512 



PHYSIOLOGICAL CHEMISTRY 



The color of the unknown should be adjusted to that of the standard both from 

 above and below the level of the latter. The matching of the colors is ordinarily 

 very easy. It is desirable to make the readings by diffused daylight if possible. 

 If electric light must be used, a sheet of smooth white paper should be interposed 

 between the colorimeter and the source of light. 



Calculation. The. reading of the standard divided by the reading of the un- 

 known gives the nitrogen hi milligrams in the volume of the urine taken. Calcu- 

 late the total nitrogen output for the 24-hour period. 



Interpretation. See page 506. 



4. Bock and Benedict's Modification of the Folin-Farmer Procedure. 

 Bock and Benedict 1 have found distillation of the ammonia more accurate than 



FIG. 165. BOCK AND BENEDICT APPARATUS. 



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

 carried out with a small Liebig 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. 171. 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 stop- 

 per. 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 508) and when partially cool 7 c.c. of water 

 are added. Into the long tube passing through the stopper suck 3 c.c. of satu- 

 rated sodium hydroxide solution and close the pinch-cock. Insert the stopper, 

 connect with the condenser 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 vigorous boiling (over a large free flame), the distilla- 

 tion being continued until a separation of salts occurs in the test-tube and the 

 mixture begins to bump. This distillation requires about two minutes. The 



1 Bock and Benedict: Jour. Biol. Chem., 20, 47, 1915. -\ 



