Nitrogen Tetroxide, N₂O₄

The formation of brown nitrous fumes under varying conditions was noted by many earlier experimenters. Thus Hales in 1727 obtained these fumes by mixing nitric oxide (nitrous air) with air. Scheele in 1777 mentions their removal from fuming nitric acid by heat; and in the same year Priestley investigated the gas obtained by the action of nitric acid on many metals, as well as the preparation by oxidising nitric oxide. Priestley proposed a method for testing " the goodness of common air " by mixing air with excess of nitric oxide over water. He was the first to notice a darkening of colour when the gas was heated,8 which we now know of course was due to dissociation of the tetroxide molecules.

Cavendish in 1781 showed the formation of nitrous fumes when electric sparks are passed through air, and these fumes dissolved in water to form nitric acid. Davy in 1800 attempted to determine the composition of nitrogen peroxide by mixing nitric oxide and oxygen and absorbing the product in water, but his results were not very successful. A more accurate method was adopted by Gay-Lussac in 1816, who measured the contraction which occurred when nitric oxide and oxygen were mixed over mercury. He concluded that 2 volumes of nitric oxide and 1 volume of oxygen produced 1 volume of nitrogen peroxide. This result is approximately true at ordinary temperatures where there is a preponderance of N₂O₄ molecules.

1. Nitrogen tetroxide, or Nitrogen Per-Oxide, is ultimately obtained by mixing nitric oxide and oxygen in the proportions of 2:1 by volume. Complete mixing is obtained by passing the gases through a tube containing broken glass, and the tetroxide may be condensed to a colourless crystalline mass in a U-tube kept at -20° C.:
   
   2NO + O₂ = N₂O₄.
2. The nitrates of the heavy metals decompose on heating into a mixture of nitrogen tetroxide and oxygen. The former may be condensed by passing through a U-tube immersed in a freezing mixture:
   
   Pb(NO₃)₂ = PbO + N₂O₄ + ½O₂.
3. The reduction of nitric acid by arsenious oxide is one of the most convenient methods for preparing quantities of nitrogen tetroxide:
   
   As₂O₃ + 4HNO₃ = 2HAsO₃ + 2N₂O₄ + H₂O.
   
   With nitric acid of specific gravity 1.40 or 1.45 a certain amount of nitrogen trioxide is also produced. Cundall's method is to add a cooled mixture containing 315 grams of fuming nitric acid and 150 grams of sulphuric acid to 250 grams of coarsely powdered arsenious oxide in a litre flask. The reaction commences slightly above room temperature, and requires external cooling. A reflux condenser is fitted to effect preliminary cooling of the gas, which is passed through a U-tube containing phosphoric oxide, and then condensed to a liquid in a U-tube surrounded by ice and salt. The distillation is stopped as soon as the liquid in the flask turns a green colour; all joints are glass-joints, cork and rubber being quickly attacked. Purification of the liquid nitrogen tetroxide is effected by treating with fuming nitric acid and excess phosphoric oxide:
   
   2HNO₃ - H₂O = N₂O₅;
   N₂O₃ + N₂O₅ = 2N₂O₄.
   
   The liquid is finally distilled, passed over powdered arsenious oxide, and reliquefied by cooling.
4. Fractional distillation of fuming nitric acid gives nitrogen tetroxide contaminated with small amounts of nitric acid and nitrogen trioxide.
5. Nitrosyl-sulphuric acid warmed with potassium nitrate evolves nitrogen tetroxide:
   
   H(NO)SO₄ + KNO₃ = KHSO₄ + N₂O₄.
6. A number of methods of preparation from nitrites have been devised:
   1. The action of nitric acid upon a nitrite:
      
      HNO₃ + NaNO₂ = NaOH + N₂O₄.
   2. The action of fuming sulphuric acid upon a fused mixture of sodium nitrate and nitrite, to which sufficient sodium bisulphate has been added to fix free sulphur trioxide:
      
      NaNO₃ + NaNO₂ + H₂S₂O₇ = 2NaHSO₄ + N₂O₄;
      2NaHSO₄ + SO₃ = Na₂S₂O₇ + H₂SO₄.
   3. Nitrogen tetroxide is produced when fused alkali nitrites are electrolysed.
   4. Iodine reacts with silver nitrite with the liberation of nitrogen tetroxide:
      
      2AgNO₂ + I₂ = 2AgI + N₂O₄.
7. A good yield of nitrogen tetroxide is obtained by passing a mixture of air and steam through a thin porcelain tube maintained at 1600° C. The hydrogen produced diffuses through the tube, and the rate of diffusion is accelerated by raising the pressure inside the tube and lowering the pressure outside.

Nitrogen tetroxide at ordinary temperatures is a brown gas with a characteristic smell, and is toxic even when inhaled in small quantities. The density shows a considerable variation with temperature owing to the dissociation:

N₂O₄ ⇔ 2NO₂.

The variation of density (referred to air=1) with temperature is given in the following table: -

Temperature, ° C.	Density
4.2	2.588
11.3	2.645
24.5	2.520
97.5	1.783
28	2.70
32	2.65
52	2.26
70	1.95
79	1.84
26.7	2.65
35.4	2.53
39.8	2.46
49.6	2.27
60.2	2.08
70.0	1.92
80.6	1.80
90.0	1.72
100.1	1.68
111.3	1.65
121.5	1.63
135	1.60
154	1.58

These data permit the calculation of the degree of dissociation "α" from the relation:

1 + α = Δ/D, i.e. α = (Δ-D)/D;

in which Δ is the theoretical density of N₂O₄ (e.g. relative to air) and D is that observed.

Since this gas shows a strong selective absorption in regions of shorter wave-length, a red line at λ = 6438Å. was chosen when the density and refractivity are both reduced to N.T.P. For NO₂, (w-1)=0.0005087; for N₂O₄, (n – 1) = 0.001123.

Solid colourless nitrogen tetroxide apparently consists of N₂O₄ molecules entirely, but the liquid contains some NO₂ molecules, and the concentration of these latter increases with rise of temperature and is followed by the deepening of colour. Thus, between the temperatures of -10° to 26° C. the liquid gradually becomes a deeper yellow, until at the boiling-point (26° C.) it assumes a distinctly orange colour. The reddish-brown vapour continues to show an increasing deepening of colour, until at a temperature of 140° C. the gas is almost black owing to its complete dissociation into the single NO₂ molecules:

N₂O₄ ⇔ 2NO₂.

The following table gives the percentages of NO₂ molecules present at varying temperatures which have been calculated from the density determinations:-

Temperature, ° C.	Density (H2=2)	NO2 Molecules (per cent.).
26.7	76.6	20
60.2	60.2	50.04
100.1	48.6	79.23
135.0	46.2	98.96
140.0	46.0	100.00

The dissociation constant of the equilibrium, N₂O₄ ⇔ 2NO₂,

is given by



and the variation of this constant with temperature following table: -

Temperature, ° C	0.0	18.3	49.9	73.6	99.8
Dissociation constant K	8.060	3.710	1.116	0.544	0.273

Dissociation constants of nitrogen tetroxide.

Fig shows these results graphically.

The thermal dissociation of nitrogen dioxide (NO₂) into nitric oxide and oxygen above 150° C. under different temperatures and pressures has been studied, with the following results:-

Temperature, °C.	Pressure, mm.	Density.	Percentage of NO2 Dissociated.
130	718.5	1.600	. . .
184	754.6	1.551	5
279	737.2	1.493	13
494	742.5	1.240	56.5
620	760.0	1.060	100.0

The variation in the density with the pressure at one temperature, namely, 49.7° C. (see table below), indicates, as was to be expected, an increase in the dissociation with decreasing pressure, since in the dissociation the number of molecules derived from a given weight of the gas is increased, and this tends to increase the pressure. The expression, which should be constant, is



When a is expressed in terms of the densities, and the whole expression is divided by constants which are independent of the dissociation, i.e. Δ and R, and also of the temperature (since this is constant), the equilibrium constant is represented by



Dissociation and pressure at 49.7° C.:

P (mm.)	0	26.8	93.75	182.69	261.37	497.75
Density	(1.590)	1.663	1.788	1.894	1.963	2.144
α	1.000	0.93	0.789	0.690	0.630	0.493
K'	106	106	112	124	130	121

Thermal dissociation of N2O4 and NO2

Thus, under these conditions, the gas is nearly 50 per cent, dissociated at a pressure of 497.75 mm.

Fig. shows the dissociation of N₂O₄ into NO₂, and of NO₂ into nitric oxide and oxygen.

Specific Heats of Nitrogen Tetroxide

The specific heat at constant pressure of nitrogen tetroxide varies considerably with temperature, owing to dissociation. It has been pointed out that the determinations of Berthelot and Ogier took no account of barometric fluctuations over different ranges, and further, that the heat of dissociation was included in the specific heat. The following table gives results obtained by use of a constant flow method which obviated the necessity of measuring the temperature change and also the water equivalent of the calorimeter:

Temperature, ° C.	Total Heat Capacity.	Heat of Dissociation.	Specific Heat.
33.73	126	114.6	11.4
41.00	146	134.2	12.0
44.00	154	141.6	12.0
55.03	176	160.8	15.2
60.90	178	163.3	14.7
63.33	179	162.4	16.6
70.70	168	153.1	14.9
80.89	143	126.9	16.1
97.51	93	75.5	17.5

These results show that the specific heat of nitrogen tetroxide is very small compared with the heat of dissociation, and also that the value C_p for N₂O₄ is slightly greater than C_v for NO₂.

The following table gives the results of a number of observers which have been collected and recalculated:-

Temperature, ° C.	Pressure, mm. Hg.	Percentage Dissociation.	Cp	Cv	γ=(Cp/Cv)
25	599	19.51	19.98	17.92	1.1154
23	304	25.00	16.96	14.93	1.1359
23	106	40.54	13 70	11.69	1.1712
22	218	28.24	17.19	15.17	1.1330
21	500	11.21	22.22	20.17	1.1020

The heat of dissociation of gaseous nitrogen tetroxide has been given by various observers as -13,000, -13,050, -12,900, -13,920, and -13,132 calories respectively.

The heat of formation of nitrogen tetroxide (containing 9 per cent, of NO₂) from 2N + 4O = N₂O₄, is -3900 calories.

In the case of completely dissociated NO₂ the value is -8125 calories, and with completely associated N₂O₄ (liquid), -2650 calories. The heat of formation of liquid nitrogen tetroxide (NO₂ = 46 grams) is -2200 calories.

The heat of vaporisation at 18° C. is -4300 calories. The latent heat of fusion varies from -32.2 to -37.2 calories per gram, which corresponds to -2960 to -3420 calories per mol. of N₂O₄. The value calculated from the depression of the freezing-point is -33.7 calories per gram. The thermal conductivity for NO₂ at 150° C. is 0.0033.

The thermal expansion of liquid nitrogen tetroxide is very regular, as is evident from the following data: -

Temperature, ° C	0	5	10	15	20
Volume	1.0000	1.00789	1.01573	1.02370	1.03196

Molecular weight determinations in acetic acid show that liquid nitrogen tetroxide is not polymerised beyond the N₂O₄ molecule.

The effect of ultra-violet light on nitrogen tetroxide has been studied, and it is found that an increase in pressure occurs which is not due to heating effect of the absorbed light. This pressure increase is due, firstly, to a photochemical equilibrium in the system:



while in the second place there is a heating effect due to the recombination of nitric oxide and oxygen as well as by absorbed radiation.

The refractive index of gaseous nitrogen tetroxide is 1.000503 at 36° C.

The spectrum of nitrogen tetroxide has been studied by Bell, who found that there was an increase in the general and selective absorption with rise of temperature. The conclusion that the absorption spectrum is due entirely to NO₂ molecules is supported by the fact that no absorption is shown by liquid nitrogen tetroxide a few degrees below 0° C.

As mentioned in the preparation, nitrogen tetroxide is easily liquefied at atmospheric pressure by condensing the gas in a freezing mixture. The liquid is pale yellow in colour and boils at 22° C.

The variation of the density of liquid nitrogen tetroxide with temperature is shown in the following table:

Temperature, ° C	-5	-4	-2	-1	0	+5	+10	+15	+21.6
Density	1.5035	1.5030	1.5020	1.5000	1.4935	1.4880	1.4770	1.4740	1.4398

The density of liquid nitrogen tetroxide between 0° and 21.5° C. may be represented by the equation

D₄ = 1.490 – 0.00215t.

The vapour pressure of liquid nitrogen tetroxide at different temperatures is given in the following table:

Temperature, ° C.	Vapour Pressure, mm. Hg.
-23.0	70
-10.0	146
-6.9	180
-0.6	256
+7.7	293
15.0	565
21.45	770
27.4	1007
39.0	1668
43.2	1982

The critical temperature of nitrogen tetroxide is 158.2° C., and the critical pressure 100±2 atmospheres.

Freezing-point curve of nitrogen tetroxide and nitrogen trioxide mixtures.

The freezing-point curve of nitrogen tetroxide and trioxide mixtures is shown in fig. Solid tetroxide or trioxide separates according to the richness of the mixture, and eventually a eutectic mixture separates at -112° C. which has the composition 7.8 per cent. N₂O₄ and 92.2 per cent. N₂O₃ (corresponding to 63.6 per cent. NO₂ and 36.4 per cent. NO). No other compound exists, therefore, between -10° and -112° C.

Solid nitrogen tetroxide exists in the form of colourless crystals, the melting-point of which is given variously as -10.1, -10.5, -10.8, and -10.95.

The variation of the vapour pressure with the temperature is given in the following table:

Temperature, ° C.	Vapour Pressure, mm. Hg.
-30	39.240
-40	9.770
-50	2.440
-60	0.605
-70	0.1510
-80	0.0360
-90	0.0093
-100	0.0023

The above values are obtained from the equation

log p = 14.9166 + θ(0.0604).

It would seem that solid nitrogen tetroxide consists entirely of N₂O₄ molecules.

Nitrogen tetroxide is not inflammable, neither does it support the combustion of feebly burning substances, e.g. a taper is extinguished. At higher temperatures, however, it is a powerful oxidising agent, owing to its decomposition into oxygen and nitrogen. Thus, freely burning carbon, sulphur, and phosphorus are converted into their oxides; carbon monoxide burns to the dioxide, and hydrogen sulphide to sulphur, with formation of nitric oxide. Hydrogen mixed with nitrogen tetroxide, and passed over platinum, copper, or nickel, reduces the tetroxide to ammonia.

Potassium, which burns with a red flame in N₂O₄, sodium, lead, and mercury are all oxidised to the nitrates, with formation of nitric oxide. Iron, cobalt, and tin, heated to 500° C., are converted into oxides, while many lower metallic oxides are further oxidised at this temperature in the gas. Calcium oxide yields the nitrate when heated to 300°-400° C., while barium oxide at 200° C. produces both nitrate and nitrite.

Certain metals, such as copper, nickel, and cobalt, when in the finely divided condition, absorb both gaseous and liquid nitrogen tetroxide, with the formation of unstable products, which decompose into their constituents on heating, and react vigorously with water, evolving nitric oxide and leaving a solution containing nitrites and nitrates. These so-called "nitro-metals " were considered to be definite compounds with formulae such as Cu₂NO₂, Ni₄NO₂, CO₂NO₂. Recent work, however, with the supposed "nitro-copper" has shown it to be merely an adsorption complex of NO₂ in Cu₂O. The amount of nitrogen peroxide absorbed by the copper is variable, and the first reaction is the oxidation of the copper to cuprous oxide:

2Cu + NO₂ = Cu₂O + NO.

The cuprous oxide then reabsorbs the tetroxide, producing a complex yNO₂.xCu₂O.

Cuprous oxide is capable of absorbing 30 per cent, of nitrogen tetroxide gas, but the liquid has no visible action.

Nitrogen tetroxide resembles nitric oxide in forming additive compounds with metallic halides. A number of such compounds have been investigated, typical examples of which are BiCl₃.NO₂, SnCl₄.NO₂, FeCl₃.NO₂, 4FeCl₂.NO₂, 4FeBr₂.NO₂.

Dry oxygen has no action upon nitrogen tetroxide, but in the presence of water, oxidation to nitric acid occurs. Ozone oxidises the tetroxide to the pentoxide.

Nitrogen tetroxide reacts with small quantities of water to produce nitrous and nitric acids:

N₂O₄ + H₂O = HNO₂ + hNO₃.

The subsequent decomposition of the nitrous acid depends upon the amount of water present. If the latter is small, then the following reaction occurs: -

2HNO₂ ⇔ N₂O₃ + H₂O.

An excess of water, however, results in the production of nitric oxide and nitric acid:

3HNO₂ ⇔ HNO₃ + NO + H₂O.

Well-cooled liquid nitrogen peroxide added to a small quantity of water produces two distinct layers. The upper light green layer consists of nitric acid with nitric oxide, while the lower deep blue layer is chiefly nitrogen trioxide:

2N₂O₄ + H₂O ⇔ N₂O₃ + 2HNO₃.

If oxygen is bubbled through these two layers, the top layer becomes orange-yellow, and contains nitric acid of concentration 95 to 99 per cent., while the bottom layer consists of nitrogen tetroxide with 5 to 10 per cent, of nitric acid.

Solutions of potassium and sodium hydroxides readily absorb nitrogen peroxide, with the formation of nitrate and nitrite. Actually, decomposition by the water present first occurs, and the neutralisation of the nitric and nitrous acids then follows. Some nitrous acid, however, always escapes neutralisation, and decomposes with evolution of nitric oxide. Maximum absorption occurs when the concentration of the alkali is about 1.5N. A 4N solution absorbs to the same extent as water, and stronger solutions are less effective than pure water. A commercially pure alkaline nitrate is produced by passing in nitrogen peroxide in excess of the quantity required for neutralisation to nitrite and nitrate. The nitrite is converted into nitrate by the nitric acid formed by the action of the peroxide on the water, accompanied by evolution of nitric oxide:

NaNO₂ + HNO₃ = NaNO₃ + [HNO₂].

Nitrogen peroxide is absorbed by aqueous solutions of sodium and potassium carbonates less vigorously than the caustic alkalies. Nitrate

and nitrite are produced with evolution of carbon dioxide less vigorously than when using caustic alkalies.

Concentrated sulphuric acid absorbs nitrogen peroxide with the formation of nitrosyl-sulphuric acid:

N₂O₄ + H₂SO₄ ⇔ NO₂.SO₃H + HNO₃.

This reaction is reversible, and proceeds from right to left with rise of temperature.

Sulphur dioxide and sulphur trioxide give a number of complex substances when acted upon by nitrogen peroxide under varying conditions. The composition of these compounds has not been elucidated, but it is stated that the chief product of liquid nitrogen tetroxide and sulphur dioxide is O(SO₂-ONO)₂, which may be regarded as the anhydride of nitrosyl-sulphuric acid.

Both dilute and concentrated nitric acid absorb nitrogen peroxide, and the maximum amount absorbed by the concentrated acid is 42.5 per cent, by weight. This corresponds to the formula N₂O₅.N₂O₄.H₂O. Solid nitrogen tetroxide and liquid ammonia at -80° C. react with explosive violence, but with gaseous ammonia at -20° C. a less vigorous reaction occurs, with the formation of a number of products such as nitrogen, nitric oxide, water, ammonium nitrite and nitrate.

Nitrogen peroxide is a useful nitrating agent for a large number of organic compounds.

Liquid nitrogen peroxide is a useful solvent for cryoscopic determinations of a number of organic compounds, the molecular elevation of the boiling-point being 13.7° C. and the molecular depression of the freezing-point 41° C.

Various constitutions have been assigned to nitrogen peroxide both in the NO₂ and N₂O₄ states. In N₂O₄ both nitrogen atoms may be assumed to be tervalent or quinquevalent:

or ;

or possibly each of the nitrogen atoms exhibits a different valency:



This formula best explains the mixed anhydride character of the tetroxide as exhibited by its reaction with water to form nitrous and nitric acids.

Difficulty is experienced, however, in the case of NO₂. Here the nitrogen atom must either be tervalent or quinquevalent with a free valency, as in



or else the nitrogen atom must function in a quadrivalent capacity:

O=N=O.

In accordance with the theories of valency, the nitrogen atom is quadricovalent with one mixed bond, thus:

Nitrogen tetroxide can be readily detected by its colour, odour, action on starch iodide solution, etc.

The estimation is accompanied by difficulties which have been mentioned in connection with nitric oxide. The absorption by aqueous caustic alkalies to form nitrite and nitrate may be used quantitatively:

N₂O₄ + 2NaOH = NaNO₃ + NaNO₂ + H₂O.

The formation of nitrosyl-sulphuric acid occurs by absorption in 85 to 95 per cent, sulphuric acid, and this may be estimated by titration with potassium permanganate.