URINE 



513 



test-tube is then disconnected from the condenser and the latter washed down 

 with a few cubic centimeters of water. The liquid in the receiving flask is 

 diluted and Nesslerized as in the FoUn -Farmer method (see page 508). 



Bock and Benedict, while holding the distillation procedure to be 

 more accurate than aspiration, do not consider that the colorimetric 

 method is equivalent to the standard Kjeldahl procedure in accuracy 

 or reliability, although usually it agrees with the latter method within 

 about 2-3 per cent, and is indispensable where very small amounts of 

 nitrogen are to be determined. According to Folin 1 and others the 

 method is capable of greater accuracy than this, and the aspiration 

 procedure gives satisfactory results. The method should be checked 

 up carefully by each new learner of the method, using pure solutions. 

 Outside air is better than laboratory air for aspiration purposes. 

 Care is needed in using the pipettes, which should be of the Ostwald 

 type and accurate. In using them allow the pipette to drain against 

 the side of the vessel for 10 seconds and then blow out clean so that 

 nothing is left behind in the tip. The reagents used must be as free 

 as possible from ammonia and must be checked up, particularly the 

 sulphuric acid and potassium sulphate. Those who have trouble in 

 using a colorimeter may substitute titration with N/5O hydrochloric 

 acid using alizarin, or better methyl red, as an indicator. 



Direct Nesslerization Method of Folin and Denis. 2 Principle. A 

 small amount of urine is digested with a. mixture of sulphuric and 

 phosphoric acids to destroy the organic matter, the digestion mixture 

 treated directly with Nessler's reagent and compared with a standard 

 ammonia solution also Nesslerized. 



Procedure. This determination requires 0.7 to 1.5 mg. of nitrogen. 

 Dilute 5, 10 or 20 c.c. of urine to 100 c.c., mix and with an Ostwald 

 pipette transfer i c.c. of the diluted urine to a large hard glass test tube. 

 With an ordinary pipette add i c.c. of the phosphoric-sulphuric acid- 

 copper sulphate mixture 3 together with a small pebble, to prevent 

 bumping. Heat over a micro burner until the water is driven off and 

 fumes become abundant within the tube. This should take place in 

 about two minutes. When filled with fumes close the mouth of the 

 test tube with a watch glass and continue the boiling at such a rate 

 that, the tube remains filled with fumes yet almost none escape. With- 

 in two minutes after the mouth of test tube was closed the contents 

 should become clear, and bluish or light green. Continue the gentle 



1 Folin: Jour. Biol. Chem., 21, 195, 1915. 



2 J 7 olin and Denis: /. Biol. Chem., 26, 486, 1916. 



3 Made by mixing 50 c.c. of 5 per cent, copper sulphate solution with 300 c.c. of 85 per 

 cent phosphoric acid and then adding 100 c.c. of concentrated sulphuric acid free from 

 ammonia. Keep well covered. 



33 



