88 METHODS FOR THE ANALYSIS OF BLOOD 



1. Determination of Urea by Urease and Distillation 



Procedure. Transfer 5 c.c. of the tungstic acid blood filtrate 

 to a clean and dry Pyrex ignition tube (capacity about 75 c.c.). 

 The graduated Pyrex tubes recommended for the non-protein 

 nitrogen determinations should never be used for urea determi- 

 nations, because they have contained Nessler solutions and Nessler 

 solutions leave behind films of mercury compounds which destroy 

 the urease. If these tubes must be used, they should first be 

 washed with nitric acid to remove the mercury films and then 

 thoroughly washed with water. Add to the blood filtrate 2 drops 

 of the pyrophosphate solution or 2 drops of a molecular o-phos- 

 phate solution (1/3 molecular monosodium phosphate plus 2/3 

 molecular disodium phosphate). Then add 0.5 to 1 c.c. of the 

 urease solution and immerse the test-tube in a beaker of warm 

 water and leave it there for five minutes. The temperature of the 

 water is not very important but should not exceed 55 C. Warm 

 water can perhaps scarcely be said to be essential, for the hydrolysis 

 is very rapid at room temperature, but it is preferable to use it. 

 If no hot water is used, continue the digestion for ten to fifteen 

 minutes or as much longer as is convenient. The ammonia formed 

 can be conveniently and quickly distilled into 2 c.c. of 0.05 normal 

 hydrochloric acid contained in the second test-tube. The second 

 test-tube should not be so heavy as the ordinary test-tubes and 

 should be graduated at 25 c.c. The test-tube which serves as the 

 receiver is held in place by means of a rubber stopper in the side 

 of which has been cut a fairly deep notch to permit the escape of 

 air (and some steam). The rubber stopper serving as a holder for 

 the receiver fits quite loosely to the delivery tube by means of 

 which the two test-tubes are connected. The delivery tube must, 

 of course, be so adjusted as to reach below the surface of the hydro- 

 chloric acid solution in the receiver before the distillation is begun. 

 Add to the hydrolyzed blood filtrate a dry pebble, 2 c.c. of 

 saturated borax solution, and a drop or two of paraffin oil; insert 

 firmly the rubber stopper carrying both delivery tube and receiver, 

 and boil moderately fast over a micro-burner for four minutes. 

 The size of the flame should never be cut down during the distil- 

 lation nor should the boiling be so brisk that the emission of 

 steam from the receiving tube begins before the end of three 

 minutes. At the end of four minutes slip off the receiver from the 



