Oxides of Nitrogen.—Five oxides of nitrogen are known, and the series is an excellent example of the Law of Multiple Propor tions : Nitrous oxide, ; nitric oxide, NO ; nitrogen trioxide, nitrogen peroxide, and nitrogen pentoxide, Nitrogen in a still higher state of oxidation exists in an unstable pernitric acid.
Nitrous oxide, was obtained by Priestley (1772) by expos ing "nitrous" air (NO) to moist iron filings. A decrease in volume occurred, and the remaining gas supported combustion better than common air. It was studied by Davy (1799), who gave it the name nitrous oxide, prepared it by the method now employed, viz., by heating ammonium nitrate and also discovered its anaesthetic properties. It is also produced by the reduction of nitric acid under certain conditions, e.g., by the action of zinc on the dilute acid. Pure nitrous oxide may be obtained by mixing solutions of hydroxylamine hydrochloride and sodium ni trite in equimolecular proportions : Nitrous oxide may be synthesized from the elements under special conditions (Chapman, Goodman and Shepherd, 1926).
The gas is colourless, and has a sweetish odour and taste. Its density is 1.9777 gm. per litre, and at i5° C one volume of water dissolves 0.7778 volume of nitrous oxide. The solution, however, is neutral, showing that the gas is not the true anhydride of hypo nitrous acid, It is more soluble in alcohol. Nitrous oxide supports combustion better than air because it decomposes into its elements at moderately low temperatures (beginning at 52o° C) and yields a gas of which one-third by volume is oxygen. It is endothermic, that is, contains more energy than the elements from which it is formed, and can be decomposed into nitrogen and oxygen by the explosion of a detonator.
Nitrous oxide on cooling or under a pressure of 5o atm. at 15° C forms a colourless liquid, which boils at —88.7° C and on rapid evaporation yields a white solid, C. The critical tem perature is 36.5° C and the critical pressure 71.66 atm. The chief use of nitrous oxide is as an anaesthetic for operations of short duration, but prolonged inhalation causes death. About 2 2 litres of the gas are required to produce insensibility and if mixed with about one-quarter of its volume of oxygen an intoxicating effect often results, hence the name "laughing gas." The decomposition of ammonium nitrate is usually employed as the manufacturing process but the temperature needs careful regulation to avoid the formation of nitrogen, ammonia and nitric oxide. One kilogram
of ammonium nitrate gives 182 litres of the gas, which is purified by passage through solutions of ferrous sulphate, caustic pot ash and milk of lime. The product is marketed in the liquid form under pressure in steel cylinders.
Nitric oxide, NO, was probably first obtained by Van Helmont, but was first recognized as a definite gas by Priestley (1772), who prepared it by the action of dilute nitric acid on copper or mer cury: When prepared in this way the gas is liable to contain free nitrogen and nitrous oxide. The pure gas is obtained by dropping a mixture of potas sium nitrite and potassium ferrocyanide solutions into dilute acetic acid : KNO, 2CH,.COOH = The gas may be collected over water or, if required pure, over mercury. It is colourless and has a density of 1.34o2gm. per litre, slightly greater than that of air. At i5° C one volume of water dissolves only o•o51 volumes of the gas. It is not very easily liquefied and has a boiling point of —1502° C and a melting point of —163.6° C. The critical tern perature is —96° and the critical pressure 64 atmospheres.
Nitric oxide is the most stable oxide of nitrogen and is only dis sociated into its elements to about 3.5% at ',coo° C. Consequently, burning substances will only continue to burn in the gas if they previously attain this temperature. A mixture of nitric oxide and carbon disulphide vapour burns with a brilliant lilac-coloured flame, very rich in actinic rays. Nitric oxide combines readily with oxygen to form the dioxide, NO,, which appears as red fumes, but a short time of contact between the gases is required for complete oxidation. The oxidation of nitric oxide is of technical importance (see NITROGEN, FIXATION OF) and has been the subject of a con siderable amount of research. Nitric oxide is absorbed by cold fer rous sulphate solution forming a black compound, but it is readily expelled again by heating. The best absorbent for the gas is a mix ture of caustic soda and sodium sulphite solutions, sodium nitroso sulphate, being formed. Nitric oxide and all the higher oxides of nitrogen are poisonous. At the temperature of liquid oxygen, nitric oxide reacts with fluorine +NO to form nitryl fluoride, the only known halide of nitric acid. It melts at —139° C and boils at —63.5° C.