| H | O; | NO | O; | NO | O; | NO2 | O; | |||||
| H | NO | NO2 | NO2 | |||||||||
| or | H2O | N2O3 | N2O4 | N2O5 | ||||||||
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.[48]
Nitrous anhydride, N2O3, corresponds[49] 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,[50] the silver salt AgNO2,[51] &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.[51 bis] The resultant salt is soluble in water, whilst the oxide of lead is insoluble. With sulphuric and other acids the solution of potassium nitrite[52] 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,[53] 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°,[54] 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.[55] Nitrous acid acts directly on ammonia, forming nitrogen and water, HNO2 + NH3 = N2 + 2H2O.[56]
As nitrous anhydride easily splits up into NO2 + NO, so, like NO2, with warm water it gives nitric acid and nitric oxide, according to the equation 3N2O3 + H2O = 4NO + 2NHO3.
Being in a lower degree of oxidation than nitric acid, nitrous acid and its anhydride are oxidised in solutions by many oxidising substances—for example, by potassium permanganate—into nitric acid.[57]
Nitric oxide, NO.—This permanent gas[58] (that is, unliquefiable by pressure without the aid of cold) may be obtained from all the above-described compounds of nitrogen with oxygen. The deoxidation of nitric acid by metals is the usual method employed for its preparation. Dilute nitric acid (sp. gr. 1·18, but not stronger, as then N2O3 and NO2 are produced) is poured into a flask containing metallic copper.[59] The reaction commences at the ordinary temperature. Mercury and silver also give nitric oxide with nitric acid. In these reactions with metals one portion of the nitric acid is employed in the oxidation of the metal, whilst the other, and by far the greater, portion combines with the metallic oxide so obtained, with formation of the nitrate corresponding with the metal taken. The first action of the copper on the nitric acid is thus expressed by the equation
2NHO3 + 3Cu = H2O + 3CuO + 2NO.
The second reaction consists in the formation of copper nitrate—
6NHO3 + 3CuO = 3H2O + 3Cu(NO3)2.