Sod!um.— Formerly secured by chemical methods at very high temperatures, the world's supply of sodium is now produced wholly by electrolysis. The operation according to the Castner process is conducted in a cylindrical steel crucible so placed in a flue that the body of it can be heated while the inverted neck, through which the cathode enters from below, remains cool. The electrolyte used is caustic soda. The anode is iron, cylindrical in form, with vertical slits which allow the free flow of the electrolyte: it surrounds the upper end of the cathode, a cylinder of wire gauze hanging between from the collecting chamber above. As the operation proceeds, molten metallic sodium is released at the cathode and rises to float on the surface of the caustic soda, being giiided by the wire gauze, which it cannot pass owing to its high surface tension. In the col lecting chamber the metal is protected by the hydrogen also set free at the cathode, and is drawn off at intervals through a trap.
The Seward and von Kugelgen process uses an electrolyte of fused sodium chloride, the melting point of which has been reduced by the addition of other salts less readily decomposed by the current.
Potassium.— There is little commercial de mand for metallic potassium. Its production. however, is entirely practicable by the Castner process described for sodium, using caustic potash as the electrolyte.
Electroplating and Electrotyping.— Elec troplating is the art of covering a metallic sur face with an adherent, electrodeposited coating of the same or some other metal, the form of the original surface being retained. The metal coating may be added purely for decorative purposes, or because of its superior resistance to wear or corrosion. Electrotyping is the art of reproducing the form of an object by electro deposition of a metal, usually copper, in a cast or molded impression of the original object. For further details of these processes, the reader is referred to the articles under these separate heatig.
Caustic .—The production of caustic soda(NaOH) and chlorine (CO by the lysis of a solution of common salt (NaCl) is readily realized experimentally (NaCI + H30 Cl H), but its successful plishment on a commercial basis is difficult cause of the secondary reactions which take place, forming a mixed product of caustic, salt, and hypochlortte of soda. These difficulties are avoided by separating the caustic soda that is formed, either by a porous diaphragm, by ing it off as soon as formed, or by absorbing the sodium deposited in mercury or melted lead. The most prominent system for the lyfic production of caustic soda and chlorine from common salt is the Castner-Kellner ess. The Castner process employed in this
country at Niagara Falls is as follows: The electrolytic tank consists of a slate box, 4 feet long, 4 feet wide and 6 inches deep, the joints being set in rubber cement. Two slate tions reaching within 1/16 inch of the bottom (under which are grooves) divide the cell into three compartments, each 15 inches by 4 feet, sealed from each other by a layer of mercury covering the bottom of the tank to a able depth. The two outside chambers through which the brine is passed are provided with graphite anodes. These compartments are vided with gas-tight covers and exhaust pipes of rubber and lead, to lead the chlorine away. The central compartment has an iron cathode, of 20 upright strips, and is filled with pure water above the mercury. Whenever the specific gravity of the water rises to 1.26, from its absorption of caustic soda, it is drawn off and fresh water supplied. The liberated gen is led from this chamber by means of pipes and used as a fuel for the concentration of the caustic. The tank is pivoted at one end on a knife blade and rests at the other on an tric, which raises and lowers that end of the tank about half an inch every minute and causes a circulation of the mercury between the outer and middle compartments. The current passes into the outer chambers, splits up the sodium chloride (common salt, NaC1) into sodium and chlorine (Na and CO, the latter is liberated at the graphite anodes and passes through the exhaust pipe to the absorption chambers where it combines with slacked lime to form bleaching powder. The sodium combines with the mer cury forming sodium amalgam, which by the rocking of the tank passes to the center cham ber, where it serves as the anode, and combines with the water to form caustic soda (NaOH) and hydrogen (H), which appears at the iron cathode. Each of these tanks uses 630 amperes at 4.3 volts. The theoretical voltage required is but 2.3, the remainder being utilized in over coming the ohmic resistance of the electrolyte and in keeping it warm. The output of this process per horse power per day is 12 pounds of caustic and 30 pounds of bleaching powder for each cell. The product contains from 97 to 99 per cent caustic, 1 to 2 per cent sodium carbonate, 0.3 to 0.8 per cent of sodium chloride, and traces of sodium sulphate and sodium sili cate. A number of other processes have been de veloped for the electrolysis of salt solutions for the production of alkali and chlorine, all based more or less on the same general principles as those utilized in the Castner process.