18 LABORATORY WORK IN PHYSIOLOGICAL CHEMISTRY. 



may be lacking altogether, facts which explain the individual 

 differences in the strengths of the reactions obtained. 



For the following tests make use of the egg-white solution 

 as an example of a typical protein: 



(a) Xanthoproteic Test. To 5 c.c. of the protein solution (or 

 dry substance) add 5 c.c. of HNO 3 . Note the white precipi- 

 tate. Boil until the precipitate is dissolved and the solution 

 becomes light yellow. Cool, and add an excess of NH 4 OH. 

 The color changes to orange. Other substances may give a 

 yellow solution with HN0 3 , but their solutions do not respond 

 orange upon neutralization with NH 4 OH. What are the 

 chemical changes of this reaction and the atomic complexes 

 involved? 



(6) Millon's Test. Add a few c.c. of Millon's reagent to 

 5 c.c. of the protein solution. The precipitate which forms 

 turns red slowly upon heating. This reaction is suitable for 

 solids, or liquids when the quantity of salts in the solution is 

 not excessive. What atomic complex in the protein molecule 

 causes this reaction? What simple substance also responds 

 to this test? 



(c) Biuret Test. Suited for testing solutions only. Place 

 5 c.c. of the protein solution in a test-tube and add an equal 

 volume of NaOH. Then add, drop by drop, a CuS0 4 solu- 

 tion so dilute that the color is hardly visible. The color 

 obtained will vary from blue violet to reddish violet, accord- 

 ing to the nature of the protein in solution. If too much 

 CuS0 4 is added the solution becomes green. A specific 

 atomic complex in the protein molecule is also accountable 

 for this reaction. What simple substance reacts positively 

 to this test? 



(d) Adamkiewicz Test. To 3 c.c. of a glyoxylic acid 

 solution (Hopkins-Cole reagent) add 1-2 c.c. of the protein 

 solution. Stratify this mixture upon 5 c.c. of cone. H 2 S0 4 . 



