102 ORGANIC ACPvlCULTURAL CHEMISTRY 



(2) Preparation of Biuret. Place a small amount of urea in a dry, 

 small test tube. Heat carefully until the urea melts. Continue to 

 heat slowly as long as gas is evolved. What is the gas? Test with 

 red litmus paper and by odor. After all ammonia is expelled, cool 

 and dissolve in water. Now add to the solution an equal volume of 

 strong KOH and a few drops of very dilute CUSO4 solution as de- 

 scribed under Experiment XVIII, 4, c. The pink color is due to the 

 reaction of the biuret, formed from the urea, and the copper sxilphate. 



Uric Acid and Purine Bases 



Uric acid is a much more complex compound related to urea. 



It contains two urea residues joined together by a carbon 



nucleus. 



NH-CO 



\ II >co 



NH-C-NH/ 



Uric acid 



It is what is known as a tautomeric compound, i.e. it exists in 

 either of two forms depending upon the conditions and the 

 character of the substance acting upon it. The tautomeric 

 form is represented by the following formula : 



N = C-(OH) 



I 

 (HO)-C C-NH 



V II ^C-(OH) 



Uric acid 



This is a tri-hydroxy derivative of a hypothetical compound 

 known as purine which has the formula, 



'N=CH 



HC C - NHv 



II ^CH 



Purine 



