Kellner-Solvay Cell

This cell has replaced Castner–Kellner cell to some extent. Kellner–Solvay cell consists of a rectangular trough, at the base of which there is a thin layer of mercury which flows from one end to the other acting as a cathode. A concentrated brine solution flows slowly through the cell in the same direction as the mercury and is maintained at a constant level.

The graphite anodes are suspended in the brine. During electrolysis, Cl ions are oxidized at the anode and chlorine gas goes out of the cell, while sodium ions are reduced at the mercury cathode forming sodium amalgam.
The sodium amalgam leaves the cell on the left and flows into an iron tank where it is decomposed by a calculated amount of water giving caustic soda, mercury and hydrogen.

Mercury so produced is sent back to the cell. The spent brine solution leaving the cell is saturated with more common salt and returned to the cell.

Using a diaphragm cell: In this method, sodium hydroxide is obtained from brine (20% NaCl solution) by electrolysis. The reactions are

At anode:

At cathode:

(from water) It may be noted here that it is H+(aq) and not Na+(aq) which is involved in the electrode reaction (reduction) at the cathode. The presence of free Na+(aq) and OH−(aq) in the solution thus yields a NaOH solution. Hydrogen gas is obtained as a by–product at the cathode. Chlorine gas is obtained at the anode.

In this method, care is taken not to allow NaOH and Cl2 to come in contact with each other. This is done to prevent the formation of sodium hypochlorite.

In modern cells, the diaphragm is replaced by a Nafion membrane.

Using Nafion membrane cell(Nelson cell): Now a days, sodium hydroxide is obtained by the electrolysis of brine (20% NaCl solution) in a membrane cell in which the cathodic and the anodic compartments are separated by a Nafion membrane. Nafion is a copolymer of tetra–fluoromethylene and penta fluoro–sulphonyl ethoxy ether. The co–polymer is supported on a teflon mesh.

The Kellner–Solvay cell

The Nelson cell for the manufacture of sodium hydroxide, chlorine and hydrogen gases are valuable by–products.