THE PROTEINS. 19 



A red to reddish- violet color develops at the junction of the 

 two liquids. Of what important complex does this reaction 

 show the presence? 



Precipitation Reactions. 



The precipitation reactions may be divided into three 

 general classes according as the protein acts as a base, as an 

 acid, or as a neutral body. To the first class belong those 

 reactions in which the weak organic acids combine with the 

 protein as a base and an insoluble salt results. The second 

 class of reactions embrace those between neutral salts and 

 the protein in which an albuminate of the base (metal) is 

 formed. In the third class the protein does not take part 

 in the reaction. Precipitation occurs when the reagent 

 which is added (in this case a salt such as MgS0 4 , Na^SO^, 

 (NH 4 ) 2 S0 4 ) has appropriated to itself sufficient of the solvent 

 to throw the protein out of its solution (salted out). 



(a) Heller's Test. If 5 c.c. of HN0 3 is placed in a test- 

 tube and a few c.c. of the albumin solution allowed to flow 

 gently down the side of the test-tube and stratify itself on 

 top of the HN0 3 , a white ring of precipitated albumin will 

 form where the two liquids meet. HC1 and H 2 S0 4 will also 

 give the same reaction, but the precipitates are soluble in an 

 excess of the reagents. 



(6) Acetic acid and potassium ferrocyanide. Make 5 c.c. 

 of the protein solution acid with acetic acid and add, drop 

 by drop, a dilute solution of potassium ferrocyanide. Note 

 result. 



(c) Make 25 c.c. of the protein solution slightly acid, 

 using dilute acetic acid. 



1. To 5 c.c. of this solution add 2 drops of tannic acid 

 solution. 



