URINE 



489 



aspiration. They connect the large Jena test-tube in which the digestion was 

 carried out with a small Liefcig condenser (made from a piece of glass tubing 

 30 by 150 mm. with two -hole rubber stoppers at each end through which pass 

 the inlet and outlet tubes and the condenser tube itself). See Fig. 156. The lower 

 end of the condenser is connected with a glass tube (or better an old pipette, to 

 prevent back suction) which reaches nearly to the bottom of the volumetric flask 

 used as a receiver. The distillation tube also has a two-hole rubber stopper. It 

 is connected with the condenser and also carries a long straight tube which reaches 

 nearly to the bottom of the test-tube, and is closed above with a piece of rubber 

 tubing and a pinch-cock. The digestion is carried out just as in the Folin-Farmer 

 method (see page 485) and when partially cool 7 c.c. of water are added. Into the 

 long tube passing through the stopper suck 3 c.c. of saturated sodium hydroxide 

 solution and close the pinch-cock. Insert the stopper, connect with the condenser 



FIG. 156. BOCK AND BENEDICT APPARATUS. 



and allow the alkali to run into the test-tube. The fluids are mixed by blowing a 

 few bubbles of air through the apparatus. The test-tube is then heated to vigor- 

 ous boiling (over a large free flame), the distillation being continued until a sepa- 

 ration of salts occurs hi the test-tube and the mixture begins to bump. This 

 distillation requires about two minutes. The test-tube is then disconnected from 

 the condenser and the latter washed down with a few cubic centimeters of water. 

 The liquid hi the receiving flask is diluted and Nesslerized as in the Folin-Farmer 

 method (see page 485). 



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 



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



