152 . A MANUAL OF BACTEEIOLOaY 



APPARATUS REQUIRED 



1. Kjeldahl flasks for both digestion and distillation. These are 

 flasks having a capacity of about 550 cc, made of hard, moderately 

 thick, and well-annealed glass. When used for the distillation, the 

 flasks are fitted with rubber stoppers and bulb tubes, as described 

 under distillation flasks. 



2. Kjeldahl digestion flasks. These are pear-shape, round-bottom 

 flasks, made of hard, moderately thick, well-annealed glass and having 

 a total capacity of about 250 ce. They are 22 cm. long and have 

 a maximum diameter of 6 cm., tapering gradually to a long neck, 

 which is 2 cm. in diameter at the narrowest part and flared a little 

 at the edge. 



3. Distillation flasks. For distillation a flask, of ordinary shape, of 

 about 550 cc. capacity may be used. It is fitted with a rubber stopper 

 carrying a bulb tube to prevent the possibility of sodium hydrate 

 being carried over mechanically during distillation. The bulbs may 

 be about 3 cm. in diameter, but the tubes are the same diameter as 

 the condenser, -^^ith which they are connected by a rubber tube. 



ANALYTICAL PROCESS 



Place from 0.7 g. to 3.5 g. of the substance to be analyzed in a 

 Kjeldahl digestion flask ; if soils are being analyzed, a 10-15 g. sample 

 is necessary. Add 30 cc. of sulphuric acid containing 1 g. of salicylic 

 acid and shake until thoroughly mixed ; then add 5 g. of crystallized 

 sodium thiosulphate ; or add to the substance 30 cc. of sulphuric acid 

 containing 2 g. of salicylic acid, then add gradually 2 g. 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 

 untU white fumes no longer escape from the flask. This requires 

 about five or ten minutes. Add approximately 0.7 g. of mercuric 

 oxide, or its equivalent in metallic mercury, and continue the boil- 

 ing 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 

 by adding small amounts of potassium permanganate until, after 

 shaking, the liquid has a permanent green or purple color. 



