Acids, Alkalis and Salts

by George Henry Joseph Adlam

The problem of making ammonia from the air is closely connected with that of making nitric acid from the same source. In some experiments the two are combined, and ammonium nitrate is produced directly. Ammonia made by the Haber process, or some modification, is mixed with atmospheric oxygen and passed through platinum gauze heated to low redness. This results in the formation of nitric oxide, which is further oxidized by atmospheric oxygen; and finally, from a mixture of oxides of nitrogen, water vapour, and ammonia, synthetic ammonium nitrate is obtained.

CHAPTER X
ELECTROLYTIC METHODS

One of the most noteworthy developments of modern chemical industry has been the increasing use of electricity as an agent for bringing about changes in matter. This has followed naturally from the reduction in the cost of electricity, due in great measure to the utilization of natural sources of energy which for untold ages had been allowed to run to waste.

This last achievement is likely to produce such a change in economic conditions that it is worth while giving a little thought to what may be called a newly-discovered asset of civilization. One example will make this clear. In the bed of the Niagara river, which flows from Lake Erie to Lake Ontario, there is a sudden drop of 167 ft. over which the water rushes with tremendous force and expends its energy in producing heat which cannot be utilized. This is a waste of energy, but it cannot be circumvented because no method has yet been found to control the waters of the Falls themselves. Nevertheless, by leading the head waters through suitable channels from the high level to the low, it is possible to use the energy to drive turbines, which, in their turn, drive dynamos which produce the current. This is merely the conversion of the energy of running water into electrical energy; and while the sun remains, this supply of energy will be forthcoming in undiminished quantity, because by the heat of the sun the water is lifted again as vapour, which descends as rain to replenish the sources from which the Niagara flows.

Electricity is employed in chemical industry in two ways. In the first place, it may be used to produce very high temperatures required for the reduction of some metallic ores, for melting highly-refractory substances, and for making steel. It is, however, rather with the second method, called electrolysis, that we are here mainly concerned.

Fig. 15. THE ELECTROLYSIS OF SALT SOLUTION

Solutions of acids, bases, and salts, and in some cases the fused substances themselves, conduct the electric current; but at the same time they suffer decomposition. This method of decomposing a substance is known as electrolysis, or a breaking up by the agency of electricity.

The apparatus required in a very simple case is shown in [Fig. 15]. It merely consists of some suitable vessel to contain the liquid; two plates—one to lead the current into the solution, the other to lead it away again—and wires to connect the plates to the poles of a battery, storage-cell, or dynamo. Each plate is called an electrode, and distinguished as positive or negative according as it is joined to the positive or negative pole of the current generator. By convention, electricity is supposed to “flow” from the positive pole of the battery to the positive electrode or anode, and then through the solution to the negative electrode or cathode, and so back to the negative pole of the generator, thus completing the circuit external to the battery.

When acids, alkalis, and salts are dissolved in water, there is strong evidence to show that they break up to a greater or less extent into at least two parts called ions. These are atoms, or groups of atoms, which have either acquired or lost one or more electrons.[5] They move about quite independently of one another and in any direction until the electrodes are placed in the liquid. Then they are constrained to move in two opposing streams—those which have acquired electrons all move towards the negative electrode, and those which have lost electrons towards the other. At the electrodes themselves, the former give up and the latter take up electrons, and become atoms again. Let us now consider a concrete example. Common salt is composed of atoms of sodium and atoms of chlorine paired. When a small quantity of this substance is dissolved in a large quantity of water, the pairing no longer obtains. The chlorine atoms move away independently accompanied by an extra satellite or electron, and the sodium atoms move away also but with their electron strength one below par. When the current is introduced into the liquid, the sodium ions travel towards the cathode and chlorine ions towards the anode, and when they reach the goal, sodium ions gain one electron and chlorine ions lose one, and both become atoms again. Chlorine atoms combine in pairs forming molecules and escape from the solution in the greenish yellow cloud that we call chlorine gas. The sodium atoms react immediately with water, forming caustic soda with the liberation of hydrogen.