The Principles of Chemistry, Volume I

by Dmitry Ivanovich Mendeleyev

[36] Ammonium nitrate, NH₄NO₃, is easily obtained by adding a solution of ammonia or of ammonium carbonate to nitric acid until it becomes neutral. On evaporating this solution, crystals of the salt are formed which contain no water of crystallisation. It crystallises in prisms like those formed by common nitre, and has a refreshing taste; 100 parts of water at t° dissolve 54 + 0·61t parts by weight of the salt. It is soluble in alcohol, melts at 160°, and is decomposed at about 180°, forming water and nitrous oxide, NH₄NO₃ = 2H₂O + N₂O. If ammonium nitrate be mixed with sulphuric acid, and the mixture be heated to about the boiling point of water, then nitric acid is evolved, and ammonium hydrogen sulphate remains in solution; but if the mixture be heated rapidly to 16O°, then nitrous oxide is evolved. In the first case the sulphuric acid takes up ammonia, and in the second place water. Ammonium nitrate is employed in practice for the artificial production of cold, because in dissolving in water it lowers the temperature very considerably. For this purpose it is best to take equal parts by weight of the salt and water. The salt must first be reduced to a powder and then rapidly stirred up in the water, when the temperature will fall from +15° to -10°, so that the water freezes.

Ammonium nitrate absorbs ammonia, with which it forms unstable compounds resembling compounds containing water of crystallisation. (Divers 1872, Raoult 1873.) At -10° NH₄NO₃,2NH₃ is formed: it is a liquid of sp. gr. 1·15, which loses all its ammonia under the influence of heat. At +28° NH₄NO₃,NH₃ is formed: it is a solid which easily parts with its ammonia when heated, especially in solution.

Troost (1882) investigated the tension of the dissociation of the compounds formed, and came to the conclusion that a definite compound corresponding to the formula 2NH₄NO₃,3NH₃ is formed, because the tension of dissociation remains constant in the decomposition of such a compound at 0°. Y. Kouriloff (1893), however, considers that the constancy of the tension of the ammonia evolved is due to the decomposition of a saturated solution, and not of a definite compound. During decomposition the system is composed of a liquid and a solid; the tension only becomes constant from the moment the solid falls down. The composition 2NH₄NO₃,3NH₃ corresponds to a saturated solution at 0°, and the solubility of NH₄NO₃ in NH₃ increases with a rise of temperature.

[36 bis] This is explained by saying that in true nitro-compounds the residue of nitric acid NO₂ takes the place of the hydrogen in the hydrocarbon group. For example, if C₆H₅OH be given, then C₆H₄(NO₂)OH will be a true nitro-compound having the radical properties of C₆H₅OH. If, on the other hand, the NO₂ replace the hydrogen of the aqueous radicle (C₆H₅ONO₂), then the chemical character varies, as in the passage of KOH into KONO₂ (nitre) (see Note [37] and Organic Chemistry).

[37] The compound ethers of nitric acid in which the hydrogen of the aqueous radicle (OH) is replaced by the residue of nitric acid (NO₂) are frequently called nitro-compounds. But in their chemical character they differ from true nitro-compounds (for details see Organic Chemistry) and do not burn like them.

The action of nitric acid on cellulose, C₆H₁₀O₅, is an example. This substance, which forms the outer coating of all plant cells, occurs in an almost pure state in cotton, in common writing-paper, and in flax, &c.; under the action of nitric acid it forms water and nitrocellulose (like water and KNO₃ from KHO), which, although it has the same appearance as the cotton originally taken, differs from it entirely in properties. It explodes when struck, bursts into flame very easily under the action of sparks, and acts like gunpowder, whence its name of pyroxylin, or gun-cotton. The composition of gun-cotton is C₆H₇N₃O₁₁ = C₆H₁₀O₅ + 3NHO₃ - 3H₂O. The proportion of the group NO₂ in nitrocellulose may be decreased by limiting the action of the nitric acid and compounds obtained with different properties; for instance, the (impure) well-known collodion cotton, containing from 11 to 12 per cent. of nitrogen, and pyro-collodion (Mendeléeff, 1890), containing 12·4 per cent. of nitrogen. Both these products are soluble in a mixture of alcohol and ether (in collodion a portion of the substance is soluble in alcohol), and the solution when evaporated gives a transparent film, which is insoluble in water. A solution of collodion is employed in medicine for covering wounds, and in wet-plate photography for giving on glass an even coating of a substance into which the various reagents employed in the process are introduced. Extremely fine threads (obtained by forcing a gelatinous mixture of collodion, ether, and alcohol through capillary tubes in water) of collodion form artificial silk.

[37 bis] The property possessed by nitroglycerin (occurring in dynamite), nitrocellulose, and the other nitro-compounds, of burning with an explosion, and their employment for smokeless powder and as explosives in general, depends on the reasons in virtue of which a mixture of nitre and charcoal deflagrates and explodes; in both cases the elements of the nitric acid occurring in the compound are decomposed, the oxygen in burning unites with the carbon, and the nitrogen is set free; thus a very large volume of gaseous substances (nitrogen and oxides of carbon) is rapidly formed from the solid substances originally taken. These gases occupy an incomparably larger volume than the original substance, and therefore produce a powerful pressure and explosion. It is evident that in exploding with the development of heat (that is, in decomposing, not with the absorption of energy, as is generally the case, but with the evolution of energy) the nitro-compounds form stores of energy which are easily set free, and that consequently their elements occur in a state of particularly energetic motion, which is especially strong in the group NO₂: this group is common to all nitro-compounds, and all the oxygen compounds of nitrogen are unstable, easily decomposable, and (Note [29]) absorb heat in their formation. On the other hand, the nitro-compounds are instructive as an example and proof of the fact that the elements and groups forming compounds are united in definite order in the molecules of a compound. A blow, concussion, or rise of temperature is necessary to bring the combustible elements C and H into the most intimate contact with NO₂, and to distribute the elements in a new order in new compounds.

As regards the composition of the nitro-compounds, it will be seen that the hydrogen of a given substance is replaced by the complex group NO₂ of the nitric acid. The same is observed in the passage of alkalis into nitrates, so that the reactions of substitution of nitric acid—that is, the formation of salts and nitro-compounds—may be expressed in the following manner. In these cases the hydrogen is replaced by the so-called radicle of nitric acid NO₂, as is evident from the following table:—

	Caustic potash	KHO.		Glycerin	C3H5H3O3.
	Nitre	K(NO2)O.		Nitroglycerin	C3H5(NO2)3O3.
	Hydrate of lime	CaH2O2		Phenol	C6H5OH.
	Calcium nitrate	Ca(NO2)2O2.		Picric acid	C6H2(NO2)3OH, &c.

The difference between the salts formed by nitric acid and the nitro-compounds consists in the fact that nitric acid is very easily separated from the salts of nitric acid by means of sulphuric acid (that is, by a method of double saline decomposition), whilst nitric acid is not displaced by sulphuric acid from true nitro-compounds; for instance, nitrobenzene, C₆H₅·NO₂. As nitro-compounds are formed exclusively from hydrocarbons, they are described with them in organic chemistry.