CH. XIII.] UREA. 335 



401. Van Slyke's method for urea. 



Principle. A small volume of the urine is treated -with an extract of the 

 Soya bean, together with a certain amount of acid potassium phosphate to 

 preserve the optimum reaction for the enzyme. The whole of the urea is 

 rapidly converted to ammonium carbonate. An excess of potassium carbonate 

 is added, and the ammonia formed from the urea, together with that from the 

 preformed ammonium salts of the urine are driven over into standard acid and 

 estimated in the way described in Ex. 399. The amount of ammonia being 

 known, the percentage of urea can be found by difference. 



Apparatus. This is exactly similar to that required for Ex. 399. A 

 duplicate set should be obtained so that the ammonia and urea determinations 

 can be conducted simultaneously. 



Urease solution. An active extract of the finely ground Soya beans* can 

 be prepared by the following method. Rub up 5 grams, of the meal with 

 50 cc. of 0-6 per cent, acid potassium phosphate (KH 2 PO 4 ) in a mortar. 

 After standing for about 15 minutes, filter through a pleated paper. The 

 opalescent solution obtained should be freshly prepared, but if kept in an ice 

 chest it seems to be stable for 2 or 3 days. 



An active dried preparation of the enzyme, known as " Arlco-urease," 

 can be obtained from the Arlington Chemical Company, Yonkers, N.Y., 

 U.S.A., but the home-made extract is very much less costly and is equally 

 efficient. 



The first batch of enzyme made from a sample of the Soya bean meal 

 should be tested by making a 2 per cent, solution of pure, dry urea, and 

 estimating the amount in 0-5 cc. of this by the method described below. The 

 result obtained should be within 4 per cent, of theory, the slight deficiency 

 being generally due to impurities in the urea. 



Method. See that the tube B (fig. 44) and the narrow tube that 

 goes into it have been well washed, and are quite free from any of the 

 alkaline carbonate used in a previous experiment. 



(i.) Measure 0-5 cc. of the urine into B, using an accurate 

 Ostwald pipette (fig. 48). If the urine is known to be a very dilute 

 one, i or even 2 cc. can be taken. 



(ii.) Add 2 cc. of the extract of Soya bean and 3 cc. of water, 

 washing the traces of urine down to the bottom of the tube with these 

 two fluids. Lightly shake to mix. 



(iii.) Add 2 drops of caprylic alcohol, to prevent subsequent 

 foaming. 



(iv.) Fit the rubber stopper with the tubes it carries. 



* Soya bean meal can be obtained from Messrs. Baird and Tatlock, 

 London. 



