DETERMINATION OF NON-PROTEIN NITROGEN 85 



filter paper and withholding the remainder until the whole filter 

 has been wet, the filtrates are almost invariably as clear as water 

 from the first drop. If a filtrate is not perfectly clear, the first 2 

 or 3 c.c. must be returned to the funnel. (Filter papers of the 

 following diameters will meet all ordinary needs; 11, 12^, 15 and 

 18| cm.) 



In the precipitation of the blood protein, a special blood pipette 

 is employed. This is simply a 15 c.c. pipette, graduated from the 

 long tip into 1 c.c. portions. It is convenient to use three such 

 pipettes, one for the blood, one for the sodium tungstate solution, 

 and one for the sulphuric acid. The water used for diluting the 

 blood may be measured with a cylinder. 



For the proper carrying through of this thod it is essential 

 that suitable sodium tungstate be employed. If the tungstate 

 contains too much carbonate, the portion of the sulphuric acid 

 will be used up and the lack of sulphuric acid will cause a loss in 

 the later determination of uric acid. A safe and convenient cri- 

 terion is to test the blood filtrate obtained with Congo red paper. 

 The reaction should be negative or at the most just perceptible. 



The carbonate content of sodium tungstate is easily determined 

 as follows: To 10 c.c. of 10 per cent solution add 1 drop of phenol- 

 phthalein and titrate with 0.1 normal hydrochloric acid. Each 

 c.c. of hydrochloric acid corresponds to 1.06 per cent of sodium 

 carbonate. The amount of acid required for the titration should 

 not exceed 0.4 c.c. 



The blood filtrates are nearly neutral, 10 c.c. of filtrate requir- 

 ing only about 0.2 c.c. of 0.1 normal sodium hydroxide when 

 titrated with phenolphthalein as indicator. If the filtrates are 

 to be kept for any length of time, more than two or three days, 

 they need some preservation. One or 2 drops of toluene or xylene 

 is adequate for the filtrate obtained from 10 c.c. of blood. This 

 method is applicable to all kinds of blood, for example, human, 

 beef, sheep, chicken, dog and rabbit. 



DETERMINATION OF NON-PROTEIN NITROGEN 



Introduce 5 c.c. of protein-free blood filtrates into a dry 75 c.c. 

 test-tube graduated at 35 c.c. and at 50 c.c. Add 1 c.c. of the 

 sulphuric-phosphoric acid mixture. Add a dry quartz pebble and 

 boil vigorously over a micro-burner until the characteristic dense 



