AGRICULTURAL ANALYSIS 



loss of nitrogen in adding the zinc dust. This was also a tedious 

 part of the operation as the zinc dust had to be added gradually. 

 Finely granulated zinc dust was tried, but in such cases the re- 

 sults were invariably low. The results were lower when using 

 chemically pure zinc dust instead of the commercial article. An 

 investigation showed that the commercial zinc dust which had 

 hitherto been used contained some zinc sulfid. This suggested 

 that hydrogen sulfid might complete the reduction as well if not 

 better than nascent hydrogen. Working on this theory, Scovell 

 and Peter made a series of experiments, using zinc dust in one 

 set of experiments and zinc sulfid in another set. 21 The results 

 on pure potassium nitrate, containing a trace of water, and 

 13.83 per cent, of nitrogen were as follows: 



Average J 3-76 



Theory l 3-&3 



The advantage of zinc sulfid over zinc dust is : First, the 

 liability of the loss of nitrogen is not so great. Second, the 

 zinc sulfid can be added all at once, and, therefore, it is less 

 troublesome and more rapid than when the zinc dust is used. 

 Third, the oxidation is more rapid and, as less salts are present, 

 the distillation is more quiet. 



In 1893 sodium thiosulfate was substituted for zinc sulfid as 

 the reducing agent in this method, not because better results 

 were obtained, but because it was found to be difficult to get 

 commercial zinc sulfid free from ammonia. The comparative re- 

 sults obtained by the different chemists using these two reduc- 

 ing agents were slightly in favor of zinc sulfid. 22 



Otto Foerster gives the following as the reaction when sodium 

 thiosulfate is used: 23 



4HNO 3 +4H 2 SO 4 +Na 2 S 2 O 3 = 

 4HO.NO 2 .SO 2 +2NaHSO 4 -f3H 2 O. 



But it would be interesting to know whether hydrogen sulfid, 

 which is also formed when salicylic acid and strong sulfuric acid 



11 Division of Chemistry, Bulletin 24, 1890 : 91. 

 ** Division of Chemistry, Bulletin 38, 1893 : 34. 

 u Zeitschrift fur analytische Chetnie, 1889, 28 : 422. 



