Sodium was ‘rst isolated by Sir Humphry Davy in 1807 in a series of experiments investigating the action on matter of the then newly discovered phenomenon of electric current [5]. A small amount of wet caustic soda placed between two platinum electrodes and heated by an applied current rapidly passes from the aqueous to the molten phase, which is then electrolyzed to produce small globules of sodium metal. However, it wasn’t until 1891 that William Castner patented a commercial process based on this reaction, having discovered that Davy’s method only works continuously if the melt temperature is held about 10°C, and no more than 20°C, above the mp of the caustic[6]. His patent consisted of an apparatus for realizing this fairly dif‘cult requirement, when 64-75% of the supplied electrical energy is dissipated as heat in the sodium hydroxide cell, and the electrolyte is a fairly poor thermal conductor compared to sodium metal. His solution included the use of a bottom-side cathode, so that the sodium rises out of the hot electrolysis zone due to the large density differential between molten sodium and sodium hydroxide (sp gr 0.817 and 1.70 at 320°C, respectively), and thick solid electrodes to provide the necessary thermal conduction as well as an iron mesh separating the cathode and anode compartments. The mesh is an essential and fastest-wearing component of the cell, its pore size is a compromise between large pores needed for good circulation to avoid unacceptable

temperature gradients in the bath and small pores that provide effective physical separation of the cathode and anode products.