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. General Name, Symbol, Number sodium, Na, 11 Chemical series alkali metals Group, Period, Block 1, 3, s Appearance silvery white Atomic mass 22. ...

How it works

Schematic diagram of the Downs' Cell

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. General Name, Symbol, Number carbon, C, 6 Chemical series nonmetals Group, Period, Block 14, 2, p Appearance black (graphite) colorless (diamond) Atomic mass 12. ... Diagram of a zinc anode in a galvanic cell. ... General Name, Symbol, Number iron, Fe, 26 Chemical series transition metals Group, Period, Block 8, 4, d Appearance lustrous metallic with a grayish tinge Atomic mass 55. ... Diagram of a copper cathode in a Daniells cell. ... An electrolyte is a substance containing free ions which behaves as an electrically conductive medium. ... This articles section called Other facts does not cite its references or sources. ... Multivalent redirects here. ...

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. Calcium chloride is a chemical compound of calcium and chlorine. ... Sodium carbonate (also known as washing soda or soda ash), Na2CO3, is a sodium salt of carbonic acid. ...

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. 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]
2. ^ ^{a b} Pauling, Linus, General Chemistry, 1970 ed. Dover Publications, pp 539-540