Acids, Alkalis and Salts

by George Henry Joseph Adlam

Ammonium Salts

Ammonium Chloride. Like all other alkalis, ammonia solution neutralizes acids, forming salts. With hydrochloric acid, it produces the white solid known as sal ammoniac or ammonium chloride. This compound is familiar as the one required to make the liquid used in a Leclanché cell, which is generally used as the current generator for electric bells.

Ammonium Carbonate, which is also called stone ammonia and salt of hartshorn, is made by subliming a mixture containing two parts chalk and one part ammonium sulphate. It is a white solid which gives off ammonia slowly and is, therefore, used as the basis for smelling salts.

Ammonium Nitrate is obtained by passing ammonia gas into nitric acid until it is neutralized. It is a white solid, which melts easily on being heated, and breaks up into water and nitrous oxide (laughing gas), which is the “gas” administered by dentists. Ammonium nitrate is also used in the composition of some explosives: for example, “ammonite” is said to contain 80 per cent. of this substance.

Ammonium Sulphate is used chiefly as an artificial manure; the amount required for this purpose throughout the world is over 1,500,000 tons every year.

Synthetic Ammonia. Though the soluble compounds of nitrogen are fairly abundant, the supply is by no means equal to the demand, because such enormous quantities are required for agricultural purposes. It has been already said that ammonia is obtained as a by-product in the distillation of coal, and it has been repeatedly pointed out that our coal supplies are far from inexhaustible; moreover, coal gas may not always be used for lighting and heating. It, therefore, becomes a very important question as to how the future supply of ammonium salts is to be maintained.

Ammonia is a very simple compound formed from the elements nitrogen and hydrogen, and, as before mentioned, the supply of free nitrogen in the air is literally inexhaustible. In recent years, the efforts of chemists have been directed towards finding a method of converting the free nitrogen of the air into some simple soluble compound. This problem is usually spoken of as the “fixation of nitrogen.”

In the Haber process, nitrogen obtained by the fractional distillation of liquid air is mixed with three times its volume of hydrogen, and this mixture is heated to between 500°C. and 700°C. under a pressure of 150 atmospheres (nearly 1 ton to the square inch) and in the presence of a contact agent. Under these conditions, nitrogen and hydrogen combine to form ammonia, which is condensed by passing the mixed gases into a vessel cooled with liquid air, any unchanged nitrogen and hydrogen being passed back again over the contact substance.

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.

To return now to practical considerations. The electrolysis of salt solution appears to be an ideally simple method of obtaining caustic soda and chlorine from sodium chloride. As a manufacturing process, it would seem to be perfect, for the salt is broken up directly into its elements and a secondary reaction gives caustic soda automatically. There is no “waste” as in the Leblanc process, and it does not require the use of any expensive intermediary substance afterwards to be recovered, as in the Solvay process. But, as very often happens when working on a large scale, difficulties arise, and these up to the present have only been partially overcome.

Some of the chlorine remains dissolved in the liquid and reacts with the caustic soda, forming other substances which, though valuable, are not easy to separate from the caustic soda. It is possible to get over this difficulty to some extent by placing a porous partition between the anode and the cathode, and in that way dividing the cell into cathodic and anodic compartments. As long as the partition is porous to liquids, it will allow the current to pass, but at the same time it will greatly retard the mixing of the contents of the two compartments. Porous partitions or cells which are in common use for batteries are made of “biscuit” or unglazed porcelain.

It must be remembered, however, that porous partitions only retard the mixing of liquids; they do not prevent it. Moreover, a further difficulty arises from the fact that chlorine is a most active substance, and therefore it is difficult to find a material which will resist its corrosive action for any length of time, and the same difficulty arises in the case of the anode where the chlorine is given off.

Castner Process for Caustic Soda. The following is the most successful electrical process for the manufacture of caustic soda yet devised. It was introduced in 1892, and is known as the Castner process. It should be noted that the use of the porous partition has been avoided in a very ingenious way.

Fig. 16. THE CASTNER PROCESS

The cell (see [Fig. 16]) is a closed, rectangular-shaped tank divided into three compartments by two non-porous partitions fixed at one end to the top of the tank, while the other end is free and fits loosely into a channel running across the tank. The floor of the tank is covered with a layer of mercury of sufficient depth to seal the separate compartments. The two end compartments contain the brine in which are the carbon anodes; the middle compartment contains water or very dilute caustic soda in which the cast-iron cathode is immersed.

The current enters the end compartments by the carbon anodes and passes through the salt solution to the mercury layer which in these compartments are the cathodes. The current then passes through the mercury to the middle compartment, and then through the solution to the cathode, thence back to the dynamo. It is important to note that in the middle compartment the mercury becomes the anode.

Chlorine is liberated at the carbon electrodes, and when no more can dissolve in the liquid it escapes and is conveyed away by the pipe P. Sodium atoms are formed at the surface of the mercury cathodes in the outside compartments and dissolve instantly in the mercury, forming sodium amalgam.

While the current is passing, a slight rocking motion is given to the tank by the cam E. This is sufficient to cause the mercury containing the dissolved sodium to flow alternately into the middle compartment, and there the sodium amalgam comes into contact with water; the sodium is dissolved out of the mercury and caustic soda is formed. Water in a regulated stream is constantly admitted to the middle compartment, and a solution of caustic soda of about 20 per cent. strength overflows.

The production of caustic soda by an electrical method still remains to be fully developed. A process which gives only a 20 per cent. solution cannot be looked upon as final. In the meantime, other methods have been tried, in some of which fused salt is used in place of brine in order to give caustic soda in a more concentrated form. For a description of these methods, the reader must consult some of the larger works mentioned in the preface. Here we can only say that very great difficulties have been encountered, particularly in the construction of a satisfactory porous diaphragm or, alternately, in devising methods in which this can be dispensed with.

Another interesting application of electrolysis is furnished by the use of copper sulphate in industry. When this salt is dissolved in water, it breaks up into copper ions (positive) and an equal number of negative ions, composed of 1 atom of sulphur and 4 atoms of oxygen (SO″4). Under the influence of the current copper ions travel to the cathode, and there by the gain of two electrons become copper atoms. Now, since copper is not soluble in copper sulphate solution, and is not volatile except at very high temperatures, it is deposited on the cathode in a perfectly even and continuous film when the strength of the current is suitably adjusted. This film continues to grow in thickness as long as the conditions for its deposition are maintained. If the current employed is not suitable, the metallic film is not coherent, and the copper may appear as a red powder at the bottom of the cell. Any other metal or impurity which might be present in the unrefined copper falls to the bottom of the tank.

Other metals are deposited electrolytically in exactly the same way. The metal to be deposited is joined to the positive pole and the article to be plated to the negative pole of the battery. Both are suspended in a solution of salt, generally the sulphate, of the metal which is to be deposited. Thus, for nickel plating, a piece of sheet nickel would be used in conjunction with a solution of sulphate of nickel or, better, a solution of nickel ammonium sulphate, made by crystallizing ammonium and nickel sulphates together. The current required is small; indeed, if it is too strong, the deposit adheres loosely to the article, and the result is, therefore, not satisfactory.

Electrotype blocks are also made by a similar process. An impression of the article to be reproduced is made in wax, or some suitable plastic material, and polished with very fine graphite or black lead, in order to give a conducting surface. It is then suspended in a solution of copper sulphate and joined to the negative pole of the battery; a plate of copper connected with the positive pole is suspended in the same solution. When a weak current is passed, copper is deposited on the black-leaded surface and grows gradually in thickness, until at length it can be stripped off, giving a positive replica of the object.