CAUSTIC ALKALIS
The Alkali Metals. The discovery of current electricity in 1790
furnished the chemist with a very powerful agency for bringing about the
decomposition of compounds. Hydrogen and oxygen were soon obtained by
passing an electric current through acidulated water; and in 1807, Sir
Humphry Davy, who is perhaps better remembered for his invention of the
miners’ lamp, isolated the metals sodium and potassium by subjecting
caustic soda and caustic potash respectively to the action of the
current.
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.
Gas liquor contains chiefly the carbonate, sulphide, sulpho-cyanide, and
chloride of ammonia, together with many other substances, some of which
are of a tarry nature. It would not be practicable to evaporate this
liquid with a view to obtaining the ammonium salts, because it is only a
very dilute solution. Hence, after the removal of tar, the liquor is
treated in such a way that ammonia is set free.
In some cases the liberation of ammonia is accomplished by blowing
superheated steam into the liquor, which sets free the ammonia which is
combined as carbonate, sulphide, and sulpho-cyanide, but not that which
is present as chloride. In other works, the gas liquor is mixed with
milk of lime, which liberates all the combined ammonia. The ammonia is
then expelled from the mixture by a current of steam or air and steam.
In both cases, the gas which is given off is passed into sulphuric acid,
whereby ammonium sulphate is formed in solution and afterwards obtained
as a solid by evaporation.
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.