Acids, Alkalis and Salts

by George Henry Joseph Adlam

Sodium and potassium are very remarkable metals. They are only a little harder than putty, and can easily be cut with a knife or moulded between the fingers. When exposed to the air, they rust or oxidize very rapidly, so much so that they have to be preserved in some mineral oil or in airtight tins. They are lighter than water, which they decompose with the liberation of hydrogen, and under favourable circumstances the hydrogen takes fire so that the metals appear to burn on the surface of the water. After the reaction is over and the sodium or potassium has disappeared, a clear colourless liquid remains which has a strongly alkaline reaction, and when this is evaporated until the residue solidifies on cooling, caustic soda or potash is obtained. For very special purposes, the caustic alkalis are sometimes made by the action of the metals on water, but for production on a large scale, less expensive methods are adopted.

Caustic Alkali is obtained from the corresponding mild alkali in the following way. The substance—washing soda, for example—is dissolved in water and the solution is warmed. Lime is stirred into this solution, and from time to time a small test portion of the clear supernatant liquid is removed and mixed with a dilute mineral acid. When this ceases to cause effervescence, the change is complete. The clear liquid is now separated from the solid matter (excess of lime together with calcium carbonate) and evaporated in a metal dish. Since the caustic alkalis are extremely soluble in water, they do not crystallize as do most of the compounds previously described. Evaporation is, therefore, carried on until the liquid which remains solidifies when cold.

Caustic Soda. To describe the process by which caustic soda is manufactured, we must return to the making of black ash. The mixture from which black ash is made contains limestone. It is heated to 1000° C., which is a sufficiently high temperature to convert limestone into lime. When the black ash is subsequently treated with water, the lime which is present converts some of the mild alkali to caustic; consequently, black ash liquor always contains both alkalis.

When the manufacturer intends to make caustic soda and not soda crystals, the composition of the black ash mixture is varied by adding a larger proportion of limestone, so that there may be an excess of lime in the black ash produced. The treatment with water is carried out as described under washing soda, and then more lime is added to convert the mild soda into caustic soda. After the excess of lime and other suspended matter has settled down, the clear caustic liquor is evaporated in iron kettles until it becomes molten caustic, which will solidify on being allowed to cool.

There are various grades of caustic soda on the market differing one from another in purity. The soap manufacturer uses caustic liquor or lye containing about 40 per cent. of caustic soda. For other purposes, the solid containing from 60 to 78 per cent. is used. Sometimes the product is whitened by blowing air through the strong caustic liquor or by the addition of a little potassium nitrate. Finally, for analytical purposes, caustic soda is purified by dissolving it in alcohol and subsequently evaporating the clear liquid.

Caustic Potash. The methods for the preparation of the corresponding potassium compound are precisely the same as those described for caustic soda; in fact, wherever the words sodium and soda occur in this chapter, the reader can always substitute potassium and potash respectively.

Caustic Lime. Apart from its use in making mortar and cement, lime is very often employed to neutralize acids. For this purpose, a suspension in water, called milk of lime, is generally used, for lime itself is not very soluble. Probably it is only the soluble part which reacts; nevertheless, as soon as this is used up, more of the solid dissolves, and in this way the action goes on as if all the lime were in solution.

Lime is also a very valuable substance in agriculture, especially on damp, boggy land, where there is much decaying vegetable matter, and on land which has been liberally manured. The soil in these cases is very likely to become acid and is then unproductive. Lime is added to “sweeten” the soil; in other words, to neutralize the acid.

Ammonia. The pungent smelling liquid popularly known as “spirits of hartshorn” is a solution of ammonia gas in water. It is a caustic alkali and, as such, is sometimes used to remove grease spots. Here, however, we shall consider ammonia only in connection with ammonium salts, some of which are used in very large quantity as fertilizers.

The principal source of ammonia at the present time is the ammoniacal liquor obtained as a by-product in the manufacture of gas for heating and lighting. Coal contains about 1 per cent. of nitrogen, and when it is distilled, some of this nitrogen is given off as ammonia, which dissolves in the water produced at the same time. This liquid is condensed in the hydraulic main and in other parts of the plant where the gas is cooled down.