37 6 



PHYSIOLOGICAL CHEMISTRY 



tube, dissolve the residue in dilute potassium hydroxide solution, and add very 

 dilute copper sulphate solution (see page 98). The purplish-violet color is due 

 to the presence of biuret which has been formed from the urea through the 

 application of heat as indicated. This is the reaction : 



NH 2 



I 



Urea, C=O 



NjH 2 

 X H 



Urea, C = O 



NH 2 



NH 2 



I 

 C=0 



\ 

 NH + NH- 



C=O 



I 

 NH 2 



Biuret. 



6. Urea Nitrate. Prepare a concentrated solution of urea by dissolving 

 a little of the substance in a few drops of water. Place a drop of this solution on a 

 microscopic slide, add a drop of concentrated nitric acid, and examine under 

 the microscope. Compare the crystals with those reproduced in Fig. 119, page 374. 



FIG. 121. UREA OXALATE. 



7. Urea Oxalate. To a drop of a concentrated solution of urea, prepared as 

 described in the last experiment (6), add a drop of a saturated solution of oxalic 

 acid. Examine under the microscope and compare the crystals with those shown 

 in Fig. 121, above. 



8. Decomposition by Sodium Hypobromite. Into a mixture of 3 c.c. of con- 

 centrated sodium hydroxide solution and 2 c.c. of bromine water in a test-tube 

 introduce a crystal of urea or a small amount of concentrated solution of urea. 

 Through the influence of the sodium hypobromite, NaOBr, the urea is decom- 

 posed and carbon dioxide and nitrogen are liberated. The carbon dioxide is 

 absorbed by the excess of sodium hydroxide, while the nitrogen is evolved and 

 causes the marked effervescence observed. This property forms the basis for 



