COMPOUNDS OF NITROCKX WITH HYDROGEN AND OXVCKN 279 



and its anhydride are oxidised in solutions by many oxidising 

 substances for example, by potassium permanganate into nitric acid. '" 

 Nitric oxide, NO. This permanent gas 58 (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 N 2 3 and 

 NO 2 are produced) is poured into a flask containing metallic copper. 59 

 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 



u O + 2NO. 



The second reaction consists in the formation of copper nitrate 

 + 3CuO=3H 2 O + 3Cu(NO 3 ) 2 . 



Nitric oxide is a colourless gas which is only slightly soluble in 

 water (^ 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. 

 The most characteristic property of nitric oxide consists in its capacity 

 for directly and easily combining with oxygen (owing to the evolution 

 of heat in the combination). With oxygen it forms nitrous anhydride 



57 The action of a solution of potassium permanganate, KMnO 4 , on nitrous acid in 

 the presence of sulphuric acid is determined by the fact that the higher oxide of man- 

 ganese, MnoO 7 , contained in the permanganate is converted into the lower oxide MnO, 

 which as a base forms manganese sulphate, MnSO 4 , and the oxygen serves for the oxida- 

 tion of the N 2 O 3 into N.,O 3 , or its hydrate. As the solution of the permanganate is of & 

 red colour, whilst that of manganese sulphate is almost colourless, this reaction is clearly 

 seen, and may be employed for the recognition and determination of nitrous acid and 

 its salts. 



58 The absolute boiling point =98 (see Chap. II. Note 29). 



59 Kammerer proposed preparing nitric oxide, NO, by pouring a solution of 

 sodium nitrate over copper shavings, and adding sulphuric acid drop by drop. The 

 oxidation of ferrous salts by nitric acid also gives NO. One part of strong hydrochloric 

 acid is taken and iron is dissolved in it (FeCl. 2 ), and then an equal quantity of hydro- 

 chloric acid and nitre is added to the solution. On heating, nitric oxide is evolved. When 

 nitric oxide is prepared by either of the above methods, the apparatus first becomes full 

 of brown fumes of nitrogen peroxide, formed by the oxygen of the air and the nitric 

 oxide, and therefore the pure gas can only be collected after it has displaced all the air 

 in the apparatus, and when the latter becomes full of colourless gas. 



