NITROGEN AND AMMONIA 235 



removed by suction through a thin capillary. The tubes are dried 

 in vacuo at 100° for 10 min. The rims of the tubes are never touched 

 with the fingers after removal from the cleaning solution and the 

 clean tubes are stored in a desiccator or in petri dishes in a room 

 where no ammonia is allowed and smoking is prohibited. To the 

 sample in a tube about 4 fx\. of the following mixture is added with 

 a constriction pipette, properly centered so that the solution deliv- 

 ered to the bottom of the tube is not drawn up between the pipette 

 and the inner wall: 



1 g. copper sulfate (CuS04-5H20) 



10 g. potassium sulfate 



0.2 g. sucrose 



5 ml. cone, sulfuric acid 



Dilute to 100 ml. 



(The sucrose is added to make sure that the reduction of the nitrog- 

 enous impurities in the reagents also takes place in the blanks.) 

 The constriction pipette is emptied under constant pressure, which, 

 for the greatest accuracy, is so adjusted that the pipette automati- 

 cally empties when the tip touches a wall or liquid surface. Before 

 use the pipette is rinsed inside and out with distilled water. After the 

 digestion solution is added to the sample the tubes are placed in a 

 vacuum desiccator over phosphorus pentoxide and the water is 

 allowed to evaporate at room temperature. To prevent creeping the 

 bulk of the water is removed at 150 mm. mercury (about 24 hr.) ; 

 then the drying is continued at 0.1 mm. mercury (another 24 hr.) . 



By means of a mouth-operated horizontal pipette with a vertical 

 delivery tip, 1 [A. cone, sulfuric acid containing 10 mg. selenium/ml. 

 is added to the charred sample in the digestion tube. (About each 

 10 mm. of the graduated length of the pipette corresponds to 1 [A.; 

 a strip of graph paper may be used for the graduation. The selenium- 

 acid solution is prepared by boiling until clear.) Proper centering of 

 the delivery tip is essential to prevent the liquid from being drawn 

 up between the tip and the wall of the tube. Should the liquid be 

 drawn up as far as the rim where contamination may occur, the 

 experiment should be discarded. It is also essential to rinse the 

 pipette both inside and outside each day before use. 



The digestion is carried out in small flasks (Fig. 78), which may 

 be made by forming a constriction in the middle of insulin ampules. 

 1 ml. cone, sulfuric acid containing 0.4 g. potassium sulfate is placed 



