Downs Cell
Definition and meaning of Downs Cell in chemistry.
A Downs cell is an electrolytic cell used to produce metallic sodium and chlorine gas by the electrolysis of molten sodium chloride mixed with calcium chloride. It is the primary industrial method for manufacturing sodium metal.
In more detail
The cell contains molten NaCl (mixed with CaCl2 to lower the melting point from 801°C to around 600°C, reducing energy cost) held between a central cylindrical carbon anode and a surrounding iron cathode, separated by an iron screen. At the cathode, Na+ ions are reduced to liquid sodium metal, which is less dense than the melt and rises through a collection hood; at the anode, chloride ions are oxidized to chlorine gas. The screen keeps the two products physically separated so the highly reactive sodium and chlorine do not recombine.
Key facts
| Field | Inorganic Chemistry |
|---|---|
| Electrolyte | Molten NaCl-CaCl2 mixture (~600°C) |
| Cathode product | Liquid sodium metal, Na(l) |
| Anode product | Chlorine gas, Cl2(g) |
Cathode reaction: Na+(l) + e- → Na(l). Anode reaction: 2Cl-(l) → Cl2(g) + 2e-. Overall: 2NaCl(l) → 2Na(l) + Cl2(g), driven by direct current from an external power source.
Frequently asked questions
Why is calcium chloride added to the Downs cell?
CaCl2 lowers the melting point of the NaCl mixture from 801°C to roughly 600°C, which reduces energy consumption; the calcium is not appreciably reduced under normal operating conditions because Na+ is preferentially discharged, though trace calcium can co-deposit and is later removed.
Why can't the Downs cell use aqueous NaCl instead of molten NaCl?
In water, sodium metal would react violently, and water is preferentially reduced at the cathode to hydrogen gas rather than sodium; a molten, water-free electrolyte is required to obtain sodium metal directly.