This is from Ullmans. I was unable to copy the drawing. It shows a diaphragm between the electrodes of steel. Try 18/10 or 18/18 Stainless. A carbon anode should also work and is used in the Downs NaCl cell. The screen keeps the O2 bubbles from reaching the central cathode and the sodium floats to the top. The cathode is fed from below and the cylindrical anode and screen project above the level of the cell. There is a duct in the annular space between the cathode and screen for the hydrogen.
............Using the same principle by which davy discovered sodium, castner developed a process for producing the metal on a large scale from molten sodium hydroxide in an electrolytic cell (Fig. (2)). This process was operated at Niagara Falls (United States) and Rheinfelden (Germany), and was the only practical method for sodium production from 1881 to the mid-1920s. Cheap hydroelectric power was available at both locations.
The Castner process yields sodium, hydrogen, and oxygen:
4 Na+ + 4 e– ¾® 4 Na (cathode)
4 OH– ¾® 2 H2O + O2 + 4 e– (anode)
2 H2O + 2 Na ¾® 2 NaOH + H2
The overall reaction is:
4 NaOH ¾® 2 Na + 2 NaOH + H2 + O2
Thus the current efficiency cannot exceed 50 %. In practice, current efficiencies of <45 % were achieved.
In the Castner cell (Fig. (2)) a cylindrical nickel anode surrounds the copper cathode concentrically. If NaOH is replaced by a molten salt containing NaCl, the collection and containment of the chlorine evolved cause problems, which are solved only when the positions of the anode and cathode are reversed, as in the Downs cell in which the anode is surrounded concentrically by the cathode.
The Castner process yields sodium, hydrogen, and oxygen:
4 Na+ + 4 e– ¾® 4 Na (cathode)
4 OH– ¾® 2 H2O + O2 + 4 e– (anode)
2 H2O + 2 Na ¾® 2 NaOH + H2
The overall reaction is:
4 NaOH ¾® 2 Na + 2 NaOH + H2 + O2
Thus the current efficiency cannot exceed 50 %. In practice, current efficiencies of <45 % were achieved.