CH. XII.] UREA. 291 



3. The test will not succeed in the presence of the salts of the heavy 

 metals, which inhibit the action of the enzyme. A high concentration of 

 buffer salts, such as phosphates or acetates, decreases the delicacy of the test, 

 by preventing considerable changes in hydrogen-ion concentration. 



344. To about 4 cc. of a I per cent, solution of urea add about 

 2 cc. of strong soda, mix and divide into two portions, A and B. 

 Boil B for 3 to 5 minutes, adding a little water from time to time to 

 replace that lost by evaporation. Cool under the tap. Add phenol 

 red to each and neutralise by the addition of hydrochloric acid, using 

 concentrated acid at first and finish by dilute hydrochloric. Apply 

 the urease test as described in the previous exercise to the two 

 solutions. A gives a strong test, whilst B gives none, or only a 

 slight one, owing to the destruction of the urea by boiling alkali. 



345. Place a little urea in a dry test-tube. Heat carefully 

 over a flame, keeping the upper part of the tube cool. The urea 

 melts and evolves ammonia, whilst a white sublimate condenses on 

 the cooler parts of the tube. Cool the tube, add a little water and 

 shake. Pour the solution into another tube and treat it with an 

 equal bulk of sodium hydroxide and a drop of copper sulphate. A 

 pink colour is produced, due to the biuret formed from the urea. 



346. Repeat the experiment, but heat more strongly till the 

 melt solidifies and becomes opaque. Cool, add two or three cc. 

 of water, boil and filter whilst still hot. Divide the solution into 

 two portions, A and B. To A add a few drops of a solution of 

 barium chloride and a single drop of diluted ammonia. A white 

 mass of barium cyanurate is formed on cooling. 



To B add some ammoniacal copper sulphate solution and boil. 

 On cooling an amethyst precipitate of copper ammonium cyanurate 

 is deposited. 



NOTE. Preparation of ammoniacal copper sulphate, i per cent, copper 

 sulphate is treated with very dilute ammonia till the precipitate that first forms 

 just redissolves. 



347. Isolation of urea from urine. Evaporate about 30 cc. 

 of urine to complete dryness, finishing the evaporation on the water 

 bath (to prevent the destruction of the urea) . Turn out the flame 

 and rub the residue with about 10 cc. of acetone till it is boiling. 

 Allow the acetone to boil, stirring all the time, till about half of it 



