[27] Phosphorus (Chapter XIX.) gives the hydride PH3, corresponding with ammonia, NH3, and forms phosphorous acid, PH3O3, which is analogous to nitrous acid, just as phosphoric acid is to nitric acid; but phosphoric (or, better, orthophosphoric) acid, PH3O4, is able to lose water and give pyro-and meta-phosphoric acids. The latter is equal to the ortho-acid minus water = PHO3, and therefore nitric acid, NHO3, is really meta-nitric acid. So also nitrous acid, HNO2, is meta-nitrous (anhydrous) acid, and thus the ortho-acid is NH3O3 = N(OH)3. Hence for nitric acid we should expect to find, besides the ordinary or meta-nitric acid, HNO3 (= ½N2O3,H2O), and ortho-nitric acid, H3NO4 (= ½N2O3,3H2O), an intermediate pyro-nitric acid, N2H4O7, corresponding to pyrophosphoric acid, P2H4O7. We shall see (for instance, in Chapter XVI., Note 21) that in nitric acid there is indeed an inclination of the ordinary salts (of the meta-acid), MNO3, to combine with bases M2O, and to approximate to the composition of ortho-compounds which are equal to meta-compound and bases (MNO3 + M2O = M3NO4).

[28] The formation of ammonia is observed in many cases of oxidation by means of nitric acid. This substance is even formed in the action of nitric acid on tin, especially if dilute acid be employed in the cold. A still more considerable amount of ammonia is obtained if, in the action of nitric acid, there are conditions directly tending to the evolution of hydrogen, which then reduces the acid to ammonia; for instance, in the action of zinc on a mixture of nitric and sulphuric acids.

[28 bis] Curtius started with benzoylhydrazine, C6H5CONHNH2 (hydrazine, see Note [20 bis]). (This substance is obtained by the action of hydrated hydrazine on the compound ether of benzoic acid). Benzoylhydrazine under the action of nitrous acid gives benzoylazoimide and water:

C6H5CONHNH2 + NO2H = C6H5CON3 + 2H2O.

Benzoylazoimide when treated with sodium alcoholate gives the sodium salt of hydronitrous acid:

C6H5CON3 + C2H3ONa = C6H5O2C2H3 + NaN3.

The addition of ether to the resultant solution precipitates the NaN3, and this salt when treated with sulphuric acid gives gaseous hydronitrous acid, HN3. It has an acrid smell, and is easily soluble in water. The aqueous solution exhibits a strongly acid reaction. Metals dissolve in this solution and give the corresponding salts. With hydronitrous acid gaseous ammonia forms a white cloud, consisting of the salt of ammonium, NH4N3. This salt separates out from an alcoholic solution in the form of white lustrous scales. The salts of hydronitrous acid are obtained by a reaction of substitution with the sodium or ammonium salts. In this manner Curtius obtained and studied the salts of silver (AgN3), mercury (HgN3), lead (PbN6), barium (BaN6). With hydrazine, N2H4, hydronitrous acid forms saline compounds in the composition of which there are one or two particles of N3H per one particle of hydrazine; thus N5H5 and N8H6. The first was obtained in an almost pure form. It crystallises from an aqueous solution in dense, volatile, lustrous prisms (up to 1 in. long), which fuse at 50°, and deliquesce in the air; from a solution in boiling alcohol it separates out in bright crystalline plates. This salt, N5H5, has the same empirical composition, NH, as the ammonium salt of hydronitrous acid, N4H4, and imide; but their molecules and structure are different. Curtius also obtained (1893) hydronitrous acid by passing the vapour of N2O5 (evolved by the action of HNO3 on As2O3) into a solution of hydrazine, N2H4. Similarly Angeli, by acting upon a saturated solution of silver nitrite with a strong solution of hydrazine, obtained the explosive AgN3 in the form of a precipitate, and this reaction, which is based upon the equation N2H4 + NHO2 = HN3 + 2H2O, proceeds so easily that it forms an experiment for the lecture table. A thermal investigation of hydronitrous acid by Berthelot and Matignon gave the following figures for the heat of solution of the ammonium salt N3HNH3 (1 grm. in 100 parts of water) -708 C., and for the heat of neutralisation by barium hydrate +10·0 C., and by ammonia +8·2 C. The heat of combustion of N4H4 (+163·8 C. at a constant vol.) gives the heat of formation of the salt N4H4 (solid) as - 25·3 C. and (solution) - 32·3 C.; this explains the explosive nature of this compound. In its heat of formation from the elements N3H = -62·6 C., this compound differs from all the hydrogen compounds of nitrogen in having a maximum absorption of heat, which explains its instability.

[29] According to the thermochemical determinations of Favre, Thomsen, and more especially of Berthelot, it follows that, in the formation of such quantities of the oxides of nitrogen as express their formulæ, if gaseous nitrogen and oxygen be taken as the starting points, and if the compounds formed be also gaseous, the following amounts of heat, expressed in thousands of heat units, are absorbed (hence a minus sign):—

N2O N2O2 N2O3 N2O4 N2O5
-21 -43 -22 -5 -1
-22 +21 +17 +4

The difference is given in the lower line. For example, if N2, or 28 grams of nitrogen, combine with O—that is, with 16 grams of oxygen—then 21,000 units of heat are absorbed, that is, sufficient heat to raise 21,000 grams of water through 1°. Naturally, direct observations are impossible in this case; but if charcoal, phosphorus, or similar substances are burnt both in nitrous oxide and in oxygen, and the heat evolved is observed in both cases, then the difference (more heat will be evolved in burning in nitrous oxide) gives the figures required. If N2O2, by combining with O2, gives N2O4, then, as is seen from the table, heat should be developed, namely, 38,000 units of heat, or NO + O = 19,000 units of heat. The differences given in the table show that the maximum absorption of heat corresponds with nitric oxide, and that the higher oxides are formed from it with evolution of heat. If liquid nitric acid, NHO3, were decomposed into N + O3 + H, then 41,000 heat units would be required; that is, an evolution of heat takes place in its formation from the gases. It should be observed that the formation of ammonia, NH3, from the gases N + H3 evolves 12·2 thousand heat units.