|Davy's original method can be modified by electrolyzing a concentrated solution of the hydroxide in contact with mercury, the resulting sodium-amalgam being decomposed by volatilizing the mercury in an atmosphere of petroleum-vapour. |
Several electrolytic methods have been employed in the isolation of the metal on the manufacturing scale. Castner's patent is worked by the Castner Kellner Co. at Wallsend-on-Tyne. It depends on the electrolysis of fused sodium hydroxide at about 330° C. The fusion is carried out in a gas-fired (G) iron furnace-pan (P), surrounded by brickwork (not shown in the figure). The metallic negative electrode ( - E) is introduced through the bottom of the pan, and its lower part is surrounded by a seal of solid sodium hydroxide (H). The positive electrode (+ E) encircles the upper part of the negative electrode, but is separated from it by a diaphragm consisting of a cylinder of wire- gauze (D) attached to the bottom of the chamber (C). Being specifically lighter than the electrolyte, the liberated sodium (S) rises to the surface. It is directed by the diaphragm (D) into the tubular iron chamber (C) placed over the negative electrode (-E), and is collected by means of ladles perforated to allow the molten hydroxide to drain off. The gas liberated escapes at the opening (O). The periodic addition of fresh sodium hydroxide enables the process to be carried on continuously. A current of 1000-1200 amperes at 4-5 volts is employed, and serves to maintain the temperature after fusion is complete. The yield of sodium is between 40 and 50 per cent, of the theoretical amount.
|Castner's apparatus for the electrolytic production of sodium |
In another electrolytic method formerly worked commercially, fused sodium chloride was employed as electrolyte. There are several practical difficulties to be overcome in carrying on this process, due partly to the corrosive nature of the chlorine liberated, and partly to the tendency to form the so-called subchloride of sodium. Either the formation of this subchloride must be prevented; or, if produced, it must not be permitted to regenerate sodium chloride by interaction with the chlorine evolved at the anode. The chlorine can be removed by contact at the anode with a heavy metal, such as lead, copper, or silver. Lowering the temperature of fusion by admixture with chloride of potassium or of an alkaline-earth-metal, or with sodium fluoride, prevents the formation of the subchloride.
Electrolytic processes are gradually displacing the older chemical methods of isolating sodium dependent on the reduction of the carbonate or hydroxide with charcoal or iron. On a small scale, magnesium can be employed as reducer. A laboratory method is the reduction of the peroxide with wood-charcoal, coke, graphite, or calcium carbide -
3Na2O2+2C = 2Na2CO3+2Na;
7Na2O2+2CaC2=2CaO + 4Na2CO3+6Na.
Hydrogen is a usual impurity in metallic sodium, and is evolved when the metal reacts with mercury. It can be removed by prolonged heating in vacuum.