MICROCHEMICAL METHODS FOR BLOOD ANALYSIS 301 



to the blood filtrate two drops of the pyrophosphate solution 

 described in the method for urea in urine (Folin), or two drops 

 of a molecular o-phosphate solution (% molecular monosodium 

 phosphate plus % molecular disodium phosphate). Then add 

 0.5 to 1 c.c. of the urease solution described in the method for 

 urea in urine (Folin) and immerse the test tube in a beaker of 

 warm water and leave it there for 5 minutes. The temperature 

 of the water is not very important but should not exceed 55 C. 

 The warm water can perhaps scarcely be said to be essential, 

 for the hydrolysis is very rapid at room temperature, but we 

 nevertheless much prefer to use it. If no hot water is used, 

 continue the digestion for 10 to 15 minutes, or as much longer 

 as is convenient. The ammonia formed can be conveniently and 

 quickly aerated into a second test tube. Add a little paraffin 

 oil. 1 or 2 c.c. of 10% sodium hydroxide are added and the 

 ammonia is aspirated into a test tube graduated at 25 c.c. and 

 containing 2 c.c. of 0.05 normal hydrochloric acid. Consult the 

 instructor as to the connections for aeration. The two test tubes 

 are connected on the same principle as the cylinder and bottle in 

 the figure given in the chapter on urine analysis (p. 323) in the 

 aeration method for ammonia (Folin). Run the air current 

 slowly for one minute and then rapidly for 10-15 minutes. 

 Rinse the delivery tube, dilute to about 20 c.c. and add 2.5 c.c. 

 of the Nessler solution. Fill to the 25 c.c. mark and compare 

 in the colorimeter with a standard containing 0.3 nig. of N in a 

 100 c.c. flask and Nesslerized with 10 c.c. of the Nessler solu- 

 tion. The standard and unknown should always be Nesslerized 

 as nearly simultaneously as practicable. 



Calculation. Multiply 20 (the height of the standard in mm.) 

 by 15 and divide by the colorimetric reading to get the urea 

 nitrogen per 100 c.c. of blood. The reasons for this calculation 

 are, of course, to be found in the fact that the standard contain- 

 ing 0.3 mg. of N is diluted to 100 c.c. while the unknown, which 

 corresponds to 0.5 c.c. of blood, is diluted to only 25 c.c. The 

 only precaution which experienced investigators are likely to 



