1892.] Formation and Decomposition of Nitrous Acid. 29 



Amount of (N0 2 ) in 1 c.c. calculated from 



the weighed quantity of silver nitrite 



taken 0'005963 gram. 



Amount of (N0 2 ) in 1 c.c. found 0'005974 



The process is, therefore, accurate in this particular case to within 

 1 part in 600. When smaller quantities of nitrous acid were to be 

 estimated, the metaphenylenediamine method was adopted, the 

 )loration produced being compared with a solution containing a 

 known quantity of nitrous acid by means of the form of tintometer 

 described in my former paper (vide supra). 



The Stability of Nitric Acid. 



It seemed worthy of investigation to determine the actual tem- 

 perature at which traces of nitrogen peroxide begin to be formed in 

 purified nitric acid of different degrees of concentration ; hitherto 

 only general statements are given in the text-books.* Samples of 

 acid of sp. gr. 1'5 and 1*4 respectively were purified by the method 

 of blowing a current of air at a temperature of 35, and thus obtained 

 quite colourless ; the latter was diluted (i) with its own volume, and 

 (ii) with three times its volume, of water. An acid of sp. gr. 1'53 

 was prepared by distillation of recrystallised nitre with sulphuric 

 acid, and the reddish-yellow acid thus obtained was redistilled with 

 an equal bulk of sulphuric acid. A number of attempts were made 

 to render this acid colourless by a current of air or carbonic acid, 

 but, notwithstanding several modifications, these proved unsuccessful. 

 Recourse was, therefore, had to distillation in vacuo in an apparatus 

 specially constructed of glass. 



The method of operation was as follows : The slightly yellow acid 

 was placed in the distillation flask, and the receiver kept cooled by a 

 freezing mixture ; at a temperature of 45, and under a pressure of 

 15 mm., the acid passed over without any visible ebullition. After 

 one-third of the liquid had been condensed, the receiver was changed, 

 and the remainder distilled over; the first portions contained a 

 greater part oi the nitrogen peroxide. The second distillate was 

 then redistilled in the same manner, and thus a perfectly colourless 

 liquid was obtained (sp. gr. 0/0 = l'541)f containing only O'Oll milli- 



* Carius (' Ber. Deut. Chem. G-es.,' 1870, p. 690) states that when pure nitric 

 acid of sp. gr. 1*51 is heated in sealed tubes to a temperature of 150 and upwards, 

 a reddish-yellow liquid is produced, with evolution of oxygen and " nitrous acid 

 gas;" above 250 the liquid contains such an abundance of nitrous acid that it 

 gives a blue colour on addition of a small quantity of water, and on further dilution 

 nitric oxide is evolved. Accordingly, the decomposition of the acid under these 

 conditions is expressible by the equation 2HNO 3 = N 2 O 3 + O 2 + H 2 O. 



f I am indebted to my colleague Dr. J. Watts for loan of apparatus and assist- 



