8 METHODS OF ANALYSIS. 



and shake until thoroughly mixed, then add 5 grams of crystallized sodium 

 thiosulphate ; or add to the substance 30 cc of sulphuric acid containing 2 

 grams of salicylic acid, then add gradually 2 grams of zinc dust, shaking the 

 contents of the flask at the same time. Finally, place the flask on the stand 

 for holding the digestion flasks, where it is heated over a low flame until all 

 danger from frothing has passed. The heat is then raised until the acid boils 

 briskly and the boiling continued until white fumes no longer escape from the 

 flask. This requires about five or ten 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 with the distillation as desciibed in the Kjeldahl 

 method. The reagents should be tested by blank experiments. 



(d) GUNNING METHOD MODIFIED TO INCLUDE THE NITROGEN OF NITRATES. 



OFFICIAL. 



(1) PREPARATION OF REAGENTS. 



Besides the reagents given under the Gunning method there will be needed 



(a) Sodium thiosulphate. 



(6) Commercial salicylic acid. 



(2) APPARATUS. 



The apparatus used is the same as that given under the Kjeldahl method, 

 page 6 (2). 



(3) DETERMINATION, 



In a digestion flask place from 0.7 to 3.5 grams of the substance to be 

 analyzed, according to the amount of nitrogen present. Add from 30 to 35 cc 

 of salicylic acid mixture, namely, 30 cc sulphuric acid to 1 gram of salicylic 

 acid ; shake until thoroughly mixed, and allow to stand from 5 to 10 minutes, 

 with frequent shaking. Add 5 grams of sodium thiosulphate and heat the 

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

 This reduces the danger of foaming. Heat very gently until foaming ceases, 

 then heat strongly until nearly colorless. Dilute, neutralize, and distil as in 

 the Gunning method. 



(e) ABSOLUTE OR CUPRIC OXID METHOD. OFFICIAL. 

 (Applicable to all nitrogen determinations.) 



(1) PREPARATION OF REAGENTS. 



(a) Coarse cupric oxid. To be ignited and cooled before using. 

 (6) Fine cupric oxid. Prepared by grinding ordinary cupric oxid. 



(c) Metallic copper. Granulated copper, or fine copper gauze, heated and 

 cooled in a current of hydrogen. 



(d) Sodium bicarbonate, free from organic matter. 



(e) Caustic potash solution. A supersaturated solution of caustic potash in 

 hot water. 



(2) APPARATUS. 



(a) Combustion tube of best hard Bohemian glass, about 66 cm long and 

 12.7 mm internal diameter. 



