8 INTRODUCTION 



Colour Reactions. 



(1) Add to a little of the solution in a test-tube a few drops of strong 

 nitric acid. A precipitate is thrown down, which becomes yellow on 

 boiling. Cool, and add strong ammonia; the colour changes to orange 

 (xantho-proteic reaction). The reaction depends upon the presence of 

 aromatic groups in the protein (in phenylalanin, tyrosin, tryptophane, 

 oxytryptophane), which are converted into nitro-compounds. 



(2) To a third portion add a drop or two of very dilute cupric sulphate 

 and excess of sodium or potassium hydroxide ; a violet colour appears 

 (Piotrowski' s test). Peptones and proteoses (albumoses) give a pink 

 (biuret reaction).* See p. 458. 



(3) To another portion add Millon's reagent ;f a white precipitate 

 comes down, which is turned reddish on boiling. If only traces of 

 protein are present, no precipitate is caused, but the liquid takes on a 

 red tinge. The reaction is due to tyrosin. It is given by all aromatic 

 substances which contain the group C 6 H 6 with at least one H replaced 

 by OH, i.e., the hydroxyphenyl group C 6 H 4 OH. 



(4) Adamkiewicz's Reaction (Hopkins' s modification). To a small 

 quantity of the albumin solution add the same bulk of dilute glyoxylic 

 acid.J Mix, and to the mixture add an equal volume of strong pure 

 sulphuric acid. A purple colour is obtained. The substance in the 

 protein molecule which gives the reaction is tryptophane (p. 360). 



(5) The Formaldehyde Reaction. Add to the albumin solution a few 

 drops of a very dilute solution of formaldehyde (i : 2,500), and then 

 allow some strong (commercial) sulphuric acid to run from a pipette 

 into the bottom of the test-tube. A purple ring appears at the surface 

 of contact. This reaction depends on the presence of tryptophane in 

 the protein. 



Precipitation Rtactiontt 



(6) Acidify another portion strongly with acetic acid, and add a few 

 drops of a solution of potassium ferrocyanide. A white precipitate is 

 obtained. Peptones do not give this reaction. 



(7) Heat a portion to 30 C. on a water-bath. Saturate with crystals 

 of ammonium sulphate; the albumin is precipitated. Filter, and test 

 the filtrate for proteins by (2). None, or only slight traces, will be 

 found. The sodium hydroxide must be added in more than sufficient 

 quantity to decompose all the ammonium sulphate. It will be best to 

 add a piece of the solid hydroxide. Peptones are not precipitated by 

 ammonium sulphate, but all other proteins are. 



(8) Add alcohol to a small quantity of the solution. The protein is 



* The reaction is also given, although more faintly, with the hydroxides of 

 lithium, strontium, and barium. It is given by all substances containing at 

 least two CONH 2 groups attached to one another (as in oxamide), or to the 

 nitrogen atom (as in biuret) , or to the same carbon atom. 



f Millon's reagent consists of a mixture of the nitrates of mercury with 

 nitric acid in excess, and some nitrous acid. To make it, dissolve mercury in 

 its own weight of strong nitric acid, and add to the solution thus obtained 

 twice its volume of water. Let it stand for a short time, and then decant the 

 clear liquid, which is the reagent. 



J A solution containing glyoxylic acid in the requisite strength can be 

 prepared by treating half a litre of a saturated solution of oxalic acid with 

 40 grammes of 2 per cent, sodium amalgam in a tall cylinder. When all the 

 hydrogen has been evolved, the solution is filtered, and diluted with twice its 

 volume of water. Oxalic acid and sodium binoxalate are also present in the 

 solution. 



