Down's Cell for the Production of Sodium |
Key Concepts
- Sodium metal can be extracted electrolytically from molten (fused) sodium chloride using a Down's Cell.
- Sodium chloride has a high melting point, ~800oC, so calcium chloride or sodium carbonate is added to reduce the melting point to around 600oC.
- The products of the electrolysis of molten (fused) sodium chloride are sodium metal and chlorine gas.
- In the Down's 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.
Down's Cell
Schematic Diagram of the Down's Cell
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Anode: (positive carbon electrode)
2Cl-(l) → Cl2(g) + 2e
Cathode: (negative iron electrode)
Na+(l) + e → Na(l)
Overall REDOX Reaction:
2Cl-(l) + 2Na+(l) → Cl2(g) + 2Na(l) |
- 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 Down's Cell requires a large energy input. About 6 volts and 30,000 amps of electricity are required.
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Schematic Diagram of
Electrode Arrangement
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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)
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