40 METHODS FOR THE ANALYSIS OF URINE 



drawn through a tube containing strong sulphuric acid before 

 it is admitted to Tube " A " so that all the ammonia of the air 

 may be absorbed. At the end of fifteen minutes aspirate for 

 about one-half minute to remove any ammonia present in the 

 free condition in "A." After this aspiration, open " A " and 

 introduce 5 c.c. of saturated potassium carbonate. Close " A " 

 at once and aspirate until all the ammonia has been removed 

 from " A " and carried over into the acid in " B." The time 

 needed for the aspiration varies for different pumps from five to 

 thirty minutes, and should be determined by trial for the par- 

 ticular apparatus used. At the end of the time needed for the 

 aeration, the pump is disconnected (care being taken to avoid 

 back suction) and the excess acid in " B " is titrated by means 

 of fiftieth-normal alkali. 



Calculation. The number of cubic centimeters of fiftieth- 

 normal acid neutralized is multiplied by the factor 0.056 to give 

 the number of grams of urea plus ammonia-nitrogen in 100 c.c. 

 of the urine. The ammonia alone may be determined at the same 

 tune as the ammonia plus urea, using the same technic except 

 that 5 c.c. of the undiluted urine, no urease, and the factor 0.0056 

 are used for the determination of ammonia alone. The ammonia 

 tubes are run in the same series as these for the urea determination, 

 using the same air current for all. 



UREA 



Marshall's Urease Method l 



Principle. This is a simple clinical method for the determi- 

 nation of urea in urine. It differs from the preceding method 

 in that instead of aspirating off the ammonia formed from the 

 urea by the action of the urease, it is titrated directly in the urine 

 mixture, thus simplifying the procedure. The method is nearly 

 as accurate as the preceding for normal urine the error being 

 only about 2 per cent which is very satisfactory for a rapid clinical 

 procedure. For diabetic urines the aeration procedure should 

 be used as such urines contain substances which render the titra- 

 tion inaccurate. 



1 Marshall: Jour. Biol. Chem., 1913, 14, 283. 



