GUNNING METHOD FOR NITRIC ACID 381 



minutes. Add now approximately 0.7 gram of mercuric oxid 

 or its equivalent in metallic mercury, and continue the boiling 

 until the liquid in the flask is colorless or nearly so. In case the 

 contents of the flask are likely to become solid before this point 

 is reached, add 10 cubic centimeters more of sulfuric acid. Com- 

 plete the oxidation with a little potassium permanganate in the 

 usual way, and proceed with the distillation as described in the 

 kjeldahl method. The reagents should be tested by blank experi- 

 ments. The object of adding the sodium thiosulfate is to prevent 

 the development of the amido-mercurous salts which require to 

 "be subsequently broken up by the addition of a sulfid. The reac- 

 tion which takes place is represented by the following formula : 



/ NH >\ 



Hg< >SO 4 + Na,S,O 3 -5H 2 O == HgS + (NH 4 )2SO 4 + Na,SO 4 



\NH/ 



-f-4H,O. The sodium thiosulfate may also be added after the 

 digestion instead of the sodium sulfid. 25 



331. Gunning Method for Nitric Acid. The essential features 

 of this modification are due to Winton and Voorhees. 26 

 The modifications of the kjeldahl method, for similar purposes, 

 furnished the material details for the gunning modified process. 

 Winton reports good results from digesting for two hours from 

 half a gram to a gram of the sample with 30 cubic centimeters 

 of sulfuric containing two grams of salicylic acid, in a flask of 

 half a liter capacity. Two grams of zinc dust are then slowly 

 added, with constant shaking, and the flask heated, at first gently, 

 until, after boiling a few minutes, dense fumes are no longer 

 emitted. Three grams of potassium sulfate are next added and 

 the boiling continued until the solution is colorless, or, if iron be 

 present, until a light straw color is produced. On cooling, when 

 the mixture begins to solidify, water is added with caution, and 

 afterwards sodium hydroxid, and the ammonia is obtained by 

 distillation. 



In the process, as conducted by Voorhees, about one gram of 



25 Neuberg, Beitrage zur chemischen Physiologic und Pathologic, 1902, 

 2 : 214. 



26 Connecticut Agricultural Experiment Station, Bulletin 112, 1892 : 3. 

 Division of Chemistry, Bulletin 35, 1892 : 86. 



