98 METHODS FOR THE ANALYSIS OF BLOOD 



ammonia by aspiration into another tube (Van Slyke and Cullen 

 apparatus, page 39 may be used) containing 15 c.c. of hundredth- 

 normal hydrochloric or sulphuric acid. Titrate the excess of acid 

 with hundredth-normal sodium hydroxide or potassium hydroxide, 

 using methyl red or alizarin as indicator. 



Calculations. Each cubic centimeter of acid neutralized 

 indicates 0.01 gram of urea per 100 c.c. of blood, or 0.00467 gm. 

 of urea nitrogen per 100 c.c. of blood. In case the blood should 

 be one of the rare samples containing over 0.15 per cent of urea, 

 all the acid will be neutralized, and it will be necessary to repeat 

 the determinations, using in the determination only 1 c.c. of blood. 

 Fresh blood contains so little ammonia that it may be disregarded. 



Benedict's Method l 



Principle. Removal of protein from the blood by coagulation 

 and addition of colloidal iron, and the colorimetric estimation of 

 uric acid in the filtrate. 



Procedure. To 100 c.c. of boiling N/100 acetic acid in a 

 casserole, 20 c.c. 2 of oxalated blood are added, and the mixture 

 is heated to boiling for a moment. Remove the casserole from 

 the flame and add 200 c.c. of boiling distilled water. Pour the 

 mixture upon a folded filter and wash the residue with 50 c.c. of 

 boiling water (heated in the same casserole in which the original 

 coagulation took place). Transfer the whole filtrate to a casserole 

 and boil down rapidly to a volume of about 25 c.c. Pour this 

 solution into a small flask roughly marked to indicate a volume 

 of 50 c.c. Transfer the contents of the casserole to the flask 

 quantitatively, with the help of two or three portions of water, 

 heating vigorously to boiling and rubbing the sides of the casserole 

 with a rubber-tipped stirring rod each time. The total volume in 

 the flask should not exceed 50 c.c. after the addition of the wash- 

 ings. Thoroughly cool the turbid solution in the flask under 



1 Benedict: Jour. Biol. Chem., 1915, 20, 629. 



2 Smaller amounts of blood may be employed, and the quantity of acetic 

 acid and water correspondingly reduced. Unless the quantity of uric acid 

 present is very large, the results are far more accurate when 20 c.c. of blood 

 are Used. 



