DETERMINATION OF UREA. 655 



saturating with alkali and the ammonia collected in standard sulphuric 

 acid. The following reagents are necessary: 



1. Sulphuric Acid. Either a mixture of equal volumes of pure 

 concentrated and fuming sulphuric acid or else a solution of 200 grams 

 phosphoric anhydride in 1 liter of pure concentrated sulphuric acid. 

 2. Caustic soda free from nitrates, 30-40 per cent solution. The quantity 

 of this caustic-soda solution necessary to neutralize 10 cc. of the acid 

 mixture must be determined. : 3. Metallic mercury or pure yellow mercuric 

 oxide. (The addition of this facilitates the destruction of the organic 

 substances.) 4. A potassium-sulphide solution of 4 per cent, whose 

 object is to decompose any mercuric amide combination which might 

 not evolve its ammonia completely during the distillation with caustic 

 soda. 5. 1/5 normal- sulphuric acid and 1/5 normal caustic soda solution. 



In performing the determination 5 cc. of the carefully measured and 

 filtered urine are placed in a long-necked KJELDAHL flask, a drop of mercury 

 or about 0.3 gram of mercuric oxide added, and then treated with 10-15 

 cc. of the strong sulphuric acid. The contents are heated very care- 

 fully, placing the flask at an angle, until they just begin to boil gently; 

 this is continued for about half an hour after the mixture becomes colorless. 

 On cooling the contents are transferred to a voluminous distil ling-flask, 

 carefully washing the KJELDAHL flask with water, and the greater part 

 of the acid is neutralized by caustic soda. A few zinc shavings are 

 added to prevent too rapid ebullition on distillation, and then an excess 

 of caustic-soda solution which has previously been treated with 30-40 

 cc. of the potassium-sulphide solution. The flask is quickly connected 

 with the condenser-tube and all the ammonia distilled off. In order 

 to prevent loss of ammonia it is best to lower the end of the exit-tube 

 below the surface of the acid, the regurgitation of the acid being prevented 

 by having a bulb blown on the exit-tube. Not less than 25-30 cc. of 

 the standard acid is used for every 5 cc. of urine, and on completion 

 of the distillation the acid is retitrated with 1/5 normal caustic soda, using 

 rosolic acid, tincture of cochineal, or lacmoid as indicator. Each cubic 

 centimeter of the acid corresponds to 2.8 milligrams nitrogen. As a 

 control and in order to test the purity of the reagents, or to eliminate 

 any error caused by an accidental quantity of ammonia in the air, we 

 always make a blank determination with the reagents. 



Among the methods suggested for the special estimation of urea, 

 that of Mo'RNER-SjOQVisT, in combination with FOLIN'S method, is per- 

 haps the most trustworthy and readily performed. For this reason 

 only this method will be given in detail, while we must refer to special 

 works for the other methods, such as BUNSEN'S method with its many 

 modifications as suggested by PFLUGER, BORLAND and BLEiBTREu. 1 



Principle of Morner-Sjoqvist's Method. 2 According to this method 

 the nitrogenous constituents of the urine, with the exception of urea, 



1 Pfluger's Arch., 38, 43, and 44. 



2 Skand. Arch. f. Physiol., 2, and Morner, ibid., 14, where the recent literature 

 may also be found. 



