3Q2 PHYSIOLOGICAL CHEMISTRY 



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 



i. Isolation from the Urine. 1 Place 800 c.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. 123, 

 page 400. 



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. 125, 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. 



FIG. ^-MELTING- 4- Crystalline Form. Dissolve a crystal of pure 

 POINT TUBES FASTENED urea in a few drops of 95 per cent alcohol and place 

 TO BULB or THERMOM- !_ 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. 123, page. 389. 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 hi 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 



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

 Hoppe-Seyler's Handbuch der Physiol. undPathol. Chem. Anal., Eighth edition, 1909, p. 145). 



2 Baryta mixture consists of a mixture of i volume of a saturated solution of Ba(NOs)2 

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



