Active Nitrogen

When the electrical discharge is passed through nitrogen at low pressures, a yellow glow is seen which persists for a short time after the gas has left the region of the discharge. Although this observation had been made some time before, it was left for Strutt in 1911 to examine thoroughly the physical properties of the afterglow and the chemical reactions of the active form, which differ widely from those of ordinary molecular nitrogen.

The production of the glow is powerfully catalysed by the presence of small quantities, about 0.1 per cent., of O₂, CO, CO₂, H₂S, SO₂, Cl₂, CH₄, and several other gases. Indeed there has been a considerable controversy as to whether it can be produced at all in absolutely pure nitrogen. Strutt prepared his nitrogen by passing the commercial gas over liquid sodium-potassium alloy, or over sodium at about 300° C., and observed the phenomena described. Tiede and Domcke found that if oxygen is completely absent from the nitrogen, i.e. if it is derived from potassium or barium azide, there is no indication that the active form is produced. Further work in collaboration seemed to show that a catalyst, such as traces of the gases mentioned, must be present for the optimum activatibn, but that in certain forms of apparatus this may be induced in pure nitrogen. It may, however, be taken as an established fact that atmospheric nitrogen freed from oxygen by combustion with an excess of phosphorus does not show the effect, nor was it observed by Strutt in the case of commercial nitrogen which had been in contact with molten sodium for several hours. In this connection it may be noted that although the glow is generally taken as the most important evidence that activation has taken place, it is not always a necessary concomitant of chemical activity.

The following method of preparation is to a great extent quoted from the description by the original investigator, and his diagram was used in drawing the apparatus.

Preparation and chemical examination of active nitrogen а, discharge tube. b, observation and reaction tube. c, tube through which gases are introduced. d, vessel for condensation. e f, spark gap. g, Dewar cylinder.

Commercial compressed nitrogen, prepared by the rectification of liquid air, is stored over water in a gas-holder in which is hung a perforated metal bucket containing phosphorus. Before use, the gas is dried by passing through an ordinary tube of phosphorus pentoxide. A stream of this nitrogen under low pressure is passed through a tube a, in which a vigorous jar discharge is maintained. The gas is then drawn by a powerful air-pump into the vessel b, where it exhibits a brilliant yellow light, which may persist for several minutes. The light, when analysed by the spectroscope, shows some, but not all, of the bands produced by nitrogen in a vacuum discharge. There is also a bluish-violet light which is masked by the yellow, but which may be seen by interposing a piece of blue glass. This light consists of two sets of bands, the β and γ series. The β bands were new, but the y bands are produced when nitrous and nitric oxides are introduced into a Bunsen flame.

The yellow and blue glows may be photographed separately by using, first an isochromatic plate with a yellow screen, and then an ordinary plate with a blue screen, or no screen, since the yellow light has only a very slight effect on a photographic plate.

Some of the chemical reactions have been conspicuously demonstrated in this apparatus. When acetylene is admitted a lilac glow is developed. The hydrocyanic acid produced may be condensed by surrounding the vessel d with liquid air, or it may be drawn through a solution of sodium hydroxide and identified by the Prussian blue reaction. When the activated nitrogen is passed through a U-tube containing cold mercury, the surface of the mercury quickly becomes fouled, and when it is distilled with water, ammonia can be detected in the distillate by the aid of Nessler solution.

The glow disappears on heating, and also under a feeble electric discharge. At ordinary temperatures it persists for about a minute. Exposure to the temperature of liquid air intensifies it, but diminishes its duration. It is greatly increased by compression, but when the former volume is restored the capacity to produce the glow is exhausted. The heat of formation is about -40,000 cals. per mol. of nitrogen. It is not affected by an electric field, but possesses a considerable electrical conductivity, comparable to that of a salted flame. The spectrum is not the same as that of ordinary molecular nitrogen; it contains a green, a yellow, and a red band.

Reactions take place readily with metals, non-metals, and compounds. Sulphur and iodine show a blue flame. In the case of sulphur some matter, probably a sulphide of nitrogen, is deposited on the glass. The yellow form of this sulphide is obtained by the action of active nitrogen on sulphur chloride. White phosphorus is changed into the red variety, and at the same time a nitride is produced. Arsenic shows a green glow and gives a nitride.

The combination with metals is accompanied by beautiful and varied glows which exhibit the line spectra of the metals. The brilliant intensification of the sodium line when the metal, just above its melting-point, is brought into active nitrogen, constitutes a delicate test for the latter. Combination also occurs with zinc, cadmium, and mercury. The nitride of mercury is explosive.

It is remarkable that neither oxygen nor hydrogen combine with active nitrogen. The extinction of the glow which is caused by ammonia or oxygen in amounts over 2 per cent., or by oxides such as those of copper or manganese, may be due to chemical action, or to a catalytic action upon the recombination of the active form.

Nitric oxide is oxidised to nitrogen trioxide, which latter may be condensed to a blue liquid in the ordinary way. The reaction probably occurs in two stages. Thus, assuming that the active form consists of or liberates atoms:

N+2NO → NO₂+N₂;
NO₂+NO → N₂O₃.

Acetylene and other carbon compounds are decomposed; the nitrogen gives cyanogen or hydrogen cyanide:

C₂H₂+2N → C₂N₂+H₂.

In the case of alkyl halides and chloroform, halogen is set free:

2CHCl₃+2N → C₂N₂+2HCl+2Cl₂.

Strutt has endeavoured to determine the proportion of nitrogen converted into the active form:

1. By absorption with phosphorus. This showed the presence of about 0.5 per cent, of the active form.
2. By causing the gas to react with excess of nitric oxide and weighing the resulting nitrogen trioxide. This showed the presence of about 2.46 per cent. Probably this divergence is to be accounted for by the difficulty in keeping the conditions of the activation constant.

Although active nitrogen does not itself combine with oxygen (which in minute amounts favours its formation, and in larger amounts catalyses the recombination of the active nitrogen), it may yet play a part in the electrical methods of preparing oxides of nitrogen.

It is well known that high-tension discharges produce both ozone and oxides of nitrogen, and are indeed necessary if a good yield of oxides is to be obtained. Since the same conditions lead to the formation of active nitrogen, it has been suggested that the oxidation may be due to an interaction between active nitrogen and ozone. In the Leetham process (1903), which was devised to prepare a bleaching gas from the air, the latter passes through an ozoniser and then through a spark gap. In an investigation of this process it was found that passage in the reverse direction was equally effective in producing oxides of nitrogen, but that each form of discharge by itself had little effect. The amount formed was estimated by means of the absorption spectrum of NO₂, which can be detected to 1 part in 1800 by observation through a very long column (64 feet) of the gas. The active nitrogen and ozone, in insufficient amount, may be formed in the spark gap, and the further amount of ozone required in the silent discharge. The active nitrogen prepared by Strutt is not oxidised by ozone, so that it is not identical with the form prepared by Lowry.

The amount of energy evolved in the decomposition of active nitrogen with the formation of 1 molecule of molecular nitrogen, N₂, was estimated by Strutt (1911) as 1.2 to 1.8 times that evolved in the decomposition of 1 molecule of NO, i.e. 26,000 to 39,000 Cals. This result was in the main confirmed by more recent experiments, one of which will be described. Active nitrogen, produced as described above, was mixed with nitric oxide and allowed to stream at a steady rate through a tube surrounded by toluene, used as the calorimetric liquid, in a Dewar flask. The temperature rose, and then attained a constant value. The same constant temperature was then attained by an electrically heated coil immersed in the same calorimeter; the electrical energy supplied per second was equal to the heat evolved per second by the reaction. It was proved by a separate experiment that this heat was evolved by that reaction which is expressed as equation (2) below, since only a small proportion of NO₂ was formed in the rapid passage (0.7 sec.) of the gases through the calorimeter. The amount of NO₂ produced by the subsequent oxidation at liquid air temperatures (equation 3) was determined by titrating the iodine liberated from a solution of potassium iodide. Every molecule of NO₂ found, corresponds, of course, to the amounts expressed by equation (2), and since the heat of decomposition of 2NO is 43,120 cals., the heat of activation E is given by the equation

22,400 cc. of NO₂≡21,560+E/2Cals.

The amounts of heat found by this method, and by one involving the catalytic decomposition of active nitrogen in the presence of oxygen, ranged between 36,700 and 46,000 Cals.; and the mean of the mean of results obtained by each of the two methods is 41,200 Cals. The authors consider that the reaction should be expressed by the following equations, in which the dash denotes the activated or metastable molecules:

1. N₂'+NO → NO'+N₂
2. NO'+NO → N₂+O₂+E+43,120 Cals.
3. 2NO+O₂ → 2NO₂

The electrical conductivity of active nitrogen suggests that it might simply be the ionised gas; but this hypothesis is not tenable for several reasons, among which may be mentioned the fact that the removal of charged particles which have survived the discharge has no effect upon the afterglow.

The chemical properties would suggest that it is triatomic nitrogen, analogous to ozone, but such an allotropic form would probably be condensed at liquid air temperatures, which does not occur so far as is known at present. Additional evidence against this view is the comparatively simple nature of the spectrum.

Rayleigh himself considered that it is probably atomic nitrogen, in support of which we may note the following:

1. The simplicity of the spectrum.
2. The groups of lines in the spectrum agree with the amounts of energy required to convert nitrogen into the atomic condition, i.e. a total of 11.4 volt-faradays, or 263,000 cals.
3. The afterglow is lengthened by heating and shortened by cooling. This behaviour is unique, and suggests that the reaction is also unique.
4. The rate of decay of the glow follows a bimolecular law.

Certain difficulties in the way of this hypothesis have been stated by those who maintain the opinion that active nitrogen consists of metastable molecules in an excited condition. The heat of activation actually observed, i.e. about 40,000, with a maximum of 46,000 Cals., is less than that calculated for the dissociation of nitrogen into atoms, which is estimated at more than 190,000 Cals., and is probably between 300,000 and 400,000 Cals.

A discharge which is of such a nature as to give only the line spectrum of nitrogen does not produce the afterglow.

These difficulties may be largely overcome by supposing that active nitrogen contains at least two distinct molecular species:

1. Metastable molecules with a lower heat of formation.
2. A much smaller proportion of atoms which combine to form high level excited molecules with a higher heat of formation.

The former are responsible for the chemical activity, the latter for the glow. These two manifestations of activity are only related to one another in a complicated manner.