Nitric oxide, NO.—This permanent gas66 (that is, unliquefiable by pressure without the aid of cold) may be obtained from all the above-described compounds of nitrogen with oxygen. The deoxidation of nitric acid by metals is the usual method employed for its preparation.
Dilute nitric acid (sp. gr. 1·18, but not stronger, as then N2O3 and NO2 are produced) is poured into a flask containing metallic copper.67
The reaction commences at the ordinary temperature. Mercury and silver also give nitric oxide with nitric acid. In these reactions with metals one portion of the nitric acid is employed in the oxidation of the metal, whilst the other, and by far the greater, portion combines with the metallic oxide so obtained, with formation of the nitrate corresponding with the metal taken. The first action of the copper on the nitric acid is thus expressed by the equation
2NHO3 + 3Cu = H2O + 3CuO + 2NO.
The second reaction consists in the formation of copper nitrate—
6NHO3 + 3CuO = 3H2O + 3Cu(NO3)2.
Nitric oxide is a colourless gas which is only slightly soluble in water (
1 / 20 of a volume at the ordinary temperature). Reactions of double decomposition in which nitric oxide readily takes part are not known—that is to say, it is an indifferent, not a saline, oxide. Like the other oxides of nitrogen, it is decomposed into its elements at a red heat (starting from 900°, at 1,200° 60 per cent. give N 2 and 2N 2 O 3 , but complete decomposition into N 2 and O 2 only takes place at the melting point of platinum, Emich 1892). The most characteristic property of nitric oxide is its capacity for directly and easily combining with oxygen (owing to the evolution of heat in the combination). With oxygen it forms nitrous anhydride and nitrogen peroxide, 2NO + O = N 2 O 3 , 2NO + O 2 = 2NO 2 . If nitric oxide is mixed with oxygen and immediately shaken up with caustic potash, it is almost entirely converted into potassium nitrite; whilst after a certain time, when the formation of nitric peroxide has already commenced, a mixture of potassium nitrite and nitrate is obtained. If oxygen is passed into a bell jar filled with nitric oxide, brown fumes of nitrous anhydride and nitric peroxide are formed, even in the absence of moisture; these in the presence of water give, as we already know, nitric acid and nitric oxide, so that in the presence of an excess of water and oxygen the whole of the nitric oxide is easily and directly converted into nitric acid. This reaction of the re-formation of nitric acid from nitric oxide, air, and water, 2NO + H 2 O + O 3 = 2HNO 3 , is frequently made use of in practice. The experiment showing the conversion of nitric oxide into nitric acid is very striking and instructive. As the intermixture of the oxygen with the oxide of nitrogen proceeds, the nitric acid formed dissolves in water, and if an excess of oxygen has not been added the whole of the gas (nitric oxide), being converted into HNO 3 , is absorbed, and the water entirely fills the bell jar previously containing the gas. It is evident that nitric oxide in combining with oxygen has a strong tendency to give the higher types of nitrogen compounds, which we see in nitric acid, HNO 3 or NO 2 (OH), in nitric anhydride, N 2 O 5 or