One condition is that the salt shall fuse at a convenient temperature-that is, at or below a red heat. Another is that, in the case of metals which are commercially used, the salts must be cheaply obtainable, and the metals easily separated from the salts.
It is interesting to note that this process led, in the hands of Sir Humphry Davy, to the discovery of the metals of the alkalies, potassium and sodium; he first prepared them by passing a current from a battery of high voltage through the hydroxide, melted on a piece of platinum foil. The metal was visible only for an instant; for it floated up from the electrode of platinum wire, and burst into flame as soon as it came into the air.
As a rule, however, the chlorides are the most convenient salts for electrolysis. From the known fact that the melting-point of a compound is lowered by the presence of an " impurity,'9 it is often found advantageous to electrolyse a mixture of chlorides rather than a pure chloride; in this case one of the elements is liberated in preference to the other. As the anode has to withstand the action of chlorine, it is always made of carbon, which does not unite with chlorine directly; the kathode may be of iron, a metal which has no tendency to form alloys with those which are prepared in this way, at least at the temperatures required. The kathode may be the iron pot in which the chloride is kept fused.
The elements which are prepared in this way are: lithium, sodium, potassium, rubidium, caesium, beryllium, magnesium, calcium, strontium, and barium. The first five are easily fusible white soft metals, which take fire when heated in air, and must therefore be kept in an atmosphere free from oxygen; they also attack water,liberating hydrogen, with formation of the hydroxide MOH. Their density is so low that they float on their fused chlorides; they must, therefore, be liberated in the interior of a bell-shaped iron electrode or of a fireclay receptacle, down which an iron kathode passes. Beryllium and magnesium are better prepared from a mixture of their chlorides with potassium chloride; the metal melts and collects at the bottom of the pot, which, in this case, may be the kathode. They are hard white metals, magnesium melting at about 7500, and beryllium about 1200°. They, too, take fire when heated in air, and burn with a brilliant flame; indeed, the chief use of magnesium is for signalling purposes. The metal is drawn, while hot, into wire, which is then rolled into ribbon; this ribbon burns with an exceedingly bright flame, producing the oxide MgO. Calcium, strontium, and barium are also white metals; they have been produced by electrolysis of their cyanides, M(CN)2, compounds which fuse at a lower temperature than the chlorides. Calcium is also produced on a large scale by electrolysing its fused chloride; an iron rod dips in the liquid, which is withdrawn as the calcium deposits. A thick stick of calcium is thus produced. These metals are very readily attacked by water, yielding the hydroxides M(OH)2. The only two which find commercial use are sodium and magnesium.
Aluminium, which is also manufactured on a large scale, is produced from its ore, bauxite, from which pure alumina, the oxide, is first prepared. The alumina is dissolved in fused cryolite, a fluoride of aluminium and sodium of the formula Na3AlF6, deposits of which occur in Greenland. On electrolysis, the aluminium sinks to the bottom of the crucible, and when a sufficient quantity accumulates it is tapped out. The " flux," as the cryolite is termed, is again melted, and a further quantity of alumina is dissolved in it. The metal is fairly hard, white, susceptible of a high polish, ductile and malleable. It is also very light (about two and a half times as heavy as water), and not easily oxidised in air at the ordinary temperature, nor is it attacked by water.
Gallium, a tin-white, hard metal, very rare, contained in some zinc ores, is deposited from a solution of its hydroxide in caustic potash. Copper prepared, as will be seen below, in a crude state by displacement, is purified by electrolysis. It is of the utmost importance to employ pure copper for the conduction of electric currents 5 for although copper is one of the best conductors, its resistance is enormously increased by the presence of a very small trace of impurity. To purify it, large rectangular blocks of crude copper are suspended close to thin sheets of pure copper in an acid bath of copper sulphate, CuSO4.Aq. The heavy block is made the anode and the thin sheet the kathode; the thick block, being deprived of electrons, dissolves as copper ions, Cu; while the cuprtony
Cu, in receiving its electrons from the kathode, deposits on the latter and increases its thickness. The impurities, arsenic, antimony, and iron, remain in solution, and a sludge is deposited containing silver and gold, besides traces of many other elements. Copper is a very malleable, ductile red metal, melting at 13300.
Objects of iron are often " nickel-plated," or covered with a thin film of nickel, a white, hard metal which preserves its lustre in air, for it is not easily oxidisable. This is done by making the object to be coated with nickel the kathode and a bar of nickel the anode; the liquid is a solution of oxalate of nickel and potassium. Iron objects are first coated with copper before nickelling. Silver and gold are best deposited from their double cyanides with potassium; these salts are used because the deposit is harder and more uniform than if a halide be used. In thus coating objects, it is of importance that the current density, i.e. the ratio of the current to the area of the surface of the object to be coated, should be considered ; if this be too high, the metal will be deposited in a loose, flocculent condition.
As an illustration of the changes which take place during such electrolysis, the deposition of silver may be chosen. The compound employed is, as stated, the double cyanide (see p. 187); its formula is-
KAg(CN)2, and the ions are K and Ag(CN)2-.
There are, however, at the same time a few ions of Ag and (CN)-. From the last, metallic silver is deposited on the kathode; and as soon as its amount is reduced, a fresh quantity is formed by the decomposition of the complex ion, Ag(CN)2. The formation and deposition of the silver ion goes on continuously until all the silver required has been deposited. Similar changes take place during the electro-deposition of nickel and of gold.
Modern electrolytic processes for obtaining chlorine and caustic soda (NaOH) from salt result in the liberation of enormous quantities of hydrogen. The salt, dissolved in water, is placed in a tank divided into two compartments by a porous diaphragm ; the anode, which consists of carbon rods, dips into one, and the kathode, which may be formed of copper plates, in the other. The ions, of course, are Na.Aq, and CI-Aq. The chlorine is liberated at the anode, and the sodium at the kathode. But as soon as the sod ion is discharged, it reacts with the water, forming caustic soda, thus: 2Na + 2HOH = 2NaOH + H2. Hence the production of hydrogen. Bromine and iodine may be liberated in the same way as chlorine, the bromide or iodide of sodium or potassium being substituted for the chloride. As fluorine at once acts on water, liberating oxygen in the form of ozone, O3, it cannot be produced from an aqueous solution of a fluoride; but Moissan found that liquid hydrogen fluoride has ionising power, so that on passing a current between poles of platinum-iridium (an alloy of metals which is less attacked by fluorine than any other conductor) through a solution of hydrogen-potassium fluoride, HKF, in pure liquid hydrogen fluoride, H2F2, at -30% fluorine is evolved from the anode as a pale yellow gas with a strong characteristic smell, somewhat resembling that of the other halogens, chlorine, bromine, and iodine; while hydrogen is evolved at the kathode, having been produced by the action of the potassium on the hydrogen fluoride. Fluorine boils at -1950, chlorine at -35°, bromine at 59% and iodine, which is a solid at atmospheric temperature, melts at 1140 and boils at 1840. The colours of these elements also show a gradation. Chlorine is greenish-yellow; bromine, red both as gas and liquid; iodine is a blue-black solid and a violet gas. These three elements are somewhat soluble in water, and more so in a solution of their soluble salts. It has recently been found that another ionising agent than water may be used. Lithium chloride is soluble in pyridine, a compound of the formula C5H5N, and may be electro-deposited on a platinum kathode from such a solution.
The metal is not attacked by pyridine; the chlorine, however, is rapidly absorbed.