NITRIC ACID AND NITRATES
Nitric acid, the _aqua fortis_ of the alchemists, must be placed next to
sulphuric acid in the scale of relative importance, because of the
variety of its uses. It is indispensable for making explosives, and is
used for the preparation of drugs and fine chemicals, including the
coal-tar dyes. The acid also dissolves many metals, forming nitrates,
which are put to several uses. Silver nitrate is the basis of marking
ink, and it is also the substance from which the light-sensitive silver
compounds required for the photographic industry are made. The important
pigments, chrome yellow and chrome red, are prepared from lead nitrate.
The solvent action of nitric acid on copper is made use of in etching
designs on copper plates. Over and above all this, it must be mentioned
that an adequate supply of “nitrate” is required for artificial manure.
Thus it can be said that with the uses of this acid and its salts are
associated our supply of daily bread, our freedom from foreign
oppression, and many of the refinements and conveniences of life.
We shall begin the study of nitric acid by taking stock, as it were, of
the natural sources of supply. The free acid is not found in Nature
except for very small traces in the air after thunderstorms. We have,
therefore, to rely entirely on that which can be obtained artificially.
Until quite recently, it could be said that there was only one method of
making the acid, namely, by the distillation of a mixture of potassium
or sodium nitrates and concentrated sulphuric acid. Now, however, nitric
acid is being made from the air, though as yet only in small quantity,
notwithstanding the great development of this method owing to war
requirements; hence, we are still mainly dependent on the naturally
occurring nitrates just mentioned.
Potassium Nitrate (nitre, saltpetre, sal prunella) is found in the soil
of hot countries, especially in the neighbourhood of towns and villages
where the sanitary arrangements are primitive. In very favourable
circumstances, it may even appear as a whitish, mealy efflorescence on
the surface of the ground. To obtain the salt, it is only necessary to
agitate the surface soil with water and, after the insoluble matter has
settled down, to evaporate the clear solution.
Potassium nitrate is required for making gunpowder, which, until quite
recent times, was the only explosive used in warfare. Continental
countries that could not afford to rely entirely on sea-borne nitre had
to make their own. The refuse of the farmyard, mixed with lime and
ashes, was made up into a heap of loose texture, which was periodically
moistened with the drainage from the stables. In the course of years,
saltpetre and calcium nitrate were formed in the surface layers, from
which they were extracted from time to time. The farmer was then allowed
to pay part of his taxes in nitrates.
Sodium Nitrate, also called caliche, Chili-saltpetre, or Chili-nitrate,
comes mainly from South America. The beds extend for a distance of about
220 miles in Chili, Peru, and Bolivia, between the Andes mountains and
the sea. The deposit is about 5 ft. thick, and its average breadth 5
miles. The crude material is treated with water in steam-heated wooden
vats. The clear solution is evaporated, and the residue obtained is
washed with the mother liquor and dried. This product may contain as
much as 98 per cent. of the nitrate.
[Illustration: Fig. 5. PREPARATION OF NITRIC ACID]
Nitric Acid. Chili-nitrate is always used for making nitric acid. It is
the more abundant of the two naturally occurring nitrates, and therefore
cheaper; moreover, weight for weight, it yields more nitric acid than
the corresponding potassium compound. A mixture of sodium nitrate and
sulphuric acid is heated in a large cast-iron retort (C, Fig. 5). The
retort is entirely surrounded by flame and hot gases to prevent the
condensation of the acid on the upper parts. If this precaution were not
taken, the acid would dissolve the iron and the life of the retort would
not be long; moreover, the product would contain ferric nitrate as an
impurity. The vapour of the acid is led away by the tube D into a series
of two-necked earthenware receivers called _bonbonnes_ (E), and there
condenses to a liquid. The lower figure shows how the leading tube of
the retort is protected from corrosion by the clay tube _a_, _b_; and
how it is connected to the first receiver by the glass tube _e_, which
is luted on at _f_. The percentage strength of the acid which distils
over depends upon that of the sulphuric acid used and on the purity of
the sodium nitrate.
Pure nitric acid is a colourless liquid 1·559 times as heavy as water,
volume for volume. It fumes strongly in air, and is a very corrosive
liquid. The pure acid of commerce is obtained by distillation of a less
concentrated acid. It is 68 per cent. pure. It is rendered free from
dissolved oxides of nitrogen by blowing air through it. When kept
exposed to light, the colour changes at first to yellow and then to
brown, because light causes a certain amount of decomposition.
Red fuming nitric acid owes its colour to the great quantity of oxides
of nitrogen dissolved in it. It is made by distilling sodium nitrate
that has been thoroughly dried with the strongest sulphuric acid; the
distillation is carried out at a high temperature, with the express
purpose of decomposing some of the nitric acid to furnish the oxides of
nitrogen. Sometimes a little powdered starch is also added to facilitate
the formation of these oxides. This variety of nitric acid is
particularly active and is used in many operations, especially in making
dyes, explosives, and other organic chemicals.
Nitric acid has all the general properties of an acid, that is, it has a
sour taste even in very dilute solution, it changes the colour of litmus
to red, and dissolves carbonates and many metals.
When the vapour of nitric acid is passed through a red-hot tube, and
also when a nitrate is strongly heated, oxygen gas is given off.
Analysis shows that the oxygen combined in pure nitric acid amounts to
76 per cent. of its weight, while that in sodium and potassium nitrates
is 56 and 50 per cent. respectively. Nitric acid and the nitrates are,
therefore, highly oxygenated compounds; moreover, under favourable
circumstances, they are rather easily broken up.
Pure nitric acid will set fire to warm, dry sawdust, and a piece of
charcoal or sulphur thrown on the surface of molten nitre takes fire
spontaneously and is quickly consumed, giving out a very vivid light.
The explanation of this is that the supply of oxygen is abundant; it is
also readily available and concentrated in a small space. We can vary
the experiment. When a mixture of 75 parts by weight of finely-powdered
saltpetre, with 15 of charcoal dust and 10 of ground sulphur, is
ignited, it burns very vigorously, and is soon consumed. This mixture
is, indeed, home-made gunpowder.
Explosives. Gunpowder was discovered in very early times by the Chinese,
but for many years the secret of its composition did not get outside the
Great Wall. In the fifth century A.D., it was apparently re-discovered
at Constantinople, and that city was for a long time defended by the use
of what is known in history as Greek Fire, an incendiary mixture very
similar to, if not actually the same as, gunpowder. But again the secret
of its composition was jealously guarded, and it was not until the
thirteenth century that it was discovered, apparently for the third
time, and introduced to Western Europe by Roger Bacon. It was used in
siege cannon early in the fourteenth century and in field guns at Crécy;
but it was apparently not employed for blasting until about 1627,
although in 1605, Guy Fawkes and his fellow-conspirators were able to
obtain it in large quantity.
From the battle of Crécy in 1346 to the beginning of the South African
campaign in 1889, gunpowder was the only explosive used in warfare.
“Villainous saltpetre” has therefore played a very important part in
shaping the course of events in the world’s history. At the present day,
gunpowder has become “old-fashioned.” In warfare, it has been superseded
by “smokeless” powders of much greater power; while for mining
operations, explosives with a much greater shattering effect have long
since taken its place.
The composition of gunpowder may vary, but on the average it contains 75
parts by weight of saltpetre to 15 of charcoal and 10 of sulphur. It is,
therefore, a mixture of two combustible substances, with a large
quantity of a third very rich in oxygen. The separate constituents are
very finely ground and afterwards thoroughly incorporated. When the
mixture is ignited, charcoal and sulphur burn very fiercely in the
oxygen supplied by the saltpetre.
The secret of the action of gunpowder lies in the extraordinary rapidity
with which combustion, started at one point, is propagated through the
whole mass. Moreover, the products of combustion are mainly gases, and
these occupy several thousand times the volume of the solid from which
they are produced. In a confined space, a gas may exert enormous
pressure when its normal tendency to expand is resisted.
Propellants. Although combustion is propagated through a quantity of
gunpowder with very great rapidity, it is not done instantaneously. The
time required is about one-hundredth of a second under ordinary
conditions, and this interval, short though it is, is very important.
When the object is to throw a projectile, the inertia of the latter has
to be overcome, that is, a certain amount of force has to be applied
before the heavy body begins to move. In order that the strain on the
breech of the gun may be as small as possible, the pressure must be
gradually developed and must reach its maximum just as the projectile
begins to move.
The time factor in the explosion constitutes the difference between what
we now call “propellants” and “high explosive.” Propellants are
explosives which develop pressure gradually, and are therefore used to
launch the projectile; high explosive develops pressure instantaneously,
and is therefore used as the bursting charge inside the shell, bomb, or
torpedo, and also in blasting operations.
Cordite, or smokeless powder, is the propellant now most used. It is
made by macerating guncotton and nitroglycerine with their common
solvent acetone. A pulp is thus made to which 5 per cent. of vaseline is
added. The mixture is then forced through a die, and in this way it is
formed into threads or rods, which harden as the acetone evaporates.
Cordite produces no smoke, because all the products of its combustion
are invisible gases.
High Explosive. _Nitroglycerine_ and _Guncotton_ are both explosives of
the instantaneous kind. The former is made by forcing glycerine, under
pressure in a very fine stream, into a mixture of fuming nitric and
concentrated sulphuric acids; the latter by soaking cotton-wool in a
similar mixture. Both products are washed with water until quite free
from acid, and subsequently dried.
Nitroglycerine is a colourless oil with a burning taste. The oil itself
is very dangerous to handle, for it is liable to explode spontaneously
even when the utmost care has been taken in its preparation. A mere spot
on a filter paper explodes with a deafening report when gently hammered
on an anvil; and one drop, when heated on a stout iron plate, blows a
hole through the plate. No use could be made of this substance for many
years after its discovery because it was so liable to explode during
transportation; now, however, it is made safer by mixing with absorbent
infusorial earth or _kieselguhr_. This mixture is known as dynamite.
Blasting gelatine, like cordite, is a mixture of nitroglycerine and
guncotton.
_Trinitrotoluene_ (T.N.T.) is made from toluene and nitric acid, and is
a type of the modern high explosive. It is a yellow crystalline
substance which melts at 79°-81·5° C., and is poured into the shell in a
molten condition. It is a remarkably stable substance, which burns
quickly when heated to 180° C.; it cannot be exploded even by hammering.
Explosion is only brought about by that of a subsidiary substance called
the detonator. The percentage composition of T.N.T. is as follows—
Carbon 33·5
Hydrogen 2·3
Nitrogen 19·5
Oxygen 44·7
100·0
The oxygen present is only just sufficient to burn the whole of the
carbon to carbon monoxide; but since carbon dioxide is also formed,
which requires twice as much oxygen for the same weight of carbon, and
since the hydrogen and nitrogen may also be oxidized, the combustion of
the carbon is not complete; and therefore the explosion of T.N.T. is
accompanied by a dense black smoke, consisting of finely divided
particles of carbon.
The explosive known as ammonal is a mixture of T.N.T., aluminium powder,
and ammonium nitrate; the function of the latter substance is to supply
more oxygen to render the combustion of the carbon of T.N.T. complete.
Nitrates and the Food Supply. Chemical analysis shows that compounds of
nitrogen enter largely into the composition of the living tissues of all
plants and animals; hence, either nitrogen itself or some of its
compounds must be assimilated by all living organisms to provide for
growth and development, and to repair wastage. Air, since it contains
approximately four-fifths of its volume of free nitrogen, is the most
obvious source of supply. At every breath, a mixture of oxygen and
nitrogen is inhaled by animals, but only part of the oxygen is used.
Practically the whole of the nitrogen is returned to the atmosphere
unchanged; it serves only to dilute the oxygen. From this it is clear
that animals do not build up their nitrogenous constituents from
elementary nitrogen.
With plants it is very much the same, for, although they obtain their
principal food, namely, carbon, from the carbon dioxide which is present
in air, it is only in a few exceptional cases that free nitrogen is
assimilated. The exceptions will be considered first, because it was
through these that we first began to learn something definite about the
great importance of nitrogen in agriculture.
Virgil, who was born in 70 B.C., wrote a poem in praise of agriculture.
Almost in the opening lines he deals with the treatment of corn land. He
advises that, in alternate years, this should either be left fallow or
sown with pulse, vetch, or lupin; but not with flax or oats, because
they exhaust the land. From this we learn that rotation of crops was one
of the established principles of good husbandry even at the beginning of
the Christian era.
It was not until the later years of the nineteenth century that any
explanation as to why rotation of crops is beneficial was put forward.
Let us first state the facts more precisely. Peas, beans, vetches,
clover, and other members of the natural order called _Leguminosae_,
which includes about 7,000 species, produce fruits rich in complex
nitrogen compounds without being dependent in any way upon nitrogen
compounds in the soil. Moreover, they do not exhaust the land as far as
these compounds are concerned; hence wheat and other grain can be grown
on the same land the following year.
It is now known that leguminous plants assimilate atmospheric nitrogen
with the help of certain bacteria. If anyone will dig up a lupin root,
he will observe[2] conspicuous wrinkled swellings or nodules at various
points on the roots. These, when examined with a high-power microscope,
are found to contain colonies of bacteria. It is these minute vegetable
organisms which assimilate nitrogen and pass on nitrogen compounds to
the larger plant. Other plants cannot assimilate what we might call raw
nitrogen; they require soluble nitrates. These they build up into
complex organic nitrogen compounds suitable for the feeding of animals
which can assimilate neither free nitrogen nor nitrates.
The Nitrogen Cycle. The supply of nitrates in the soil needs continually
to be renewed by the addition of decaying vegetable matter, stable or
farmyard manure, or Chili saltpetre. The natural manures contain organic
nitrogen compounds which were built up during the life of some animal or
plant. They are not immediately available as food for other plants,
because they are, as it were, the end products of life, and are not
soluble in water. But Nature provides for this. The manures decay,
forming humus, and ultimately ammonia, one of the simplest of inorganic
nitrogen compounds. Ammonia is then transformed to nitrites by certain
bacteria present in the soil, while other bacteria change nitrites into
nitrates. Both of these organisms are quite distinct from the root
nodule bacteria of the _Leguminosae_.
The nitrates pass into the plant in solution, and then begins again that
wonderful cycle of changes which we have described. This is perhaps made
clearer by the following diagram.
[Illustration: Fig. 6. THE NITROGEN CYCLE]
It now remains to show why artificial manures also are necessary. Let us
consider what happens to a piece of ground which is left uncultivated.
Although nothing is taken from it in the way of a crop, yet it very
quickly deteriorates, and the soil becomes infertile through the loss of
nitrogen compounds. This is explained by the fact that nitrates are
soluble in water, and so they get washed away from the top soil. In
addition to this, the nitrogen which is returned to the land forms quite
an insignificant fraction of that which is taken from it, for we waste a
great deal of organic nitrogen. The difference on both these accounts
has, therefore, to be made up by the addition of artificial manures
containing soluble nitrates.
The natural supply of nitrate is very limited. According to a report of
the Chilian Government published in 1909, the nitre beds of that country
were expected to last for less than a century at the current rate of
consumption. Wheat, above all things, will not grow to perfection on
soil which is deficient in nitrate. In 1908, Sir William Crookes called
attention to the difficulty which might be experienced in the near
future in supplying the people of the world with bread. Statistics
showed that wheat was grown on 159,000,000 acres out of a possible
260,000,000. The average yield is 12·7 bushels per acre. By 1931, it is
calculated that the population of the world will be 1,746,000,000; and
to supply these with bread, wheat would have to be grown on 264,000,000
acres, that is, 4,000,000 acres beyond the total available wheat land.
The remedy which Sir William Crookes suggested in order to avoid famine
was to raise the average yield from 12·7 to 20 bushels per acre by the
application of an additional 12,000,000 tons of Chili saltpetre per
annum. In view of the possible exhaustion of the supply of this
substance, this would only mean a postponement of the evil day. The
position, however, is now modified to a great extent because undeveloped
deposits of sodium nitrate are known to exist in Upper Egypt, and the
making of nitric acid from the air, which in 1908 was put forward as a
suggestion, is now an accomplished fact.
Nitric Acid from Air. The supply of nitrogen in the air is truly
inexhaustible; it amounts to about 7 tons for every square yard of the
earth’s surface, which is about 200,000,000 square miles. It is quite
evident that anything man may do in the way of taking nitrogen from the
air will make no perceptible difference to its composition.
Every time a flash of lightning passes between a cloud and the earth,
oxygen and nitrogen combine in the path of the spark, producing oxides
of nitrogen. These dissolve in water, and are washed into the earth as a
very dilute solution of nitric acid. As long ago as 1785, H. Cavendish
imitated this natural phenomenon. A reference to the diagram (Fig. 7)
will show how nitric acid can be made from the air on a small scale. The
globe contains air under slightly increased pressure. The platinum wires
or carbon rods are connected with the terminals of an induction coil,
which in its turn is connected to accumulators supplying the current
required.
When the coil is put into action, a spark passes across the gap between
the ends of the carbon rods. With a larger coil and a more powerful
battery, there is an arching flame which can be blown out and
re-lighted. This is actually nitrogen burning in oxygen. The result in
either case is the same; the air in the globe sooner or later acquires a
reddish-brown colour due to oxides of nitrogen, which, when shaken with
water, form a very dilute solution of nitric acid.
The same process is now carried out on a large scale. Air is driven by
fans through a very powerful electric arc, whereby 1·5 to 2 per cent. is
converted into nitric oxide. This combines spontaneously with more
oxygen to form nitrogen peroxide, which, when dissolved in water, gives
a very dilute solution of nitrous and nitric acids.
[Illustration: Fig. 7. NITRIC ACID FROM AIR]
The absorption of the oxides of nitrogen is carried out systematically.
The mixed gases, after passing through the arc, are passed through a
series of towers filled with acid-resisting material over which a stream
of water is flowing. The solution of nitric acid so obtained is very
dilute, but by using the liquid over and over again, a moderately strong
solution is ultimately produced. This is collected in granite tanks and
neutralized with lime, forming calcium nitrate or Norwegian saltpetre,
as it is now called.
This is a new industry and a rapidly-growing one; in the course of five
years (1905-1909) the annual output of Norwegian or “air” saltpetre
increased from 115 to 9,422 tons. Mountainous countries like Norway and
Switzerland are perhaps in a specially favoured position with respect to
this industry. Rapid streams and waterfalls, in conjunction with
turbines, are used for driving the dynamos, and in this way electricity
is produced at very low cost. It is interesting, however, to note that a
plant for the manufacture of nitric acid from air has now been
established in Manchester.