Nitrous Oxide, N₂O

Priestley discovered nitrous oxide in 1772 when experimenting with the action of moist iron filings on nitric oxide (nitrous air):

2NO + Fe + H₂O = N₂O + Fe(OH)₂.

He found that a gas remained which he termed " diminished nitrous air," and that this gas behaved very much like ordinary air. Five years later Priestley found that the same gas was obtained by the action of nitric acid on certain metals, especially tin and zinc.

Berthollet prepared the gas in 1785 by heating ammonium nitrate, but it was left to Davy (1799) to make detailed investigations as to its properties. He showed that the combustions of a candle, phosphorus, sulphur, carbon, and iron wire were similar to those in oxygen. The gas was neutral to an extract of red cabbage, possessed a sweet taste and slight odour, and there was no diminution in volume when it was mixed with oxygen or nitric oxide. Davy discovered that inhalation produced an exhilaration, and he foresaw its use as an anaesthetic, notably in dentistry.

The name " nitrous oxide " was first given by Davy, who also analysed the gas by burning carbon in a measured volume. Priestley and Lavoisier performed the volumetric analysis in a more accurate manner, and showed that nitrous oxide contained its own volume of nitrogen with one-half its volume of oxygen.

The gas may be prepared by the following methods: -

1. Decomposition of ammonium nitrate by heat:
   
   NH₄NO₃ = N₂O + 2H₂O.
   
   This method is a common process for generating nitrous oxide, but the mixture becomes explosive unless the salt is free from organic impurities. It is advisable to melt the nitrate first to remove external moisture, and the decomposition begins at 170° C. Purification from nitric oxide and chlorine (from ammonium chloride) is effected by washing with ferrous sulphate and caustic soda solutions respectively. A modification is to mix the ammonium nitrate with sand, pass the gas through ferrous sulphate or sodium sulphide solutions, and finally dry by washing with an emulsion of ferrous sulphate in concentrated sulphuric acid.
2. Another method is by decomposing an equimolecular mixture of ammonium sulphate and sodium nitrate at 240° C., when a quiet, regular stream of nitrous oxide is evolved.
   
   A number of methods have been described for the preparation of nitrous oxide by reduction of nitric acid or nitrates.
3. Nitric acid is reduced by means of stannous chloride. A solution is made of 5 parts of stannous chloride, 10 parts of hydrochloric acid (sp. gr. 1.21), and 0.9 part nitric acid (sp. gr. 1.38), which on heating to boiling gives a regular stream of nitrous oxide:
   
   2HNO₃ + 4SnCl₂ + 8HCl = 4SnCl₂ + 5H₂O + N₂O.
4. Another method consists in the reduction of a nitrate by heating with anhydrous formic acid:
   
   2KNO₃ + 6H.COOH = N₂O + 4CO₂ + 2H.COOK + 5H₂O.
   
   The reaction is started by heating, but once the gas begins to come off the source of heat is removed; the gas is collected over a solution of caustic potash (20 per cent.) at 40° C., whereby the carbon dioxide is absorbed.
5. Oxalic acid may also be used as a reducing agent. A solution of potassium nitrate, to which sulphuric acid has been added until the solution contains 20 per cent., is heated with oxalic acid:
   
   2KNO₃ + H₂SO₄+ 4(COOH)₂= N₂O + 8CO₂ + K₂SO₄ + 5H₂O.
6. Nitrous acid when reduced with hydrazine yields a mixture of nitrous oxide and ammonia:
   
   HNO₂ + N₂H₄ = N₂O + NH₃ + H₂O.
   
   Hydroxylamine may also be used as a reducing agent.
7. Sodium nitrite solution is treated with cooled concentrated hydroxylamine-hydrochloride solution, when the nitrous acid liberated by the hydrochloride is reduced by the hydroxylamine:
   
   HNO₂ + NH₂OH = N₂O + 2H₂O.
8. It is also possible to isolate nitrous oxide produced directly by the union of its elements in the nitrogen-oxygen flame. Spectroscopic examination will determine that in a certain part of the flame nitrous oxide is the chief product, and rapid cooling at this point will give a 25 per cent, yield of the gas.

Nitrous oxide is a colourless gas with a pleasant odour and sweetish taste. When inhaled in small amounts it produces a feeling of exhilaration, while in larger quantities anaesthesia results, whence its extended application in minor operations, as in dentistry.

Its litre density at N.T.P., as determined by different investigators, is given as 1.9780, 1.9777, and 1.9774 grams. Compared with air as unity the density is 1.5301 and 1.5297.

Nitrous oxide is easily liquefied, and thus would be expected to show considerable deviation from Boyle's Law. The variation of volume with pressure has been studied by Rayleigh at low pressures:

Pressure Ratio	Volume Ratio
75 mm.:150 mm.	2.100066:1
½ atm.:1 atm.	2.100327:1
1 atm.:2 atm.	2.100651:1

The coefficient of expansion of nitrous oxide is 0.0037067. The thermal conductivity at 0° C. has been given as 0.0000350, 0.00003515, and 0.00003530. The value at 10° C. is 0.00003723, and at 100° C. is 0.0000506. The ratio of the specific heat at constant pressure to that at constant volume at 15° C. is 1.300 according to Partington and Shilling, who have calculated this value from all reliable work. The value at 0° C. is 1.31, and at 100° C. 1.27. The change of molar heat with temperature has been determined from 0° to 1000° C.:

C_v = 6.629 + 0.00694t – 0.0₅241t²;
C_p = 8.659 + 0.00677t – 0.0₅222t².

The viscosity of nitrous oxide is less than that of air according to the following table, which shows the values of the coefficient of viscosity, η, in absolute (C.G.S.) units: -

η ×107.	Temperature, ° C.
1366	0
1441	15
1845	100

The refractive index of nitrous oxide for sodium light is 1.0000516.

Solubility

The following table gives the absorption coefficient, β, of nitrous oxide in water:-

Temperature, ° C.
0	1.305
5	1.1403
10	0.9479
15	0.7896
20	0.6654
25	0.5752

In its solubility relations nitrous oxide obeys Henry's Law. The gas is much more soluble in ethyl alcohol, as is seen by the results of Knopp:

t° C	0	5	10	15	20	25
β	4.178	3.844	3.541	3.268	3.025	2.853

The solubilities of nitrous oxide in aqueous salt solutions, and also in solutions of aqueous glycerol, are lower than those in water.

Faraday liquefied the gas at 0° C. under a pressure of 30 atmospheres. The liquid is colourless, mobile, and has a very faint odour of burnt sugar.

The refractive index is very low, the values for the sodium D line being 1.193 at 0° C. and 1.3305 at 90° C.

The critical data are given in the following table: -

Critical constants of Nitrous Oxide

Critical Temperature, °C.	Critical Pressure, Atm.	Relative Critical Volume	Critical Density, Gram/c.c.
35.4	75	. . .	. . .
36.4	73.07	0.0048	0.41
38.8	77.5	0.00436	0.454
36.5	71.66	. . .	. . .

The boiling-point of liquid nitrous oxide under 760 mm. pressure is given as –88.7° C., -87.9° C.

The variation of the boiling-point with the pressure is given in the following table: -

Variations of the boiling-point if nitrous oxide with the pressure

Pressure	Boiling-point, °C.
200 mm	-90.1
730 mm	-89.3
760 mm	-88.7
4.6 atm	-62
11.02 atm	-40
13.19 atm	-34
30.75 atm	0
41.20 atm	12
45.30 atm	16
49.40 atm	20

Densities of Liquid nitrous oxide at various temperatures

Temperature, ° C.	Density
-20.6	1.003
-7.3	0.953
-2.2	0.912
0	0.9105
10	0.856
17.5	0.804
32.9	0.640
36.3	0.572

Surface Tension

The surface tension at -89.3° C. is 26.323 dynes per cm. The molar weight of the liquid, calculated at the critical temperature of 36° C., is 43.52.

The calculated molecular weights of both liquid and gaseous nitrous oxide from the critical data are 43.26 and 43.78, which shows that nitrous oxide is monomolecular in both states.

Heat of Formation

The molecular heat of formation of gaseous nitrous oxide is -17.740 Cals., this value being the mean of the values obtained by the combustion of hydrogen in nitrous oxide and carbon monoxide in nitrous oxide. Berthelot's value is -20.600 Cals. The heat of formation of liquid nitrous oxide is given as –18.000 Cals.

The molecular heats of volatilisation at -20° C., 0° C., and 35° C. are given as 2900, 2600, and 400 Cals. respectively.

Evaporation in vacuo of a mixture of liquid nitrous oxide and carbon bisulphide causes the temperature to fall to -140° C.

A snow-like mass is obtained by allowing liquid nitrous oxide to evaporate rapidly under reduced pressure. Solid nitrous oxide melts at -102.3° C. The relationship between melting-point and vapour pressure is shown in the following table:-

Variations of the melting-point of nitrous oxide with the pressure

Pressure, mm.	Melting-point, ° C.
650	-91
600	-91.9
500	-93.9
400	-96.4
300	-99.5
200	-103.7
150	-106.7
100	-110.8
50	-117.2
30	-121.1
15	-127.0
7	-131.3
4	-138.9
1	-144.1

The equation of the curve is given by the expression:

log P = -1096.72/t + 1.75 log t + 0.0005t + 4.8665.

This is one of the few examples of a gas reaction which takes place in the gas phase, at any rate in a silica vessel, as is shown by the fact that increase of surface (by the addition of powdered silica) has no effect on the velocity constant. Since this latter is inversely proportional to the concentration the reaction is bimolecular; thus

2N₂O = 2N₂ + O₂.

Initial p (mm.)	296	139	52.5
Half-life (secs.)	255	470	860

The velocity constant is found to diminish with rise of temperature:

T	1125	1085	1053	1030	1001	967	838
log10(k×103)	4.064	3.575	3.223	2.940	2.580	2.133	0.040

From these results the " heat of activation" of nitrous oxide molecules is calculated from the Arrhenius equation

,
.

The mean value of A is 58.500 Cals. per mol.

The outstanding chemical property of nitrous oxide is its ability to support vigorously the combustion of many substances. This is because of its ready decomposition into its elements, which begins at 500° C. and is complete at 900° C., and all combustions in this gas are combustions in oxygen:

2N₂O = 2N₂ + O₂.

It will be seen that, after decomposition, the mixed gases contain one-third their volume of oxygen as compared with one-fifth in air.

A glowing splint is rekindled, and a taper burns brilliantly in the gas. Brightly burning sulphur, phosphorus, and carbon burn similarly as they do in oxygen. Sulphur which is only just burning is extinguished in the gas, as the temperature is not sufficient to cause decomposition. Sodium, potassium, and some other metals burn in nitrous oxide to produce in the first place peroxides, which when further heated yield nitrites and nitrates:

Na₂0₂ + 2N₂O = 2NaNO₂ + N₂.

At lower temperatures nitrous oxide is a less vigorous oxidising agent than oxygen. Thus below 350° C. copper only forms cuprous oxide, Cu₂O, while potassium gives an oxide, K₂O₃, which absorbs further oxygen on being brought into the air. Nitrous oxide, in common with the other oxides of nitrogen, is completely and quantitatively decomposed by passing the gas over red-hot copper.

Hydrogen reduces nitrous oxide in the presence of platinum black, finely divided palladium, or reduced nickel.

An unstable crystalline hydrate, N₂O + 6H₂O, is formed between nitrous oxide and water at a low temperature. Although by its formula nitrous oxide is the anhydride of hyponitrous acid, H₂N₂O₂, it will not combine with water to form this acid.

The most important use of nitrous oxide is as an anaesthetic. When mixed with oxygen and inhaled in small doses it produces a temporary exhilarating effect - hence the name " laughing gas." Larger quantities of the mixed gases produce temporary unconsciousness, which generally lasts for about forty seconds. The amount of oxygen varies from 5 to 25 per cent., as the use of nitrous oxide alone involves danger of asphyxia.

It is important that the gas should be free from chlorine, nitric oxide, etc., when used for purposes of anaesthesia.

Nitrous oxide may be distinguished from oxygen by its much greater solubility in water, its faint odour, and the fact that it does not react when brought into contact with nitric oxide.

Its ready reduction by hydrogen into nitrogen and water in the presence of finely divided metals may be used for its estimation, or it may be exploded with hydrogen in a Hempel apparatus. Volumetrically, decomposition by an electrically heated iron wire which leaves a residual volume of nitrogen may be used. Gravimetrically, nitrous oxide may be decomposed by passing over red-hot copper and weighing the oxygen removed.