URINE 



375 



alterations in metabolism, e.g., myxedema, and in others as a result 

 of changes in excretion, as in severe and advanced kidney disease. A 

 pathological increase is found in a large proportion of diseases which 

 are associated with a toxic state. In marked acidosis it may be con- 

 siderably decreased relative to the total nitrogen (see Ammonia). 



EXPERIMENTS ON UREA 



1. Isolation from the Urine. 1 Place Sooc.c. of urine in a precipitating jar, 

 add 250 c.c. of baryta mixture, 2 and stir thoroughly. Filter off the precipitate 

 of phosphates, sulphates, urates, and hippurates 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 con- 

 tains the urea contaminated with pigment. Decolor- 

 ize 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 micro- 

 scope and compare them with those shown hi Fig. 1 18, 

 page 372. 



2. Solubility. Test the solubility of urea, prepared 

 by yourself or furnished by the instructor, in water 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 ordinary melting-point tube, sealed 

 at one end, introduce powdered urea. Fasten the tube 

 to the bulb of a thermometer as shown in Fig. 120, and 

 suspend the bulb and its attached tube in a small beaker 

 containing sulphuric acid. Gently raise the tempera- 

 ture of the acid by means of a low flame, stirring the 

 fluid continually, and note the temperature 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 re- 

 produced in Fig. 118, page 372. 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 i32C. and liberates 

 ammonia. Continue heating until the fused mass begins to solidify. Cool the 



x The method based upon the precipitation by nitric acid is also satisfactory (see 

 HoppQ-Seylei'sHandbuchderPhysiol. undPathol. Chem. Anal., Eighth edition, 1909, p. 145). 



2 Baryta mixture consists of a mixture of i volume of a saturated solution of Ba(NO 3 )j 

 and 2 volumes of a saturated solution of Ba(OH) 2 . 



FIG. 1 20. MELTING- 

 POINT TUBES FASTENED 

 TO BULB OF THERMOM- 

 ETER. 



