Note 67 — Chapter VI · TTHE COMPOUNDS OF NITROGEN WITH HYDROGEN AND OXYGEN

If the hydrate HNO3 corresponds to N2O4, the hydrate HNO, hyponitrous acid, corresponds to N2O, and in this sense N2O is hyponitrous anhydride. Hyponitrous acid, corresponding with nitrous oxide (as its anhydride), is not known in a pure state, but its salts (Divers) are known. They are prepared by the reduction of nitrous (and consequently of nitric) salts by sodium amalgam. If this amalgam he added to a cold solution of an alkaline nitrite until the evolution of gas ceases, and the excess of alkali saturated with acetic acid, an insoluble yellow precipitate of silver hyponitrite, NAgO, will he obtained on adding a solution of silver nitrate. This hyponitrite is insoluble in cold acetic acid, and decomposes when heated, with the evolution of nitrous oxide. If rapidly heated it decomposes with an explosion. It is dissolved unchanged by weak mineral acids, whilst the stronger acids (for example, sulphuric and hydrochloric acids) decompose it, with the evolution of nitrogen, nitric and nitrous acids remaining in solution. Among the other salts of hyponitrous acid, HNO, the salts of lead, copper, and mercury are insoluble in water. Judging by the bond between hyponitrous acid and the other compounds of nitrogen, there is reason for thinking that its formula should he doubled, N2H2O2. For instance, Thoune (1893) on gradually oxidising hydroxylamine, NH2(OH), into nitrous acid, NO(OH) (Note 25), by means of an alkaline solution of KMnO4, first obtained hyponitrous acid, N2H2O2, and then a peculiar intermediate acid, N2H2O3, which, by further oxidation, gave nitrous acid. On the other hand, Wislicenus (1893) showed that in the action of the sulphuric acid salt of hydroxylamine upon nitrite of sodium, there is formed, besides, nitrous oxide (according to V. Meyer, NH3O,H2SO4 + NaNO2 = NaHSO4 + 2H2O + N2O), a small amount of hyponitrous acid which may be precipitated in the form of the silver salt; and this reaction is most simply expressed by taking the doubled formula of hyponitrous acid, NH2(OH) + NO(OH) = H2O + N2H2O2. The best argument in favour of the doubled formula is the property possessed by hyponitrous acid of forming acid salts, HNaN2O2 (Zorn).

According to Thoune, the following are the properties of hyponitrous acid. When liberated from the dry silver salt by the action of dry sulphuretted hydrogen, hyponitrous acid is unstable, and easily explodes even at low temperatures. But when dissolved in water (having been formed by the action of hydrochloric acid upon the silver salt), it is stable even when boiled with dilute acids and alkalis. The solution is colourless and has a strongly acid reaction. In the course of time, however, the aqueous solution also decomposes into nitrous oxide and water. The complete oxidation by permanganate of potash proceeds according to the following equation: 5H₂N₂O₂ + 8KMnO₄ + 12H₂SO₄ = 10HNO₃ + 4K₂SO₄ + 8MnSO₄ + 12H₂O. In an alkaline solution, KMnO₄ only oxidises hyponitrous acid into nitrous and not into nitric acid. Nitrous acid has a decomposing action upon hyponitrous acid, and if the aqueous solutions of the two acids be mixed together they immediately give off oxides of nitrogen. Hyponitrous acid does not liberate CO₂ from its salts, but on the other hand it is not displaced by CO₂.