UREA 215 



revolve the mixture in -the flask for 1 min. and set aside for 30 

 min. before use. 



PROCEDURE 



1. Fix a 15 cm. length of uniform capillary tubing (0.35 mm. 

 inner diameter) to the stage of the binocular microscope so that one 

 end is in the optical field and attach the water manipulator to the 

 other end. 



2. Introduce a 6 mm. column of distilled water with a capillary- 

 pipette and draw it back 1 mm. from the end. 



3. Introduce a 3 mm. column of tungstic acid supernatant (page 

 198) or other urea soln. to be analyzed, draw it in away from the 

 end, and seal the end with plasticine. 



4. Accurately measure the length of the air column between the 

 two liquid columns with a filar micrometer temporarily substituted 

 for the right ocular which contains a disc micrometer. Two successive 

 readings must agree within 1 micrometer scale division (5 fx). 



5. Replace the right ocular, cut off the end of the tube sealed 

 with plasticine, move the urea column back to the end of the capil- 

 lary with the water manipulator, and add 3 mm. of sodium hypo- 

 bromite soln. from a blunt-tipped pipette freshly filled just before 

 use. Should gas bubbles appear immediately upon the addition of 

 the reagent, discard the tube, and prepare fresh reagent. 



6. Move the liquid column away from the end of the tube and 

 seal with plasticine. 



7. Carefully remove the tube from the water manipulator by 

 cutting it about 6 cm. from its end, revolve it between thumb and 

 forefinger for a few sec. and set aside in a nearly vertical position 

 upon a plasticine mount. 



8. After 2 hr. again revolve the tube for a few sec, place on the 

 microscope stage and accurately measure the length of the air column 

 with the filar micrometer. 



9. Run a blank determination with distilled water and subtract 

 the increase in the length of the air column from that found above. 

 For each milligram per cent of urea nitrogen the increase averages 

 four scale divisions (20 /a) ; the increase in the blank averages five 

 divisions. Hence a soln. containing 10 milligram per cent urea 

 nitrogen should give an increase of 45 divisions. 



