URINE. 



269 



FIG. 87. 



EXPERIMENTS ON UREA. 



1. Isolation from the Urine. Place 800 c.c. of urine in a pre- 

 cipitating jar, add 250 c.c. of baryta mixture 1 and stir thoroughly. 

 Filter off the precipitate of phosphates, sulphates, urates and hip- 

 purates and evaporate the filtrate on a water-bath to a thick syrup. 

 This syrup contains chlorides, creatinine, organic salts, pigments 

 and urea. Extract the syrup with warm 95 per cent alcohol and 

 filter again. The filtrate contains the urea contaminated with pig- 

 ment. Decolorize the filtrate by boiling with animal charcoal, filter 

 again and stand the filtrate away in a cold place for crystallization. 

 Examine the crystals under the microscope and compare them with 

 those shown in Fig. 85, page 266. 



2. Solubility. Test the solubility of urea, prepared by yourself 

 or furnished by the instructor, in the ordinary 



solvents (see p. 23) and in alcohol and ether. 



3. Melting-Point. Determine the melting- 

 point of some pure urea furnished by the 

 instructor. Proceed as follows : Into an ordi- 

 nary melting-point tube, sealed at one end, 

 introduce a crystal of urea. Fasten the tube 

 to the bulb of a thermometer as shown in Fig. 

 87, p. 269, and suspend the bulb and its at- 

 tached tube in a small beaker containing sul- 

 phuric acid. Gently raise the temperature of 

 the acid by means of a low flame, stirring 

 the fluid continually, and note the tempera- 

 ture at which the urea begins to melt. 



4. Crystalline Form. Dissolve a crystal 

 of pure urea in a few drops of 95 per cent 

 alcohol and place 1-2 drops of the alcoholic 

 solution on a microscopic slide. Allow the 

 alcohol to evaporate spontaneously, examine 

 the crystals under the microscope and compare 

 them with those reproduced in Fig. 85, p. 266. 

 Recrystallize a little urea from water in the 

 same way and compare the crystals with those 

 obtained from the alcoholic solution. 



5. Formation of Biuret. Place a small amount of urea in a 

 dry test-tube and heat carefully in a low flame. The urea melts at 



1 Baryta mixture consists of a mixture of one volume of a saturated solution 

 of Ba(NO 3 ) 2 and two volumes of a saturated solution of Ba(OH) : . 



MELTING-POINT TUBES 

 FASTENED TO BULB OF 

 THERMOMETER. 



