96 ORGANIC AGRICULTURAL CHEMISTRY 



parts by weight of concentrated HNO3 (sp. gr. 1.42) and then dilute 

 with two voL H2O. To a small amount of solid protein in a test 

 tube add a few drops of Millon's reagent. Warm. A yellow or 

 white color which turns red on heating indicates protein. The test 

 works best on solid protein. The test is obtained with any sub- 

 stance containing the hydroxy-phenyl group (C6H4(OH)) so that non- 

 proteins like tyrosine, phenol or carboHc acid (CeHsOH) and thymol 

 give a positive test. 



(6) Xanthoproteic Reaction. To a small amount of protein in a 

 test tube add concentrated nitric acid. A white precipitate turning 

 yellow indicates protein. Cool and add ammonium hydroxide. The 

 color changes to orange. The reaction depends upon the presence of 

 the phenyl group (C5H5— ) so that any phenyl-containing compound 

 such as tyrosine, phenylalanine and tryptophane, phenol, etc., gives a 

 positive test. The yellow color produced when nitric acid stains the 

 flesh is due to this reaction. 



{c) Biuret Reaction. To a 5 c.c. solution of protein add an equal 

 volume of concentrated KOH or NaOH. Mix thoroughly. Make a 

 very dilute solution of copper sulphate by adding a few drops (2 to 5) 

 of ordinary 10 per cent solution copper sulphate to a test tube full of 

 water. The dilute CUSO4 should be only very faintly blue. Now 

 add a few drops of this dilute CUSO4 to the alkahne protein solution 

 and warm. A pink-violet color indicates protein. The reaction is 

 due to substances containing two amino (NH2) groups. These groups 

 may be joined together directly or with an intermediate carbon 

 group. Biuret, the substance which gives the name to the reaction, 

 is formed from urea by the loss of ammonia (see Experiment XIX). 



Test by the above reagents : 



(a) Egg albumin, fresh, {e) Milk casein or curd, 



{h) Dilutesolution of egg albumin, (J) Wheat gluten, 



(c) Egg albumin, dry, (g) Dried blood, 



{d) Egg albumin, coagulated {h) Horn or hair or finger nail. 

 (cooked) , 



(5) Precipitation Tests, {a) To 5.0 c.c. of dilute mercuric chloride 

 (corrosive sublimate) solution, add a little egg albumin solution. 

 Note the precipitation of mercury albuminate, {h) Repeat the ex- 

 periment, using lead acetate solution instead of mercuric chloride. 

 The precipitate here is lead albuminate. These two experiments 



