Nitrogen trioxide (nitrous anhydride), was discovered by Glauber. When nitric acid (56%) is distilled with arsenious oxide or starch, and the vapours are cooled with a freezing mixture, a dark blue liquid is obtained which is crude nitrogen trioxide : On evaporation, the liquid decomposes almost completely into nitric oxide and nitrogen per oxide, but the two gases recombine on cooling: H. B. Baker has shown that when the liquid is dried by long exposure to phosphorus pentoxide it may be evaporated without decomposition, but the molecules in the vapour are Nitrogen trioxide is the anhydride of nitrous acid, but only traces of this are formed by the action of water, since it is very unstable and the blue solution decomposes, partly into water and the anhydride (to which the colour is due) and partly with evolution of nitric oxide and formation of nitric acid : Although the gas is largely composed of nitric oxide and nitrogen peroxide, it is rapidly absorbed by solutions of alkalis with forma tion of nitrites, e.g., sodium nitrite, and by concentrated sulphuric acid with formation of nitroso-sulphuric acid ("chamber crys tals"), SO2(OH)•0.N0.
Sodium nitrite, is an important salt used in many organic preparations, e.g., of dyestuffs, and is manufactured by heating molten sodium nitrate with metallic lead, by reducing sodium nitrate in presence of sodium hydroxide with sulphur or, usually, by absorbing higher oxides of nitrogen obtained by the oxidation of air in the arc process or by the oxidation of ammonia, in sodium hydroxide or carbonate solutions. Nitrites are reducing agents, being converted to nitrates, but they also liberate iodine from potassium iodide, being reduced to nitric oxide.
The chloride of nitrous acid, nitrosyl chloride, NOC1, is a yel low gas formed by the direct union of nitric oxide and chlorine, by the action of phosphorus pentachloride on sodium nitrite, or by heating nitroso-sulphuric acid with sodium chloride.
Nitrogen peroxide (dioxide or tetroxide), or is formed as a reddish-brown gas by the direct union of nitric oxide and oxygen. The later stages of this reaction are somewhat slow. It is more conveniently obtained by heating lead nitrate, and condensing the nitrogen peroxide to a liquid in a tube cooled in a freezing mixture, or (most con veniently and in a very pure state) by heating chamber crys tals with potassium nitrate: 2N0,. Nitrogen peroxide in a freezing mixture solidifies to nearly colourless crystals, melting at —9.04°C to a honey-yellow liquid. With rise of temperature this becomes reddish-brown and boils at 21.9° C, giving a reddish-brown vapour. The colour of the gas becomes deeper on heating; at 140°C it is nearly black. These colour changes are due to the existence of two forms of nitrogen peroxide : a colourless form, and a strongly coloured form, NO,. The conversion of N,O, into NO, occurs in the liquid; in
the vapour the change may be followed by the decrease in the vapour density due to dissociation : 2NO2. The dis sociation is complete at 140°C; at higher temperatures de composes into NO and oxygen. On cooling the reverse changes occur.
Nitrogen peroxide supports the combustion of strongly burning phosphorus; it is decomposed at the temperature of the flame with formation of oxygen. A mixture of the gas with hydrogen is reduced to ammonia in contact with heated platinum. In contact with water, nitrogen peroxide is absorbed with the production of nitrous and nitric acids; the former then decomposes with forma tion of and NO as explained above. The absorption of the gas with the formation of nitric acid, in presence of air or oxygen, which is an important industrial process, is therefore attended with difficulty. In presence of alkalis, nitrogen peroxide is ab sorbed with production of nitrite and nitrate : Liquid nitrogen peroxide forms a violently explosive mixture with petrol or other liquid hydrocarbons, and bombs filled with the mixture have been used in aerial bombard ment. The gas, largely diluted with air, is used in bleaching flour.
Hyponitrous acid, is produced in the form of a salt by reducing nitrites with sodium amalgam (Divers, 1871), The free acid is ob tained in the form of explosive crystals by the decomposi tion of the silver salt with hydrogen chloride in dry ether. The acid and its salts are powerful reducing agents. Salts of an acid (nitrohydroxylamic acid), intermediate between hyponitrous acid and nitrous acid, are also known. There is an isomeride of hypo nitrous acid, nitramide, Nitrogen pentoxide (nitric anhydride), first pre pared by Deville (1849), is best obtained by the removal of water, by means of phosphoric oxide, from cooled, concentrated nitric acid : The product is distilled in a cur rent of ozonized oxygen, dried over phosphoric oxide and con densed in a vessel cooled by solid carbon dioxide and ether. An alternative method is to pass ozonized oxygen through cooled, liquid nitrogen tetroxide : Crystals of nitrogen pentoxide are stable below o°C but are very hygroscopic. At ordinary temperatures, especially on exposure to light, slow decomposition occurs into nitrogen peroxide and oxygen, while on heating the crystals melt with decomposition at 29.5°C. Sudden heating causes explosion. Nitrogen pentoxide dissolves in water forming nitric acid. Phosphorus and potassium burn in the liquid pentoxide on warming, and charcoal burns if previously ignited.