Nitrogen Trioxide, N₂O₃

1. The mixing of nitric oxide and oxygen in any proportion below -110° C. results in production of nitrogen trioxide:
   
   4NO + O₂ ⇔ 2N₂O₃.
   
   For example, when nitric oxide is passed through liquid oxygen at -185° C. a green solid, (N₃O₄)_x, is first formed, which at a slightly higher temperature is converted into blue N₂O₃.
   
   An impure form of the trioxide is produced as a bluish liquid when a mixture of nitric oxide and nitrogen peroxide is cooled to -30° C.:
   
   NO₂ + NO ⇔ N₂O₃.
2. The reduction of nitric acid by arsenious oxide yields nitrogen trioxide:
   
   4HNO₂ + As₄O₆ = 2N₂O₃ + 4HAsO₃.
   
   Lunge stated that nitric acid of specific gravity 1.35 gives pure nitrogen trioxide; but Ramsay and Cundall used acid of specific gravity 1.5 and carried out the preparation as follows: The mixture of acid and arsenious oxide is warmed on a water-bath until the reaction starts, when the source of heat is removed and, if necessary, the retort (or distilling flask) is cooled. The fumes are passed over fused calcium nitrate (or chloride) and phosphoric oxide and condensed in a U-tube immersed in a freezing mixture of ice and salt. The bluish-green liquid is then redistilled in an atmosphere of nitric oxide, passed over phosphoric oxide, and again liquefied at - 10° C., when the product is a deep blue liquid which, however, contains traces of nitrogen tetroxide.
3. By passing a spark discharge through liquid air a flocculent green precipitate is obtained, which, after evaporation of the liquid air, leaves the trioxide as an amorphous blue powder, melting at -111° C.
4. The action of water upon nitrosyl-sulphuric acid (chamber crystals) results in the formation of nitrogen trioxide:
   
   + H₂O = 2H₂SO₄ + N₂O₃.
5. Nitrogen tetroxide yields the trioxide when decomposed by a small quantity of water:
   
   N₂O₄ + H₂O = HNO₃ + HNO₂;
   
   2HNO₂ = N₂O₃ + H₂O.
6. Liquid nitrogen tetroxide, when saturated with nitric oxide, and subsequently cooled, yields dark blue crystals of nitrogen trioxide:
   
   N₂O₄ + 2NO = 2N₂O₃.
   
   An impure form of the trioxide is produced as a bluish-green liquid when a mixture of gaseous nitrogen peroxide and nitric oxide is cooled to -30° C.
7. Mineral acids decompose sodium nitrite with the liberation of nitrogen trioxide. Thus when a 20 per cent, solution of sodium nitrite is decomposed by concentrated sulphuric acid, a greenish colour is produced, and a gaseous mixture of nitrogen peroxide and nitric oxide in the proportions to form the trioxide is liberated:
   
   2NaNO₂ + H₂SO₄ = Na₂SO₄ + N₂O₃ + H₂O.
8. The action of a diluted nitric acid (5N or 6N) upon copper gives nitrogen trioxide entirely as the gaseous product.

Nitrogen trioxide under ordinary conditions is a brown gas which behaves as though it were a mixture of molecular proportions of nitrogen peroxide and nitric oxide. Much investigation has taken place as to the real existence of gaseous nitrogen trioxide. No marked contraction occurs when gaseous nitrogen peroxide and nitric oxide are mixed, as should happen if either of the following reactions took place:-

NO₂ + NO = N₂O₃;
N₂O₄ + 2NO = 2N₂O₃.

A small diminution, however, does occur, which is consistent with the presence of 3 per cent, of undissociated nitrogen trioxide.

The vapour density of gaseous nitrogen trioxide obtained from the liquid, which has been subjected to prolonged drying, indicates that the gas is made up of a mixture of N₄O₆ and N₂O₃ molecules. Traces of moisture dissociated the larger into the smaller molecules.

The boiling-point of liquid nitrogen trioxide is approximately -2° C., although after prolonged drying the boiling-point may rise to 43° C. Under ordinary pressures the liquid is stable up to -21° C. According to Baume and Roberts, the boiling-point of pure N₂O₃ is - 27° C. at 760 mm. The variation of the density of liquid nitrogen trioxide with the temperature is shown in the following table:-

Temperature, ° C.	-8	-4	-1	0	1	2
Density	1.4640	1.4555	1.4510	1.4490	1.4485	1.4470

The melting-point diagram of the system N₂O₃ - NO₂ is normal, and shows a single eutectic in the neighbourhood of pure N₂O₃.

The dissociation of nitrogen trioxide under various conditions of temperature and drying has been studied by Jones. He concludes that three distinct reactions proceed:

1. N₄O₆ ⇔ N₂O₃ + NO₂ + NO;
2. NO₂ + NO;
3. 2NO₂ ⇔ N₂O₄.

The first reaction (a) occurs when liquid nitrogen trioxide is vaporised after having been dried as completely as possible. Vapour density measurements showed that dissociation into equal volumes of nitrogen trioxide, nitrogen peroxide, and nitric oxide was complete at 140° C. At the same time, however, there would be present a few NO₂ or N₂O₄ and NO molecules, (b), which was due to incomplete drying. These " wet " molecules would take no part in the first reaction, neither would they combine to form nitrogen trioxide. Association of NO₂ to N₂O₄ molecules will occur if sufficient moisture is present, (c).

Dry liquid nitrogen trioxide is blue, but if moisture is present a green liquid results, owing to the mixture of wet NO₂ molecules with the blue trioxide. At very low temperatures all specimens of trioxide become blue even if slightly wet because of the complete association of any wet NO₂ molecules to give colourless N₂O₄.

In the gaseous state both N₄O₆ and N₂O₃ are colourless when pure.

Nitrogen trioxide is oxidised completely to the peroxide with air or oxygen when in the gaseous state. In the liquid state oxidation only occurs if the temperature is below -100° C.

In many of its reactions nitrogen trioxide reacts as a mixture of the peroxide and nitric oxide. Some metals are converted into their oxides with the formation of nitric oxide or nitrogen, according to whether they are at a red heat or more strongly ignited. Hydrogen sulphide is oxidised to sulphur, and sulphur dioxide, in the presence of moisture, undergoes the same reactions as with nitrous acid, but no reaction occurs when quite dry. Sulphur trioxide in carbon bisulphide solution forms sulpho-nitrous anhydride, (SO₃)₅(N₂O₃)₃.

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

2H₂SO₄ + N₂O₃ = 2H(NO)SO₄ + H₂O.

Nitrogen trioxide mixes with a small amount of water at 0° C. to give a bluish-green liquid which effervesces with the liberation of nitric oxide, and contains both nitric and nitrous acids. With a large excess of water no evolution of gas occurs, and the relatively dilute solution, which contains nitrous acid, is fairly stable.

According to Baker, the trioxide does not dissolve as such in water, but sinks to the bottom and decomposes into nitric oxide and nitrogen peroxide, which latter oxide dissolves in the water to form both nitric and nitrous acids.

Dry sodium and potassium hydroxides completely absorb nitrogen trioxide, with the exclusive formation of nitrite:

2NaOH + N₂O₃ = 2NaNO₂ + H₂O.

If there is any moisture present, however, decomposition into nitrous and nitric acids occurs first, and neutralisation takes place with the formation of nitrate as well as nitrite, but a certain amount of the nitrous acid decomposes, with evolution of nitric oxide, before neutralisation.

Chloroform forms a blue additive compound with nitrogen trioxide at -20° C., which decomposes at ordinary temperatures with the evolution of nitric oxide and nitrogen peroxide.