Downs cell

The Downs process is a method for the commercial preparation of metallic sodium, in which molten NaCl is electrolyzed in a special apparatus called the Downs cell.

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How it works

  The Downs cell uses a carbon anode and iron cathode. The electrolyte is sodium chloride that has been fused to a liquid by heating. Although crystaline sodium chloride is a poor conductor of electricity, fusing it mobilizes the sodium and chloride ions, which become charge carriers and allow conduction of electric current.

Some calcium chloride and/or sodium carbonate is added to the electrolyte to reduce the temperature required to keep the electrolyte liquid. Sodium chloride normally melts at 804 °C, but the mixture can be kept liquid at temperatures around 600 °C.

The anode reaction is:

2Cl– → Cl2 + 2e–

–1.358 volts

The cathode reaction is:

2Na+ + 2e– → 2Na

+2.712 volts

for an overall reaction of

2Na+ + 2Cl– → 2Na + Cl2

4.070 volts

The calcium does not enter into the reaction because its reduction potential of 2.87 volts is higher than that of sodium. Hence the sodium ions are reduced to metallic form in preference to those of calcium.^[1] If the electrolyte contained only calcium ion and no sodium, it would be calcium metal produced as the cathode product (which indeed is how metallic calcium is produced).

Both the products of the electrolysis, sodium metal and chlorine gas, are less dense than the electrolyte and therefore float to the surface. Perforated iron baffles are arranged in the cell (see the diagram to the right) to direct the products into separate chambers without their ever coming into contact with each other.^[2]

Although theory predicts that a potential of a little over 4 volts should be sufficient to cause the reaction to go foward, in practice potentials of up to 8 volts are used. This is done in order to achieve useful current densities in the electrolyte despite its inherent electrical resistance. The overvoltage and consequent resistive heating contributes to the heat required to keep the electrolyte in a liquid state.

The Downs process also produces chlorine as a byproduct, although chlorine produced this way accounts for only a small fraction of chlorine produced industrially by other methods.^[2]

References

1. ^ Sodium Production by Electrowinning. corrosion-doctors.org. Retrieved on 2007-12-20.
2. ^ ^{a b} Pauling, Linus, General Chemistry, 1970 ed. Dover Publications, pp 539-540