698 SAUNDERS :

LXXX1.-The Decomposition of

By HORACE LEONARD S by Heat.

Ammonium Nitrate

.UNDERS.

THIS research was undertaken owing to the many reports regarding the manner in which ammonium nitrate decomposes when heated. These reports agree in giving the main course of the reaction as NH,NO, + N,O -k 2H,O, but nitric oxide, nitrogen peroxide, free ammonia, and free nitric acid have also been found. Measure- ments are given only in one instance, where the reaction was carried out at a red heat (Favre and Silbermann, Ann. chhirn. phys., 1852, [iii], 36). It is well known that if ammonium nitrate is too strongly heated when used for preparing nitrous oxide, red fumes are produced, and soon will follow a rather violcnt explosion. Hence there must be some change in the nature of the reaction at this point, either with regard to time or temperature. If the nitrate should contain small quantities of chlorides, the gas evolved invariably contains chlorine. The problem thus becomes :

1. To ascertain the nature of the decomposition of pure ammonium nitrate at moderate temperatures.

2. To ascertain how the presence of small quantities of commonly occurring impurities modifies this decomposition.

3. To ascertain the nature of the explosive decomposition. The method selected for estimating the nitrous oxide was

explosion with excess of hydrogen. Ammonium nitrate of commerce is liable to contain chlorides and

sulphates of ammonia and sodium and also sodium nitrate in quan- tities up to 1.5 per cent. Therefore the influence of each impurity must be investigated. The first part of this research was directed to the decomposition of the specially purified nitrate. It was dried at 100" for two days and reduced to a fine powder. In all the experiments, unless otherwise stated, 40 grams of material were used.

Outline of the Method. The ammonium nitrate is heated in a large tube, H (Fig. l),

which is placed in a thermostat, containing an oil of high boiling point, the temperature of which is given by the thermometer, D. Stirring is effected by the automatic fan, E , and the mill wheel, G. The temperature of the nitrate is given by a second thermometer, J , the bulb of which is immersed in the molten mixture. The products of the reaction are led away to a condensing tube, 5, where an acid liquid collects, which may be drawn off by turning the tap. From here the gas passes on to M or N , as may be necessary. N contains

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THE DECOMPOSITION OF AMMONIUM NITRATE: BY HEAT. 699

sodium hydroxide to absorb acid vapours or chlorine (p. 703). After this, the gas is collected over mercury in the vessel, 0. This gas may now be submitted to analysis in the Haldane apparatus. The hydrogen required in these experiments is passed into the burette, Y , which contains alkaline pyrogallol to absorb traces of

diffused oxygen. Further, the rubber joints through which the hydrogen passes are varnished with shellac to minimise diffusion. Haldane’s apparatus is slightly modified ; the stem of P is graduated in 1/100 c.c., the explosion chamber is made double the usual size, and a new form of spark gap substituted, where one electrode forms an arc a t the centre of which the other is situated. (When

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700 SAUNDERS

the ordinary form was used, some of the explosions were very violent, often shattering the tube.)

By suitably turning the taps 3, 5 , 6, a sample of the gas is drawn over into P and measured, the liquid in the compensating tube being meanwhile adjusted bo the mark on V . The gas is passed to the explosion chamber 8, which was previously full of mercury, and the tap 7 closed. The capillary tubes from 5 onwards are washed out by drawing hydrogen through them, and then filled again with mercury. By turning taps 4, 5 , 6, hydrogen is admitted into P, and its volume measured as before. It is then passed into X, and mixing effected by passing the gases from X to P and back again five times. The tap 7 is closed, the pressure reduced, and the mixture exploded by a spark from an induction coil. After the lapse of a few minutes to allow for cooling, the gases are passed back into P and measured. The level in V should remain undis- turbed when this tube is again put in communication with R ; if this is not the case, a correction may be applied up to 0.5 per cent. or the experiment is repeated. The proportion of hydrogen used is adjusted so that it is always just in excess of that required; a large excess limits the completeness of the reaction. All the preliminary experiments were made in the above apparatus,

including the series in which rises of temperature were measured (chloride mixtures, p. 705). In order to obtain satisfactory results, it was found that there must be no cork or rubber a t any place where heated products of the reaction must pass. Thus the tubes H , K , L were fused together, and H was fitted with a ground-glass stopper to facilitate the introduction of the nitrate.

The gases were tested for nitrous oxide, nitric oxide, nitrogen peroxide, ammonia, nitrogen, and oxygen, the last alone being invariably absent. Samples of the gas evolved a t 240" were passed through ferrous sulphate solution for half an hour, but no colour change was observed; hence the quantity of nitric oxide must have been very small. Any nitrogen peroxide will have been absorbed in the sodium hydroxide in N , but the amount was very minute. The nitrous oxide was found as stated above, and the free nitrogen given by difference. The estimation of the nitric oxide and nitrogen peroxide was carried out in the following way: the sodium hydroxide tube was removed and the gases were passed through an acidified solution of potassium iodide, t h t volume of the gas being afterwards measured. The liberated iodine was titrated with thiosulphate, and the volume of nitrogen peroxide corresponding with the iodine calculated. The nitric oxide and nitrogen peroxide were then estimated together. A known volume of the gas was collected over mercury, and a few

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THE DECOMPOSITION OF AMMONIUM NITRATE BY HEAT. 701

C.C. of dilute potassium iodide solution were introduced into the gas, which was then shaken. Owing to the sohbility of the nitrous oxide, a slight vacuum was produced, and air was then admitted so that the nitric oxide was oxidised, and liberated iodine. This process was repeated several times, and the iodine titrated as before. The nitric oxide was then found by difference. The results are given in Tables I and 11.

TABLE I. Nitrogen peroxide. Nitric oxide.

C.C. of gas Nitrogen peroxide. C.C. of gas (NO, + NO). Nitric oxide. taken. Per cent. taken. Per cent. Per cent.

500 0.001 100 0.0024 0.001 9 7 9 7 7 7 0.0022 9 7

9 9 7 9 0.0024 7 7

7 9 9 7 id0 0-0023 Y Y

1 9 7 7 7 9 0.0024 7 9

The following results show a series of time reactions, readings being taken half-hourly after the apparatus was free from air.

TABLE 11. Percentage of nitrous oxide in the gas evolved from the sure

1

Temp. 233'

220"

234"

262"

nitrate. Vol. of

Vol. of gas. * hydrogen. * Contraction. 712 703 726 732 7 15 710 722 706 712 715 709 713 7 05 717 743 720 705 724 721 7 00 712 715 718 727 713 720 720 718 738

867 794 790 683 781 707 882 717 904 700 766 693 908 708 923 694 918 695 850 69 7 901 693 923 699 7 84 686 83 1 700 857 729 857 707 896 691 918 704 912 706 912 682 923 695 912 700 973 706 916 716 910 701 902 708 965 705 953 702 980 723

* In hundredths of a C.C.

Nitrous oxide. Per cent.

97.5 97-2 97.4 98-0 98 97.7 98.1 98.3 97.6 97.5 97.7 98.1 97-3 97.6 98.1 98-2 98.1 97.3 98 97.4 97.6 98 98.3 98.5 98.3 98.3 98 97.8 97-9

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702 SAUNDERS :

Decomposition of the Pure Nitrate [up to 260"l. Pure dry ammonium nitrate decomposes only slowly at 200",

liberating 20 C.C. of gas per 50 grams of nitrate per hour. The nitrate first melts a t 169", then dissociates into nitric acid and ammonia, and begins to evolve a gas containing 98 per cent. of nitrous oxide. Free nitrogen is always present; the quantity is nearly 2 per cent. up to 260" and considerably exceeds this a t higher temperatures (see p. 710). Nitrogen peroxide and nitric oxide are also present, the average amount between 220" and 260' being 0.001 per cent. of each.

The liquid products of the reaction contain nitric and nitrous acids and a small quantity of ammonium nitrate, which, being volatile, is carried over with the gas.

There is evidence that, after dissociation, the action between the ammonia and nitric acid proceeds even in the condensing tube; a glass rod held at the mouth of the heating tube is quickly coated with a deposit of nitrate, whereas in the condensing tube only a trace of ammonia is found,

The following equations represent the course of the decom- position :

1. NH,NO, f/ NH, + HNO, 2. NH,NO, -+ N,O + 2H,O 3. 5NH, + 3HN0, + 9H,O + 4N2,

the second denoting the main change. In support of the third we have the fact that the liquid which collects is always acid, showing that an excess of ammonia is used up in the reaction; also free nitrogen is always present.

The rate of evolution of gas increases with rise of temperature and up to 250" is steady. Above this point, the gas is often evolved in rushes, and in two instances a slight explosion took place just above 260". The experiments were therefore not carried further in this form of apparatus.

The InJluence of Impurities. (a) ChEorides.-The commonest impurities in commercial am-

monium nitrate are the chlorides of ammonia and sodium. In order to study the modifying influence of these substances, a series of mixtures was made containing quantities of chloride ranging from 0-1 to 2 per cent. In all cases the " impurity " was specially purified by recrystallisation. A known quantity of the mixture is introduced into the tube H . The temperature of the thermostat having been adjusted, the tube is inserted, and the gases are collected in the ordinary manner. The tube L acts as a condenser,

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THE DECOMPOSITION OF AMMONIUM NITRATE BY HEAT. 703

and the gases, after passing this, flow through the tube containing sodium hydroxide, where chlorine is absorbed. The surface of the mercury in 0 is covered with a layer of sodium hydroxide solution to ensure complete absorption of chlorine. The remaining gases are then submitted to analysis as before. In order to make measurements of the quantity of chlorine present, the gas is passed through the tube M into a solution of potassium iodide, the remain- ing gas being collected over mercury and its volume measured. The percentage of chlorine may then be calculated.

FIG. 2. Graph showing compocritwn of gaa when ammonium chloride is prwent.

250'

240

230 w' 2 g 220 w

P 210

200

190

Chlorine per cent. Nitrow, oxide per cent.

These experiments were conducted as before in two complete series: in the first, general observations were made and rises of temperature noted; in the second, where the joints were glass- sealed, the final readings were taken. The reason for making the two series is as follows. When the temperature of the mixture is gradually raised until the reaction sets in, 'there is a sudden evolution of gas which contains chlorine; then the action suddenly slows down. On account of the rush of gas a t first obtained, it was thought that there might be an initial rise of temperature, and on introducing a second thermometer into the reacting mixture this was found to be the case. This rise depends on the quantity of the reacting substances and the temperature of the thermostat. It was therefore not possible to keep the temperature actually

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704 SAUNDERS :

constant, and the following method was adopted-the thermostat was set to a definite temperature, and by means of a second thermometer the initial rise was measured. Once this preliminary reaction was a t an end, the temperature gradually fell to that of the thermostat. The initial reaction was usually completed in from five to thirty minutes. Table 111 gives the rises (the differ- ences of columns 2 and 3) and induction periods for various mixtures.

An examination of the relative amounts of chloride in the original and final mixtures from which the gas was being evolved showed

FIG. 3. Graph showing composition of gas when sodium chloride is present.

250'

240

230 6 % s g 220

P c 210

200

190

Chlorine per cent. Nitrous oxide per cent.

that the chloride was decomposed faster than the nitrate. Hence it might be expected that the final composition of the gas would be the same as that evolved from the pure nitrate, provided that the heating were continued long enough.

Five sets of experiments were made, using 0.1, 0-25, 0.5, 1, and 2 per cent. of chloride respectively. These were conducted at different temperatures, and in each case measurements were made of the quantity of nitrous oxide, chlorine, and nitrogen in the gas evolved during the preliminary reaction. The results are shown in tables IV and V and illustrated in Figs. 2 and 3. According to the above experiments, the actual temperature of the molten mixture was some 10" to 20" higher than that here represented.

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THE DECOMPOSITION OF AMMONIUM NITRATE BY HEAT. 705

This rise was a somewhat variable quantity, depending on the mass and other factors. Therefore the same weight of material was used on each occasion and the temperature of the thermostat taken as the basis of calculation.

The modifying influence of sulphates and sodium nitrate was next investigated.

TABLE 111. Temp. of the

Impurity thermo- per cent. stat. 1

0.1 NH,Cl 215'

0.25 ,, l t 3 205

0.5 ,, 215 185 205

1 9 ,

2;o

Temp. Period

mixture. duction. of the of in-

224" - 226 -

222 - 236 10min. 198 25 ,, 222 5 ,) 225 8 ,, 211 15 ,, 215 15 ,, 218 27 ,,

- 1 hr. 20m.

Temp. of the

Impurity thermo - per cent. stat. I

0.1 NaCl 210"

2 ,, 2i)o 230

1 (NH4&304 210 2 9 ,

2i)o

Temp. of the

nixture. 222" 219 218 223 214 213 23 3 2 60 221 2 15 225

Period of in-

duction.

TABLE IV. Table showing the variation of the amount of chlorine in the gas

with different percentage admixtures of impurity and with temperature.

Temp. 192" 212 236 196 206 234 194 205 230 191 205 236 195 210 230

Impurity per cent. 0-1 NaCl

9 ,

d.i5

0 -2 9 ,

Y Y

3 ,

1 Y ,

Y ,

2 9 ,

1 9

Chlorine per cent.

0.6 0-96 1.9 1-6 1-9 2-7 1.6 2.2 3-85 2.0 2-5 3.8 2.4 3.5 -

Temp. 193" 212 230 190 203 235 190 205 230 191 205 235 196 205 230

Impurity per cent. 0.1 NH,Cl

9 9

0% 5

0 S Y i

7 ,

Y ,

Chlorine per cent.

0.8 1.5 2-2 1.2 1.5 2.2 1.3 2.0 3.1

2-45 4.1 2-35 3.1 5.75

-

General Nature of the Results obtained with Chloride Mixtures.- It is difficult to state definitely the temperature a t which the decomposition commences. Taking as a typical example the behaviour of 50 grams of a mixture containing 1 per cent. of ammonium chloride, the reaction sets in at 199" if the temperature

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706 SAUNDERS :

TABLE V. Table showing the composition of the gas (by avcrages) with relation

to temperature. Ammonium chloride.

1mpWit-y . .. - per cent. Temp. Cl,. N,O. N,.

0.1 193' 0.8 70.0 29.2 212 1.5 59.9 38-6 230 2.2 53.3 44.5

0.25 190 1.2 75.5 23-3 205 1.5 64.2 34.3 235 1.75 52.4 45-4

0.5 190 1.3 73.8 24.9 205 2.0 62.2 35.8 230 3.1 48.5 48.4

1 191 1.9 70.0 28.1 205 2.45 59.0 38-55 235 4.1 43.3 52.6

2 196 2.4 61.0 36-6 205 3.1 52.2 44.7 230 5-75 37.0 57.25

- -_ Temp.

192O 210 235 196 206 230 194 205 230 191 205 236

195 210

Sodium chloride. -. _I_, \.--

a,. N,O. N,. 0.6 70.6 28.8 0.95 59.0 40.05 1.9 52-8 45.4

1.6 69.4 29.0 1-9 59-8 38.3 2.7 50.5 46.8

1.6 71.2 27.2 2.2 59.8 38.0 3-6 47.8 40.6 2.0 71.6 26.4 2-6 59.0 39.4 3.8 43.3 62.9

2.4 65.4 32.2 3.4 5 4 4 42-2

The graphs are constructed from these tables.

is raised directly. If the same mixture is kept a t NO", the action will start after one and a quarter hours; and the temperature may now be lowered to 175" without causing it to stop completely. At 210°, the decomposition is not unduly rapid, but at higher temperatures the mixture froths violently and is carried over by the rush of gas.

When any mixture is raised to a particular temperature there is at first a passive period, varying from a few minutes to one and a half hours (Table 111), depending on the amount of impurity present, on the temperature, and possibly on the presence of free nitric acid (compare Veley, Chem. News, 1883, 41, 299). Then a new reaction sets in, chlorine is evolved, and in some cases the amount of nitrous oxide falls by 50 per cent., whilst the percentage of nitrogen increases. This is accompanied by a marked rise of temperature, and directly this has attained a maximum the flow of gas suddenly slackens. During the first period the composition of the gas remains constant for any particular mixture, but as soon as the temperature begins to fall, most of the chloride having decomposed, the composition of the gas gradually approximates to that given by the pure nitrate a t the same temperature, the process taking several hours.

The action of the chlorides of ammonia and sodium is very similar, that of the former being the more marked. In all cases the amount of chlorine present depends on the amount of impurity

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THE DECOMPOSITION OF AMMONIUM NITRATE BY HEAT. 707

and on the temperature. The action of the chloride is certainly catalytic in character, 1 part in 1,000 being quite sufficient to alter entirely the main course of the decomposition.

Of the three reactions mentioned on p. 702, the last assumes an important part in the case of chloride mixtures, especially when the temperature rises above 215", the chloride accelerating this reaction and not the second. Further, the acid liquid which collects in the condensing tube now contains hydrochloric acid also, and this may be formed by the interaction of chlorine and ammonia-3C1, + 2NH, + N, + 6HC1, thus accounting for the presence of some of the nitrogen. The chlorine is probably formed by the inter- action of the hydrochloric and nitric acids set free by dissociation.

( b ) Ammonium Su1phate.-With a 1 per cent. mixture the tem- perature may be raised to 200" before the action sets in, with an initial rise of 10". The action is very sluggish, the more so as the proportion of sulphate is increased; with a 2 per cent. mixture the lowest working temperature is 215", when the initial rise is only 5". The gas evolved sometimes contains a trace of ammonia, but otherwise there is no change in its composition.

(c) Sodium Su1phntc.-A 1 per cent. mixture does not influence the reaction between 220" and 250".

(d) Sodium Nitrate.-Mixtures with 1 per cent. and 2 per cent. of sodium nitrate are without characteristic action.

Explosive Decornposition. In carrying out this series of experiments, considerable modi-

fications of both apparatus and method were found to be necessary. A small, thin-walled, iron cylinder, G (Fig. 4), was used, in which the nitrate was heated. It is fitted with a cover containing a glass observation tube, D, plugged with glass wool, and a hole just large enough to take the tube F . The other part of the apparatus consists of two flasks, A and B, thoroughly dried and connected by means of capillary tubing with a three-way tap, all joints being sealed with paraffin wax. The apparatus having been tested for the maintenance of a vacuum, the flasks are exhausted in turn by means of a Fleuss pump.

A charge of nitrate is inserted and the temperature gradually raised : white fumes, which consist of sublimed ammonium nitrate, appear in the observation tube, D ; during this interval, nitrous oxide is evolved as usual and drives out the air from the apparatus. Further heating causes red fumes to appear, and almost imme- diately there follows an explosion which blows out the plug E . The end of P is now inserted well into the cylinder so that the

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708 SAUNDERS :

end reaches under the tube D . By turning the tap C , first one and then the other flask is filled with the gas from G, the nitrous oxide having been driven out when the explosion occurred. The object of the tube D is to provide a means of ascertaining that there is no back-rush of gas when the flasks are being filled. Thus there are two samples of gas ready for analysis. From preliminary experiments, it was found that the gases contained large quantities of nitric oxide and nitrogen peroxide, whilst the amount of nitrous oxide seemed to be very small. Hence a new method of analysing the gases was employed. The end of the tube F is inserted in a known volume of standard permanganate solutlion acidified with

FIG. 4. FIQ. 5. fl

sulphuric acid, and the tap opened, when a little of the solution enters the flask. The solution is decolorised, a partial vacuum produced, and the tap is again opened; this process is repeated, the permanganate being each time allowed to enter and well shaken, until the solution is no longer decolorised, the unchanged per- manganate showing that the absorption of nitric oxide is complete. The residual gas is now measured a t atmospheric pressure. The end of the tube F is connected with a suitable gas burette, N (Fig. 5), and the reservoir, 0, is lowered so that the air in the capillaries is drawn into N and then expelled through 8 and T. The flask, B, is put in communication with N , and the reservoir, R, lowered until the mercury stands a t the zero mark. The tap, R, is closed, the levels of the mercury are adjusted, and the true

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THE DECOMPOSITION OF AMMONIUM NITRATE BY HEAT. 709

volume is found. Knowing the volume of the flask and the volume of the liquid it contains, the true volume of the gas may be calcu- lated. This sample of gas is then used for the estimation of nitrous oxide. There will be a slight error due to the solubility of this gas in the permanganate, but it will never exceed 1 per cent. under the conditions of these experiments, and a correction may be applied. The nitrous oxide is estimated as described on p. 700. The method of estimating nitric oxide and nitrogen peroxide is carried out as follows. The remainder of the permanganate is admitted to the flask, the tap opened, and the cork removed. The whole of the permanganate is then titrated with AT/2-ferrous sulphate until it is colourless. It is best to use a slight excess to dissolve any precipitate that may be formed, and to determine the end- point by back-titration. The volume of permanganate reduced by the sample of gas is thus determined. The total combined nitrogen is found by the method of Bowman and Scoft. A known volume of the above solution (permanganate and ferrous sulphate) is run with careful cooling into ten times its volume of concentrated sulphuric acid, and then titrated with ferrous sulphate until a brownish-pink colour appears. At this stage the whole of the nitrates produced by the oxidation with permanganate has been reduced to nitrous acid. The colour change requires 0.2 c.c., which is subtracted from the volume observed. I n these experi- ments, there is yet another source of error-the presence of nitric acid vapour and ammonium nitrate among the gaseous products of decomposition. To determine what correction has to be made, the second bulb, A , filled with the same sample of gas, is utilised. By means of the three-way tap a little dilute sodium hydroxide solution is allowed to enter the bulb; this dissolves any nitrate, and combines with the nitrogen peroxide and nitric acid. I n order that the nitric oxide shall not interfere with the result (owing to oxidation and then combination with the alkali), it is removed by means of the pump, and air admitted. Then another estimation is carried out as before.

By the permanganate titration, the amount of nitrogen peroxide is calculated. Then, by the ferrous sulphate titration, the total nitrate is found. If this is in excess of that demanded by the amount of nitrogen peroxide present, the corresponding correction to be used in the analysis of the contents of B is applied. I f a = C.C. of NIB-permanganate reduced by the gas in the first titration, and b = C.C. of NIB-ferrous sulphate required in the second titration, after subtracting the above correction, then the volumes of nitrogen peroxide and of nitric oxide a t N.T.P. are 2-8 (3b - 2a) C.C. and 2-8 (2a - b ) C.C. respectively.

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710 THE DECOMPOSITION OF AMMONIUM NITRATE BY HEAT.

TABLE VI. Explosive decomposition.

In each case the amounts of nitric oxide and nitrogen peroxide are calculated froni the formula, and the remaining gases calculated to N.T.P. and percentages taken.

No. of expt. :- 1 C.C. of N/2-KMn04 ............ 16-28 C.C. of N/2-FeS04 ............ 14.3 Vol. of NO, ..................... 29 Vol. of NO ........................ 51.2 Vol. of remaining gases ...... 76 N20 in ), - t NO2 per cent. .................. 18.6 NO y y .................... 32.7 N,O ,, .................... - N, .. y y (by diff.) ...... 48.7

. . . . . . . .

2 3 4 5 15.7 8.9 16.0 15.7 13-3 7.5 14.6 14.6 23.7 12.9 33 34.7

76 42 so 82 6 * 8 * 17 -t

15.8 15.4 20-4 20.6' 32-7 34.5 30.2 30.8 3- 1 4 8.4 -

47.5 46.1 41.0 48.5

60-3 28.9 48.7 52

* These mixtures refused to explode, so sparks were passed until there was no further contraction.

t The quantity of nitrous oxide is less than 5 per cent. and impossible to estimate under the conditions of the experiment; i t is therefore included with the nitrogen.

As may be seen from these results, the decomposition at the lowest temperature a t which an explosion will occur is very different from that which occurs normally. The amount of nitrous oxide is greatly diminished, and the gas contains nitrogen peroxide, nitric oxide, and nitrogen in the approximate ratio 2 : 4 : 5 .

Percentage composition Percentage composition of the gas, neglecting

N,O. calculated for the

ratio 2 : 4 : 5. Nitrogen peroxide ......... 18.4 18.2 Nitric oxide .................. 32.7 36.3 Nitrogen ..................... 46.3 45.5

The agreement is sufficiently close, considering the nature of the experiments, to justify the statement that the main course of t,he reaction is :

8NH,NO, -?- 16H,O + 2N0, + 4N0 + 5N,. The explosion is accompanied by a yellow flame not unlike that

of ammonia burning in oxygen.

Summary of Results. Pure ammonium nitrate decomposes into nitrous oxide and

water to the extent of 98 per cent., the course of the decomposition remaining unaltered over a range of temperature from 210" to 260".

,4t some point near 300" other oxides of nitrogen are evolved, $he action proceeding explosively.

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NOTE ON THE EFFECT OF ELECTROLYTES, ETC. 711

Nitrogen is always present; with the pure nitrate up to 260', 2 per cent.; immediately after explosion, 46 per cent.; in the case of chloride mixtures, 30-50 per cent.

At the moment of explosion, the nitrate decomposes in a totally different manner, giving nitrogen peroxide, nitric oxide, and nitrogen in the ratio 2 : 4 : 5 .

With the pure nitrate, the rate of evolution of the gas is dependent on the temperature and on the mass of material; but when any impurity is present, this does not necessarily hold good.

With chlorides, a quantity as small as 0.1 per cent. has a very marked influence on the decomposition, chlorine being invariably present in the gas evolved.

The quantity of chlorine is proportional to the amount of chloride and to the temperature.

When chlorides are present, the rate of evolution of gas is very much accelerated, the addition of 1 per cent. producing a result comparable with that caused by a rise of temperature of 25-30" in the case of the pure nitrate.

The higher the initial temperature, the poorer the gas is in nitrous oxide for any given mixture.

At any given temperature the amount of chlorine falls off as time goes on, and the composition of the gas approximates to that given by the pure nitrate a t the same temperature.

The liquid products of the reaction in these cases always contain hydrochloric acid as well as nitric acid.

Sulphates are without characteristic action. Sodium nitrate in small quantities does not influence the decom-

position at temperatures below 250".

This work was undertaken at the suggestion of Dr. E. P. Perman. I desire to express my indebtedness to him for his helpful criticism.

UNIVERSITY COLLEGE, CARDIFF. [Received, November 26th, 1921.1

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