234 PHYSIOLOGICAL CHEMISTRY 



albumin solution with an equal volume of glyoxylic acid solution. 

 Add an equal volume of concentrated sulphuric acid, pouring it 

 down the inside of the test tube so as to make a layer beneath 

 the aqueous solution. A reddish violet or purple ring will 

 develop at the juncture of the two liquids. Shake the tube gently 

 so as to mix the two layers. The color will spread throughout 

 the entire solution. 



This test is often performed by adding glacial acetic acid in 

 place of a solution of glyoxylic acid, as glyoxylic acid is usually 

 present in glacial acetic acid as an impurity. It is more satis- 

 factory, however, to use a solution of glyoxylic acid, which may 

 be prepared by reducing oxalic acid with sodium amalgam or 

 magnesium. The reaction is due to tryptophane, which is present 

 in most proteins. 



v. Molisch Test. The student is already familiar with the 

 Molisch test from his work on carbohydrates. It is given by 

 many proteins and is supposed to indicate the presence of a car- 

 bohydrate group in the protein molecule. To about 3-4 c.c. of 

 albumin solution add 2-3 drops of 15% alcoholic <x naphthol. 

 Incline the tube and pour down the side a few cubic centimeters 

 of concentrated sulphuric acid to form a layer at the bottom 

 of the tube. Observe the red ring. 



2. Precipitation Tests. 



i. By Heat. Heat about 3 c.c. of albumin solution to boiling. 

 If no precipitation occurs, add 1 drop of 1% acetic acid and boil 

 again, repeating the process until coagulation occurs. Repeat 

 the experiment, adding about 1 c.c. saturated sodium chloride 

 solution to the albumin solution. The presence of salts is fa- 

 vorable for coagulation. 



In removing the protein from a solution the acid is added after 

 heating, as otherwise acid metaprotein, which does not coagulate 

 on boiling, may be formed. Proteoses, peptones, casein and a few 

 other proteins are not coagulated by heat. Heat another portion 

 of albumin solution, first adding 1-2 drops of concentrated 



