DECOMPOSITION AND SUBLIMATION OF AMMONIUM NITRITE. 345

XLIV.-The Decomposition and Sublimation o f Ammonium Nitrite.

BY PRAF ULLA CHANDRA RAY. THE decomposition of ammonium nitrite in its aqueous solution has been investigated by Arndt (Zeitsch. physikal. Chem,, 1901, 39, l), Veley (Trans., 1903, 83, 736), and others. The main object which these chemists had in view was to study the rate of evolution of nitrogen, the conclusion arrived a t being that the evolution of the gas is in conformity with the law of mass action for a unimolecular reaction. During the last two years and more, I have been a t intervals occupied with the problem of a simple synthesis of hydrazine from ammonium nitrite according to the equation :

NH,*NO, NH, + = 1 +2H20+2N0,

NH,*NO, NH,

but although this expectation has not yet been realised, some interesting results have been obtained, which appear to be worthy of record. It will be shown below that an aqueous solution of ammonium nitrite, when heated in a vacuum at temperatures varying from 3 7 O to 70°, decomposes only partly according to the well-known equation : NZ€,*NO, = N, + 2H20, a considerable portion of the salt (sometimes the major portion) subliming into the upper and cooler parts of the tube.

The method of carrying on the investigation was as follows: A solution of ammonium nitrite was prepared by double decom- position between recrystallised silver nitrite and pure ammonium chloride, and sometimes between barium nitrite and ammonium sulphate. Care was taken that the solution of ammonium nitrite contained in every case a very slight excess of silver nitrite or of barium nitrite, so that on the addition of dilute hydrochloric or sulphuric acid respectively, a faint opalescence was noticed. This was to ensure the total absence of ammonium chloride or of Etmmonium sulphate in the solution of ammonium nitrite.

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346 RAY : THE DECOMPOSITION AND SUBLIMATION

About 2 to 4 C.C. of the solution of the salt thus prepared were introduced into a glass tube of 1.3 cm. diameter and 63 cm. length. As the liquid bumps during the earlier stages of heating, the tube was provided with a trap to guard against the contingency of even a trace of the liquid being carried over into the Sprengel pump. The lower part of the tube was immersed in a beaker containing water, into which a thermometer was plunged. The details of a few typical experiments are given below.

Experiment I.-Two C.C. of a solution of ammonium nitrite were taken; an evaluzbtion by the ‘( urea ” method gave for 1 C.C. of the solution 7.7 C.C. of nitrogen a t 23O and 760 mm. (=a 2 per cent. solution). During the first stage of heating, the temperature was kept a t 30° to 92O; only water distilled over, and when a certain degree of concentration had been attained, minute bubbles began to be given off. The temperature was gradually raised to 70°, to complete the decomposition. A. distinct sublimate was, however, invariably noticed in the upper part of the tube. The sublimate was washed out and made up to ct given bulk. An aliquot portion of it liberated iodine copiously from a solution of potassium iodide acidified with dilute sulphuric acid ; another portion yielded nitrogen by the (‘urea” method, and nitric oxide by the Crum- Frankland method, the volumes of the gases being identical. The ammonia in the base was also estimated by nesslerisation, and it was found that the ratio of nitrogen in the base to that in the acid radicle was 1 : 1.

It was thus proved that the sublimate was a nitrite pure and simple. The gas collected in the Sprengel pump reservoir was tested with a solution of ferrous sulphate. A pale brown ring was obtained, but the amount absorbed was scarcely measurable.

The volume of the gas collected as above was 13-6 c.c., and that derived from the sublimate was 1.6 C.C. ; total v0lume=15~2 C.C. a t 2 3 O and 760 mm., the calculated volume of the gas at the same temperature and pressure being 15.4 C.C. : difference = 0.2 c.c.*

Baperiment ZZ.-Four C.C. of the solution, the same as used in experiment I, were taken, and the gases were also measured a t the same temperature and pressure.

Volume of the gas from the Sprengel pump.. ............................ 26.8 C.C.

> 9 ,> ,, sublimate by the (‘urea” method ... 3.2 ,, Total volume ............ 30.0 C.C.

* I t may bc pointed out here that , although, by the ‘‘ urea” method, only the radicle NO, is attacked, the amount of nitrogen set free is really doubled, and hence the volume of nitrogen as yielded by the decoinposition of ammonium nitxite by heat is identical with that yielded by the ‘( urea ” method.

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OF AMMONIUM NITRITE. 347

The calculated amount is 30-8 C.C. The deficiency, as will be shown below, is due to traces of the sublimate being drawn into the “ Sprengel ” tube.

In the present experi- ment the other conditions were exactly the same as in the preceding ones, but the temperature of the bath was kept a t 57-60°.

3.6 ,,

Experiment ZIZ.-Liquid taken = 2 C.C.

The gas collected in the Sprengel reservoir .................. 9.7 C.C. The gas from the sublimate by the “urea” method .........

Total volume ............... 13.3 C.C.

The calculated amount of nitrogen in 2 C.C. of liquid= 15.4 C.C. : deficiency=2.1 C.C. I n fact, the lower the temperature a t which the decomposition is effected the larger is the amount of the sublimate formed, and of the portion of it which finds its way into the “ Sprengel” tube.

Experiment 1V.-The ammoniuni nitrite solution used in this case was prepared by the interaction of barium nitrite and ammonium sulphate.

The volume of the nitrite solution was 4 C.C. (=0.7 per cent. solution, approximately). The temperature of the bath was slowly raised to 3 5 O , but as the distillation of the water was very slow at that temperature, it was gradually raised to 40-45O, and kept constant a t 4 5 O . It was noticed, as before, that when the solution had attained a definite degree of concentration, minute bubbles of nitrogen began to be given off; but as the rate of decomposition was very slow, the temperature was raised to 50°. After a time, when the water had almost evaporated, a thick, semi-solid magma was obtained, the salt separating out in fine needles and bubbles of gas being given off continuously. Owing to the fact that water is one of the products of its decomposition, the salt always remained moist. When, finally, the temperature of the bath was raised to 70°, most of the salt volatilised, a small portion only undergoing decomposition. The gas was collected in two separate instalments, the first being that given off during crystallisation and sublimation, and the second portion being that yielded by heating the sublimate itself. I n order that the sublimate may be seen to advantage, it is necessary t o heat the upper portion of the tube just enough to prevent moisture being deposited, which by dissolving ammonium nitrite would mask the real nature of the sublimate. The sublimate was sometimes deposited as a thin incrustation, and sometimes in t.he shape o€ fine flakes, and it could be moved up and down the tube by simply heating from outside with a naked flame. To effect as completc a decomposition of the sublimate as possible, the entire length of the tube was kept heated; but

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348 DECOMPOSITION AND SUBLIMATION OF AMMONIUM NITRITE.

in spite of this precaution a small fraction of it invariably made its way into the " Sprengel " tube.

In the present experiment the first instalment of gas amounted to 1.9 c.c., and the second to 6.8 C.C. : total volume=8'7 C.C. ;Four C.C. of a solution of ammonium nitrite gave by the '' urea " process 10.8 C.C. of nitrogen at 2 3 O and 760 mm. !L"he deficiency is accounted for by the ammonium nitrite which entered into the " Sprengel." The first instalment of gas gave no coloration with a solution of ferrous sulphate; but the second portion, when simi- larly treated, gave a dark brown ring with a diminution in volume of 0.4 c.c., and when it was shaken up with water no further absorp- tion took place, thus proving the presence of nitric oxide and the absence of nitrous oxide. The ratio of nitrogen to nitric oxide

is thus equal to E4= 16. 0.4

It is unnecessary to multiply instances of the decomposition of the salt, the nature of the decomposition products being sufficiently indicated by the above typical experiments. It remains to lay stress on the fact that the lower the temperature a t which the solution is evaporated the larger is the yield of the crystallised salt.

It has been stated by several observers that the deliquescent crystals of this salt explode on heating to 60-70°, and that even acid solutions of it sometimes undergo similar decompositions (Berthelot, Bull. Soc. chim., 1874, [ii], 21, 55 ; Sorensen, Zei t sch . anorg. Chern., 1894, 7,33). I n one experiment the tube was detached from the " Sprengel," and the crystals at the bottom were heated directly with a burner.

The question naturally arises : Does ammonium nitrite volatilise as such , or is it simply a case of dissociation? If the latter hypothesis is accepted, it follows that nitrous acid can exist by itself at least for the time being. It has been shown, with regard to ammonium chloride, that under diminished pressure and in an atmosphere of ammonia it volatilises without dissociation (Neuberg, Ber., 1891, 24, 2543); and Baker has also proved that if ammonium chloride is free from every trace of moisture, it has the normal vapour density even under ordinary atmospheric pressure. Experi- ments have been undertaken to throw light on this point, but for the present the question must be left an open one. It should also be remembered in this connexion that Berthelot has shown that i f ammonium nitrate is gently heated (the temperature being below ZOOo), it decornposcs into water and nitrous oxide, but a portion of the salt a t the same time sublimes unchanged (Compt. rend., 1876, 82, 932). He seems inclined t o regard it as a case of sublimation without preliminary dissociation.

No explosion, however, took place.

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SMYTHE : BENZYL SULPHOXLDE. 349

Sztmrnary. From the foregoing investigation, it follows that when an aqueous

solution of ammonium nitrite is heated in a vacuum a t 3740°, only a very small portion of the salt decomposes according to the equation : NH,=NO, = 2H,O -+ N,, most of the salt crystallising out. If the temperature is now gradually raised to 70°, slow decom- position goes on according to the above equation, but the major portion sublimes unchanged, When this sublimate is heated by means of a naked flame, the gaseous products are nitrogen and nitric oxide, the latter often amounting to as much as 6 per cent.

I avail myself of this opportunity to express my sincere thanks to Mr. Atul Chandra Ghosha, M.A., for the care and diligence with which he carried out some of the details of the experiments.

CHEMICAL LABORATORY, PRESIDESCY COLLEGE,

CALCUTTA.

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