Page:Encyclopædia Britannica, Ninth Edition, v. 5.djvu/525

OF NITROGEN.] CHEMISTKY 513 not exhibit any tendency to enter into combination with other bodies. Nitric Oxide, NO; MoL wt., 29 96. This gas is one of the products of the action of nitric acid on metals ; the gas obtained on dissolving copper in moderately concentrated nitric acid contains at first about 97 per cent, of nitric oxide, but as the action proceeds and cupric nitrate accumulates in the solution, the proportion of nitric oxide evolved diminishes, whilst that of nitrous oxide increases. This appears to be due to the alternate reduction of the cupric nitrate to cuprous nitrate 2Cu(N0 3 ) 2 + H 2 = Cu(N0 + 2HN0 Cupric nitrate. Cu 2 (N0 3 ) 2 Cuprous nitrate. and reconversion of the latter into cupric nitrate, as by the action of nitric acid on cuprous nitrate nitric oxide mixed with a large proportion of nitrous oxide is produced. Pure nitric oxide may be prepared by gently heating a mixture of a ferrous salt, such as ferrous chloride or ferrous sulphate, with hydrochloric or dilute sulphuric acid and potassium or sodium nitrate. The nitric acid liberated from the nitrate is reduced by the ferrous salt, which is oxidized to a ferric salt, and nitric oxide is evolved 6FeCl 2 + 6HCl Ferrous chloride. 2HN0 3 = 2NO + 3Fe 2 Cl 6 + 4H 2 . Nitric acid. Nitric oxide. Ferric chloride. Nitric oxide is a colourless, transparent, uncondensable gas, almost insoluble in water; it is the most stable of the oxides of nitrogen, and may even be exposed to a red heat without undergoing decomposition. A lighted taper and phosphorus just kindled are extinguished by it, but if the phosphorus be burning vigorously when introduced into the gas the temperature is then sufficiently high to enable it to decompose the gas into its elements, and it continues to burn as brilliantly as in pure oxygen. Nitric oxide immediately combines with oxygen when mixed with it, forming deep red fumes of higher oxides of nitrogen. It is perfectly absorbed by a solution of ferrous chloride or sulphate, which it turns black; when the solution is heated, most of the nitric oxide is expelled from it unchanged, and in this way nitric oxide may be separated from other gases insoluble in solutions of the ferrous salts. Nitric oxide is absorbed by nitric acid, and apparently nitrous acid is produced HN0 3 + 2NO + H 2 = 3HNO 2 . It combines directly with chlorine, forming the compound NOC1, or nitrosyl chloride, which is also produced together with chlorine when a mixture of concentrated nitric and hydrochloric acids the so-called aqua regia is heated HNO 3 + 3HC1 = NOC1 + C1 2 + 2H 2 O . Nitrosyl chloride is most readily prepared, however, by gently heating a mixture of dry sodium chloride and the lead chamber crystal compound (p. 504) SQ {O.NO +Naa = NOC1 + so (ONa 2 ( OH 2 1 OH. It is an orange-yellow coloured gas, which readily liquefies; the liquid has a deep orange colour, and boils at about - 8 C. It is decomposed by water, forming hydrochloric and nitrous acids NOC1 + Nitrosyl chloride. HOH = NO. OH + Nitrous acid. HC1. Nitrous Anhydride, N 2 O 3 . The properties of this compound have not yet been satis factorily ascertained. Apparently, it is produced, together with nitric peroxide, when nitric oxide is mixed with oxygen, and when nitric acid is heated with arsenious anhydride ; it is stated that it may be prepared in a pure state by passing a mixture of nitric peroxide with an excess of nitric oxide through a heated tube, and that it may be condensed to a deep blue coloured liquid, which begins to boil at about 2 C., but is decomposed into nitric oxide and peroxide. It combines readily at ordinary temperatures with oxygen, forming nitric peroxide. Nitric Peroxide, N 2 O 4 ; MoL wt., 91 86. Nitric peroxide may be obtained by combining nitric oxide with oxygen, and by heating lead nitrate 2Pb(N0 3 ) 2 = 2N 2 4 + 2PbO + O 2 . Lead nitrate. Nitric peroxide. Lead oxide. At low temperatures it forms colourless prismatic crystals, which melt at - 9 C. ; at this temperature the liquid oxide is almost colourless, but it assumes a yellow colour, increasing in depth up to its boiling point. It boils at about 25 C., yielding a brownish-red vapour, the depth of which also increases with the temperature, until at 40 C. it is so dark as to be almost opaque. The vapour of nitric peroxide, however, probably even at tem peratures below its boiling point, is a mixture of the com pound N 2 O 4 with simpler molecules of the composition NO 2 ; the proportion of the latter increases as the tem perature rises, and at temperatures above 150 C. the decomposition into NO 2 is apparently complete. The compound N0 2 is not decomposed at a dull red heat. Nitric peroxide has a pungent suffocating odour, and is quite irrespirable. It is decomposed by water with pro duction of nitrous and nitric acids : N 2 O 4 + H 2 O = HNO 2 + HN0 3 . Chlorine is without action on nitric peroxide in the cold, but when a mixture of the two gases is passed through a heated tube the compound NO 2 C1 or nitrylic chloride is formed. It is a pale yellow liquid, which is decomposed by water into nitric and hydrochloric acids. Nitric Anhydride, N 2 5 . This compound may be obtained directly from nitric acid by the action of phosphoric anhydride 2HNO S Nitric acid. P 2 5 = N0 5 Nitric Phosphoric anhydride. anhydride. - 2HP0 3 . Metaphosphoric It may also be obtained by the action of chlorine on silver nitrate 2AgN0 3 + 2Cl 2 = AgN0 3 + N0 2 Cl=N 2 O 5 + AgCl. Nitric anhydride crystallizes in transparent prisms, which melt at about 30 C. ; it is very unstable, par ticularly in the fused state, and gradually decomposes even at ordinary temperatures into oxygen and nitric peroxide. Water converts it into nitric acid : N 2 O 5 + H 2 O = 2HN0 3 . Nitrous Acid, HN0 2 . In the pure state this compound is unknown to us, but its salts, the nitrites, may readily be prepared. Thus, on heating sodium or potassium nitrate to redness until the gas which is evolved begins to contain nitrogen, a residue which consists chiefly of sodium or potassium nitrite is obtained ; these nitrites are soluble in alcohol, and may by its aid be separated from the nitrates, which are insoluble. A mixture of nitrate and nitrite is also pro duced on passing the gas evolved when nitric acid is heated with arsenious anhydride, starch, or sugar into a solution of potassium or sodium hydroxide, and when oxygen is added to nitric oxide which is in contact with a solution of an alkali. On adding silver nitrate solution to a concentrated solution of the impure nitrite, silver nitrite, AgNO 2 , which is difficultly soluble in water, is precipitated ; this salt may be purified by crystallization

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