Sodium is important in many technical processes and is therefore
prepared in considerable quantity. Almost all of it is now made by
electrolysis of the fused sodium chloride, using the Downs cell (see
Figure 6.1). The graphite anode is cylindrical and is surrounded by
the steel gauze diaphragm and the concentric cylindrical cathode
(also of steel). The electrolyte is usually a mixture of sodium chloride
and calcium chloride; the latter is added to reduce the m.p. of the
sodium chloride to approximately 800 K. (Some calcium is therefore
liberated with the sodium.) The gap between anode and cathode is
kept as small as possible to reduce resistance: the heat developed
by the current maintains the temperature of the cell. Chlorine is set
free at the anode surface, rises into the nickel cone and can be
collected. Sodium, liberated at the cathode, is prevented by the
diaphragm from passing into the anode region; the molten sodium
collects under the circular hood and rises up the pipe, being assisted