SPIEGEL'S MODIFICATION 407 



B, and the temperature observed, /, while / denotes the tension 

 of the aqueous vapor in millimeters of mercury pressure at the 

 observed temperature, t. The tension of the aqueous vapor at 

 temperatures from zero to 26, expressed in millimeters of mer- 

 cury, is given in the following table: 



Tension in mm. 

 mercury. 



15-3 

 16.3 

 17.4 

 18.5 

 19.6 

 20.9 

 22.2 



23-5 

 25.0 



From the gas volume corrected by the above formula the nitric 

 acid is calculated as follows : 



One cubic centimeter of nitric oxid weighs at o and 760 milli- 

 meters barometric pressure 1.343 milligrams. 



Since two molecules of NO (molecular weight 60) correspond 

 tc one molecule of N 2 O 5 (108), we have the following equa- 

 tion: 60 : 108 i.343:x. Whence x =: 2.417 milligrams, the 

 weight of nitric acid (N 2 O 5 ) corresponding to one cubic centi- 

 meter of nitric oxid. 



354. Spiegel's Modification. Spiegel noticed inaccuracies in 

 the results of the ferrous chlorid method of estimating nitric acid 

 when carbon dioxid is used, which sometimes amounted to three 

 per cent, of the nitric acid present in the sample. The following 

 suggestions are made by him for the improvement of the pro- 

 cess : 43 



As regards the use of carbon dioxid in the operation, the first 

 difficulty consists in obtaining it entirely free from air. By the 

 use of small pieces of marble, which, before being placed in the 

 kipp apparatus are kept for a long while in boiling water, a pro- 

 duct is obtained which, after 30 minutes of moderate evolution, 

 leaves only a trace of unabsorbed gas in contact with potash-lye. 

 The apparatus used is illustrated in Fig. 24. 



45 Berichte der deutschen chemischen Gesellschaft, 1890, 23 : 1361. 



