8 METHODS OF ANALYSIS [Chap. 



TOTAL NITROGEN. 

 Kjeldahl Method Modified to include the Nitrogen of Nitrates. — Official. 



24 REAGENTS. 



(a) Zinc dust. — Impalpable powder. Granulated zinc or zinc filings will not 

 answer. 



(b) Sodiu7n thiosulphate. 



(C) Commercial salicylic acid. 



The other reagents and standard solutions are described under 16. 



25 APPARATUS. 



The apparatus used is described under 1 7. 



26 DETERMINATION. 



Place 0.7-3.5 grams, according to the nitrogen content, of the substance to be 

 analyzed in a Kjeldahl digestion flask. (1) Add 30 cc. of sulphuric acid containing 



1 gram of salicylic acid, shake until thoroughly mi.xed, allow to stand for at least 

 .30 minutes, and then add 5 grams of crystallized sodium thiosulphate and digest as 

 directed below; or, (2) Add to the substance 30 cc. of sulphuric acid containing 2 

 grams of salicylic acid, allow to stand at least 30 minutes and then add gradually 



2 grams of zinc dust, shaking the contents of the flask at the same time and digest 

 as follows: 



Place the flask on the stand for holding the digestion flasks and heat over a low 

 flame until all danger from frothing has passed. Then raise the heat until the acid 

 boils briskly and continue the boiling until white fumes no longer escape from the 

 flask. This requires about 5-10 minutes. Add 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 cc. more of sulphuric acid. Complete 

 the oxidation with a little potassium permanganate in the usual way and proceed 

 as directed under 18. The reagents should be tested by blank experiments. 



Gunning Method Modified to include the Nitrogen of Nitrates. — Official. 



27 REAGENTS AND APPARATUS. 



The reagents and standard solutions are described under 16, 17, 19 and 24. 



28 DETERMINATION. 



Place 0.7-3.5 grams, according to the nitrogen content, of the substance to be 

 analj^zed in a digestion flask. Add 30-35 cc. of salicylic acid mixture (30 cc. of sul- 

 phuric acid to 1 gram of salicylic acid) ; shake until thoroughly mixed, and allow to 

 stand for at least 30 minutes with frequent shaking. Add 5 grams of sodium thio- 

 sulphate and heat the solution for 5 minutes; cool; add 10 grams of potassium sulphate 

 and heat very gently until foaming ceases, then strongly until nearly colorless, 

 and proceed as directed under 21 . 



Absolute or Cupric Oxid Method. — Official. 



29 REAGENTS. 



(a) Coarse cupric oxid. — Ignite and cool before using. 



(b) Fine cupric oxid. — Grind (a). 



