Note 31 — Chapter VI · TTHE COMPOUNDS OF NITROGEN WITH HYDROGEN AND OXYGEN

Dalton, Smith, Bineau, and others considered that the hydrate of constant boiling point (see Chapter I., Note 60) for nitric acid was the compound 2HNO3,3H2O, but Roscoe showed that its composition changes with a variation of the pressure and temperature under which the distillation proceeds. Thus, at a pressure of 1 atmosphere the solution of constant boiling point contains 68·6 p.c., and at one-tenth of an atmosphere 66·8 p.c. Judging from what has been said concerning solutions of hydrochloric acid, and from the variation of specific gravity, I think that the comparatively large decrease in the tensions of the vapours depends on the formation of a hydrate, NHO3,2H2O (= 63·6 p.c.). Such a hydrate may be expressed by N(HO)5, that is, as NH4(HO), in which all the equivalents of hydrogen are replaced by hydroxyl. The constant boiling point will then be the temperature of the decomposition of this hydrate.

The variation of the specific gravity at 15° from water (p = 0) to the hydrate NHO₃,5H₂O (41·2 p.c. HNO₃) is expressed by s = 9992 + 57·4p + 0·16p², if water = 10,000 at 4°. For example, when p = 30 p.c., s = 11,860. For more concentrated solutions, at least, the above-mentioned hydrate, HNO₃,2H₂O, must be taken, up to which the specific gravity s = 9570 + 84·18p - 0·240p²; but perhaps (since the results of observations of the specific gravity of the solutions are not in sufficient agreement to arrive at a conclusion) the hydrate HNO₃,3H₂O should be recognised, as is indicated by many nitrates (Al, Mg, Co, &c.), which crystallise with this amount of water of crystallisation. From HNO₃,2H₂O to HNO₃ the specific gravity of the solutions (at 15°) s = 10,652 + 62·08p - 0·160p². The hydrate HNO₃,2H₂O is recognised by Berthelot on the basis of the thermochemical data for solutions of nitric acid, because on approaching to this composition there is a rapid change in the amount of heat evolved by mixing nitric acid with water. Pickering (1892) by refrigeration obtained the crystalline hydrates: HNO₃,H₂O, melting at -37° and HNO₃,3H₂O, melting at -18°. A more detailed study of the reactions of hydrated nitric acid would no doubt show the existence of change in the process and rapidity of reaction in approaching these hydrates.