The group NO2 of nitro-compounds in many cases (like all the oxidised compounds of nitrogen) passes into the ammonia group or into the ammonia radicle NH2. This requires the action of reducing substances evolving hydrogen: RNO2 + 6H = RNH3 + 2H2O. Thus Zinin converted nitrobenzene, C6H5·NO2, into aniline, C6H5·NH2, by the action of hydrogen sulphide.
Admitting the existence of the group NO2, as replacing hydrogen in various compounds, then nitric acid may be considered as water in which half the hydrogen is replaced by the radical of nitric acid. In this sense nitric acid is nitro-water, NO2OH, and its anhydride dinitro-water, (NO2)2O. In nitric acid the radical of nitric acid is combined with hydroxyl, just as in nitrobenzene it is combined with the radical of benzene.
It should here be remarked that the group NO3 may be recognised in the salts of nitric acid, because the salts have the composition M(NO3)n, just as the metallic chlorides have the composition MCln. But the group NO3 does not form any other compounds beyond the salts, and therefore it should he considered as hydroxyl, HO, in which H is replaced by NO2.
[38] The nitro-compounds play a very important part in mining and artillery. Detailed accounts of them must be looked for in special works, among which the works of A. R. Shuliachenke and T. M. Chelletsoff occupy an important place in the Russian literature on this subject, although historically the scientific works of Abel in England and Berthelot in France stand pre-eminent. The latter elucidated much in connection with explosive compounds by a series of both experimental and theoretical researches. Among explosives a particularly important place from a practical point of view is occupied by ordinary or black gunpowder (Chapter XIII., Note [16]), fulminating mercury (Chapter XVI., Note 26), the different forms of gun-cotton (Chapter VI., Note [37]), and nitro-glycerine (Chapter VIII., Note [45], and Chapter XII., Note [33]). The latter when mixed with solid pulverulent substances, like magnesia, tripoli, &c., forms dynamite, which is so largely used in quarries and mines in driving tunnels, &c. We may add that the simplest true nitro-compound, or marsh gas, CH4, in which all the hydrogens are replaced by NO2 groups has been obtained by L. N. Shishkoff, C(NO2)4, as well as nitroform, CH(NO2)3.
Fig. 49.—Decomposition of nitrous oxide by sodium.
Nitric acid may be entirely decomposed by passing its vapour over highly incandescent copper, because the oxides of nitrogen first formed give up their oxygen to the red-hot metallic copper, so that water and nitrogen gas alone are obtained. This forms a means for determining the composition both of nitric acid and of all the other compounds of nitrogen with oxygen, because by collecting the gaseous nitrogen formed it is possible to calculate, from its volume, its weight and consequently its amount in a given quantity of a nitrogenous substance, and by weighing the copper before and after the decomposition it is possible to determine the amount of oxygen by the increase in weight. The complete decomposition of nitric acid is also accomplished by passing a mixture of hydrogen and nitric acid vapours through a red-hot tube. Sodium also decomposes the oxides of nitrogen at a red-heat, taking up all the oxygen. This method is sometimes used for determining the composition of the oxides of nitrogen.
[40] The application of this acid for etching copper or steel in engraving is based on this fact. The copper is covered with a coating of wax, resin, &c. (etching ground), on which nitric acid does not act, and then the ground is removed in certain parts with a needle, and the whole is washed in nitric acid. The parts coated remain untouched, whilst the uncovered portions are eaten into by the acid. Copper plates for etchings, aquatints, &c., are prepared in this manner.