514 CHEMISTRY [NITROGEN COMPOUNDS. from a large quantity of hot water, and from it other nitrites may be prepared in a pure state by double decomposition with the corresponding chlorides ; thus AgN0 2 + KC1 = AgCl + KN0 2 . Silver nitrite. Potassium chloride. Silver chloride. Potassium nitrite. Although oxygen and nitrogen do not directly combine, when a succession of electric sparks is passed through a mixture of the two gases confined over water, red fumes are produced which dissolve in the water, forming nitric and nitrous acids. The nitrite and nitrate present in the atmosphere and in rain-water are doubtless formed in this manner. On adding an acid to a nitrite, nitrous acid is liberated, but at once decomposes with evolution of nitric oxide KNO 2 + HC1 = HN0 2 + KC1 3HNO 2 = 2NO + HNO 3 + OH 2 . Nitrous acid can only exist, in fact, in presence of a large quantity of water, or of nitric acid. It parts with its oxy gen more readily, and is therefore a more powerful oxidizing agent, than nitric acid. It also appears to act more readily than nitric acid upon many metals. Thus, pure nitric acid is almost without action upon silver, but on passing a few bubbles of nitric oxide gas into the acid, and thus produc ing a minute quantity of nitrous acid, the metal is at once attacked, and is dissolved more and more rapidly the longer the action continues, doubtless because the hydrogen dis placed from the acid by the silver reduces nitric acid to nitrous acid, so that the more silver is dissolved the richer the solution becomes in nitrous acid. Platinum also, which is insoluble in nitric acid, is dissolved by nitrous acid. On some hydrogenized carbon compounds nitrous acid exerts an action similar to that of nitric acid, and causes the displacement of hydrogen by the monad compound radicle NO, producing n&rojo-compounds. Thus C 6 H 5 . OH + NO . OH = C 6 H 4 (NO) . OH + H 2 . Phenol. Nitrous acid. Nitrosophenol. Nitrous acid readily absorbs oxygen, and is converted into nitric acid. The nitrites are also converted into nitrates when exposed in the moist state to air. Nitrous acid is easily distinguished from nitric acid by its power of liberating iodine from iodides, by the readiness with which it destroys the blue colour of a solution of indigo at ordi nary temperatures, and by its decolorizing a solution of potassium permanganate, nitric acid being without the power of liberating iodine from iodides, or of bleaching permanganate solution. Hyponitrous Acid, HNO. When sodium is added to a solution of sodium nitrate, the hydrogen displaced from the water by the sodium re duces the nitrate to nitrite, which in its turn undergoes further reduction to the hyponitrite, NaNO. On rendering the solution slightly acid with acetic acid, and adding silver nitrate, a yellow pulverulent precipitate of silver hyponi trite, AgNO, is produced. It is insoluble in water, and almost insoluble in acetic acid, but is soluble in either dilute nitric or sulphuric acid, and without immediate decomposi tion. Moderately diluted nitric, sulphuric, or hydrochloric acid decomposes it with the evolution of nitrogen, and the production of apparently both nitrous and nitric acids in the solution. It is immediately oxidized by concentrated nitric acid. A solution of the sodium salt acidified with acetic or hydrochloric acid decolorizes potassium perman ganate ; it does not liberate iodine from iodides, however, but on the contrary decolorizes a solution of iodine. When the solution acidified with acetic acid is heated, nitrous oxide is evolved. Hydroxylamine, NH 2 (OH). This compound is formed when nitric acid is added to a mixture of tin and hydrochloric acid, the hydrogen pro duced by the action of the tin on the hydrochloric acid re ducing the nitric acid. It may also be formed by the direct union of nitric oxide with hydrogen, namely, by passing a stream of nitric oxide gas through a series of glass cylinders containing tin and hydrochloric acid, together with a little platinum chloride solution, whereby hydrogsn is produced at ordinary temperatures. It is a very unstable substance, and can be obtained only in solution, but well-crystallized hydroxylammonium salts are formed by its union with acids. Thus, the hydro- chloride, NH 3 C1(OH), or NH 2 (OH) . HC1, crystallizes from alcohol in long spicular crystals, and from water in large irregular monoclinic prisms ; it melts at 100 C., but decom poses, with violent evolution of gas, into nitrogen, ammo nium chloride, water, and hydrochloric acid. A solution of hydroxylamine has an alkaline reaction, and precipitates many metallic salts ; it decomposes quickly if concentrated, and gradually if dilute, with evolution of nitrogen and formation of ammonia 3NH 2 (OH) = N 2 + NH 3 + 30H 2 . Hydroxylamine is readily reduced tc ammonia by the action of the nascent hydrogen from sodium amalgam and water. The results of Thomson s thermochemical investigation of various nitrogen compounds are collected in the following, table : Reaction. Units of heat developed or absorbed. Remarks. (N, H 3 ).... 26,710 Ammonia gas. (NH 8 , Aq) 8,440 (N, H 3 , Aq) 35,150 Ammonia - (NH 3 Aq, HClAq)... (NH,Aq, H 2 S, Aq) (N,M 4 ,Cl,Aq) ... (N, H 4 , Br, Aq)... (N,H 4 , I, Aq) (N,H S ,S, Aq) (N,H,, Cl)... 12,270 6,190 86,740 75,800 60,580 50,600 90 620 (N, H,, Br)... 80 180 Crystalline salts (N, H-, I) .. 64 130 formed from (NH 3 , HC1) 41,910 the gaseous (NH 3 ,HBr) (NH 3 , HI) 45,030 43 460 constituents. Nitrous (N,. 0) . 18 320 oxide Nitric (NO, 0) 19 57 Gaseous. peroxide (N0 2 , Aq) 7,750 !(2N0 Aq, 0)... 18,300 -KT-i Nitric acid (N 2 2 , 0,, Aq) (N0 2) 0,H, Aq)... 72,940 51,080 It will be observed that a considerable amount of heat would be absorbed in the formation of nitrous oxide from its elements; consequently, when this gas is decDmposed into its elements, heat is developed, and on this account it is readily decomposed by burning bodies. This fact also explains the non-formation of nitrous oxide from its ele ments. Probably, heat would also be absorbed in the for mation of nitric oxide from its elements. The absorption of heat is perhaps necessary, because the amount of energy in the form of heat which can be developed by the com bination of nitrogen with oxygen to form nitrous and (1) nitric oxides is less than must be expended in separating the atoms of oxygen of the oxygen molecules from each other, and the atoms of nitrogen of the nitrogen molecules from
each other.