PROTEINS 233 



CONH 2 . NH . CONH 2 , a substance obtained by heating urea to 

 180 C. 



ii. Millon's Reaction. To a few cubic centimeters of albumin 

 solution add 2-3 (no more) drops of Millon 's reagent (1 pt. by 

 weight of Hg. dissolved in 2 pts. of cone. HN0 3 and diluted with 

 two volumes of water). A yellowish or white precipitate forms. 

 Heat the solution carefully. The precipitate will turn pink or 

 red. Repeat the test, first adding sodium chloride solution to the 

 albumin solution. The test fails, that is, the precipitate no 

 longer turns pink. This fact should be borne in mind in testing 

 for protein in a liquid containing sodium chloride. Shake up a 

 small amount of dry casein with water and apply the Millon test. 

 The test is given by solid proteins as well as by proteins in solu- 

 tion. Perform the test on a dilute solution of phenol. A beau- 

 tiful red color results. The reaction is given by substances con- 

 taining a hydroxyphenyl group C 6 H 4 . OH. Most proteins 

 contain tyrosine, a substance possessing this grouping, and it is 

 because of the presence of this substance that the proteins give 

 the Millon reaction. 



iii. Xanthoproteic Reaction. To a few cubic centimeters of 

 egg albumin solution add concentrated nitric acid. Warm the 

 mixture until the whitish precipitate has dissolved. The solu- 

 tion is yellow. Cautiously add ammonia until the reaction is 

 alkaline and observe the deepening of the color to orange. Re- 

 peat the test using a small portion of dry casein. Invert the 

 stoppered bottle of concentrated nitric acid slightly so as to get 

 a drop on the glass stopper. Carefully apply the stopper mois- 

 tened with nitric acid to a small area on the palm of the hand. 

 In a moment pour on the yellow spot a drop or so of ammonia. 

 Observe the orange spot. Rinse off the hand carefully under the 

 tap. The test is given by substances containing a benzene ring 

 and is due to the formation of certain nitro-compounds. Most 

 proteins contain amino acids in which there is a benzene ring, 

 and thus will respond to this test. This is also true of the pro- 

 teins of the skin. 



iv. Adamkiewicz, or Hopkins-Cole Reaction. Mix 2-3 c.c. of 



