Downs Cell for the Production of Sodium Chemistry Tutorial

Key Concepts

• Sodium metal can be extracted electrolytically from molten (fused) sodium chloride using a Downs Cell.
• Sodium chloride has a high melting point, ~800^oC, so calcium chloride or sodium carbonate is added to reduce the melting point to around 600^oC.
• The products of the electrolysis of molten (fused) sodium chloride are sodium metal and chlorine gas.
• In the Downs Cell a circular iron cathode surrounds the carbon anode and the products of the electrolysis are separated by steel mesh or gauze to prevent them coming into contact and reforming sodium chloride.
• The amount of sodium produced can be calculated using Faraday's Laws of Electrolysis.

Downs Cell

Schematic Diagram of the Downs Cell:

Schematic Diagram of Electrode Arrangement:

• At the carbon anode chloride ions are oxidized to form chlorine gas.

  Chlorine gas is less dense than the molten (fused) sodium chloride so it rises to the surface and can be captured.

• At the iron cathode sodium ions are reduced to sodium atoms.

  Sodium metal is less dense than the molten (fused) sodium chloride so it rises to the surface and can be captured.

• Adding calcium ions in the form of a salt such as calcium chloride does NOT affect the reduction of sodium ions because Ca²⁺ is a weaker oxidant than Na⁺.
• The overall REDOX reaction in the Downs Cell requires a large energy input. About 6 volts and 30,000 amps of electricity are required.

Uses for Sodium

• sodium vapour street lights
• liquid sodium used as a heat exchange medium in fast breeder nuclear reactors
• manufacture of zirconium (Zr), titanium (Ti), sodium cyanide (NaCN), sodium peroxide (Na₂O₂), sodium hydride (NaH)