84 METHODS FOR THE ANALYSIS OF BLOOD 



freed from protein and the protein-free nitrate is used for the 

 determination of the constituents designated above. 



Method of Drawing Blood. Attach, by means of a short piece 

 of pure gum tubing, an hypodermic needle about 1 mm. in diameter 

 and 25 mm. in length, previously sterilized and paraffined, to the 

 tip of a suitable graduated pipette. Introduce into the upper end 

 of the pipette, which must be perfectly clean and dry, a small 

 pinch of powdered potassium oxalate, and allow it to run down 

 into^the tip and the needle. An excess of oxalate is to be avoided. 

 Citrate should be employed only in minimum amounts. Attach 

 a piece of rubber tubing to the upper end of the pipette, and to 

 this a mouthpiece consisting of a short tapering glass tube. Place 

 a pinchcock over the rubber tube near the top of the pipette. To 

 draw the blood, insert the needle into the vein or artery and regulate 

 the flow by means of the pinchcock and suction. The exact 

 quantity of blood desired may thus be obtained without any 

 waste or clotting. 



Preparation of Protein-free Blood Filtrates. Transfer a 

 measured amount of blood into a flask having a capacity of 15 to 

 20 times that of the volume taken. Dilute the blood with 7 vol- 

 umes of water and mix. With an appropriate pipette add 1 

 volume of 10 per cent solution of sodium tungstate (Na2W04.2H2O) 

 and mix. With another suitable pipette add to the contents in 

 the flask (with shaking) 1 volume of 2/3 normal sulphuric acid. 

 Close the mouth of the flask with a rubber stopper and give a few 

 vigorous shakes. If the conditions are right, hardly a single air- 

 bubble will form as a result of the shaking. Much oxalate or citrate 

 interferes with the coagulation and later with the uric acid deter- 

 mination; 20 mg. of potassium oxalate is ample for 10 c.c. of blood. 

 Citrate, except in the minimum amount, is to be avoided. When the 

 blood is properly coagulated, the color of the coagulum gradually 

 changes from pink to dark brown. If this change does not occur, 

 the coagulation is incomplete, due to too much oxalate or citrate. 

 In such an emergency the sample may be saved by adding 2 normal 

 sulphuric acid drop by drop, shaking vigorously after each addi- 

 tion and allowing the mixture to stand for a few minutes before 

 adding more, until the coagulation is complete. Pour the mixture 

 on a filter large enough to hold the entire contents of the flask and 

 cover with a watch glass. If the nitration is begun by pouring 

 the first few c.c. of the mixture down the double portion of the 



