic solution is a 25% of NaCl solution. The anode consist of a number of titanium plates whereas cathode is a steam of flowing mercury. The Castner-Kellner’s cell can be shown diagrammatically in the following figure.

Figure

Working Of Castner-Kellner’s Cell

Sodium chloride dissociates in water to give Na⁺ and Cl^- ions as:

2NaCl ® 2Na⁺ +2Cl^-

The Cl^- ions migrate towards titanium plates, it gains electron and convert into chlorine gas.

2Cl^- ® Cl₂ + 2e^- (Oxidation)

In castener-Kellner’s process, H⁺ ions are not easily discharged due to high voltage of H⁺ ions, on the contrary Na⁺ ions are easily discharge over mercury surface. The sodium, thus liberated dissolves in mercury forming an amalgam.

2Na⁺ + 2e^- ® 2Na (Reduction)

Na + Hg ® Na/Hg (Amalgum)

The mercury containing dissolved sodium is sent to another chamber called Denuder where sodium reacts with water forming sodium hydroxide and hydrogen. Denuder is packed with graphite blocks as hydrogen is easily liberated over graphite surface.

2Na/Hg + 2H₂O ® 2NaOH + H₂ + 2Hg

The mercury is recycled to dissove more of sodium.

The solution, which flows out from denuder, is a NaOH solution, which is evaporated to dryness.