Downs cell

Operation

The Downs cell uses a carbon anode and an iron cathode, with molten sodium chloride as the electrolyte. Although solid sodium chloride is a poor conductor of electricity, when molten, the sodium and chloride ions are mobilized. These become charge carriers and allow conduction of electric current.

Some calcium chloride and/or chlorides of barium (BaCl2) and strontium (SrCl2), and, in some processes, sodium fluoride (NaF){{cite web

The anode reaction is:

: 2Cl− → Cl2 (g) + 2e−

The cathode reaction is: : 2Na+ + 2e− → 2Na (l)

for an overall reaction of

: 2Na+ + 2Cl− → 2Na (l) + Cl2 (g)

As calcium has a lower reduction potential than sodium (-2.87 volts, compared to -2.38 volts for sodium). the calcium does not react, whilst the sodium ions are reduced. If the sodium ions were absent, calcium metal would be 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 to direct the products into separate chambers, preventing them from making contact.

Although theory predicts that a potential of a little over 4.07 volts should be sufficient to cause the reaction to go forward, in practice potentials of up to 8 volts are used to achieve useful current densities in the electrolyte despite its inherent electrical resistance. The overvoltage and consequent resistive heating also provides additional heat, helping to keep the electrolyte molten.

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.

References

References

1. link. (2018-07-25 U.S. Patent no. 1,501,756 (filed: 1922 August 18 ; issued: 1924 July 15).)
2. (1959). "Electrolytic Manufacture of Chemicals from Salt". Oxford University Press.
3. "Sodium Production by Electrowinning". corrosion-doctors.org.
4. Pauling, Linus, ''General Chemistry'', 1970 ed. Dover Publications, pp 539-540