CHAPTER VI TTHE COMPOUNDS OF NITROGEN WITH HYDROGEN AND OXYGEN

The above explains the action of nitrogen peroxide on water at low temperatures. N₂O₄ then acts on water like a mixture of the anhydrides of nitrous and nitric acids. The first, N₂O₃, may be looked on as water in which each of the two atoms of hydrogen is replaced by the radicle NO, while in the second each hydrogen is replaced by the radicle NO₂, proper to nitric acid; and in nitrogen peroxide one atom of the hydrogen of water is replaced by NO and the other by NO₂, as is seen from the formulæ—

In fact, nitrogen peroxide at low temperatures gives with water (ice) both nitric, HNO₃, and nitrous, HNO₂, acids. The latter, as we shall afterwards see, splits up into water and the anhydride, N₂O₃. If, however, warm water act on nitrogen peroxide, only nitric acid and monoxide of nitrogen are formed: 3NO₂ + H₂O = NO + 2NHO₃.

Although NO₂ is not decomposed into N and O even at 500°, still in many cases it acts as an oxidising agent. Thus, for instance, it oxidises mercury, converting it into mercurous nitrate, 2NO₂ + Hg = HgNO₃ + NO, being itself deoxidised into nitric oxide, into which the dioxide in many other instances passes, and from which it is easily formed.⁵⁵

Nitrous anhydride, N₂O₃, corresponds⁵⁶ to nitrous acid, NHO₂, which forms a series of salts, the nitrites—for example, the sodium salt NaNO₂, the potassium salt KNO₂, the ammonium salt (NH₄)NO₂,⁵⁷ the silver salt AgNO₂,⁵⁸ &c. Neither the anhydride nor the hydrate of the acid is known in a perfectly pure state. The anhydride has only been obtained as a very unstable substance, and has not yet been fully investigated; and on attempting to obtain the acid NHO₂ from its salts, it always gives water and the anhydride, whilst the latter, as an intermediate oxide, partially or wholly splits up into NO + NO₂. But the salts of nitrous acid are distinguished for their great stability. Potassium nitrate, KNO₃, may be converted into potassium nitrite by depriving it of a portion of its oxygen; for instance, by fusing it (at not too high a temperature) with metals, such as lead, KNO₃ + Pb = KNO₂ + PbO.⁵⁹ The resultant salt is soluble in water, whilst the oxide of lead is insoluble. With sulphuric and other acids the solution of potassium nitrite⁶⁰ immediately evolves a brown gas, nitrous anhydride: 2KNO₂ + H₂SO₄ = K₂SO₄ + N₂O₃ + H₂O. The same gas (N₂O₃) is obtained by passing nitric oxide at 0° through liquid peroxide of nitrogen,⁶¹ or by heating starch with nitric acid of sp. gr. 1·3. At a very low temperature it condenses into a blue liquid boiling below 0°,⁶² but then partially decomposing into NO + NO₂. Nitrous anhydride possesses a remarkable capacity for oxidising. Ignited bodies burn in it, nitric acid absorbs it, and then acquires the property of acting on silver and other metals, even when diluted. Potassium iodide is oxidised by this gas just as it is by ozone (and by peroxide of hydrogen, chromic and other acids, but not by dilute nitric acid nor by sulphuric acid), with the separation of iodine. This iodine may he recognised (see Ozone, Chapter IV.) by its turning starch blue. Very small traces of nitrites may be easily detected by this method. If, for example, starch and potassium iodide are added to a solution of potassium nitrite (at first there will be no change, there being no free nitrous acid), and then sulphuric acid be added, the nitrous acid (or its anhydride) immediately set free liberates iodine, which produces a blue colour with the starch. Nitric acid does not act in this manner, but in the presence of zinc the coloration takes place, which proves the formation of nitrous acid in the deoxidation of nitric acid.⁶³ Nitrous acid acts directly on ammonia, forming nitrogen and water, HNO₂ + NH₃ = N₂ + 2H₂O.⁶⁴