368 PRACTICAL ORGANIC AND BIO-CHEMISTRY 



B. COAGULATION REACTIONS. 



(1) Heat. 



On heating some of the solution an opalescence occurs, with 

 perhaps a slight precipitate on the surface of the glass. But on faintly 

 acidifying it, or another portion, with 1-2 drops of dilute acetic acid 

 and again heating, a cloudiness and then a flocculent precipitate of 

 coagulated protein is formed. This precipitate is not soluble in dilute 

 acids and alkalies in the cold, but it gradually dissolves on heating 

 with caustic soda. 



Coagulation did not occur at first as the reaction was alkaline ; it 

 only occurs when the solution is faintly acid. 



(2) Alcohol. 



A precipitate is formed if excess of alcohol be added to some of the 

 solution. This precipitate is at first capable of re-solution in water, 

 but on prolonged contact with alcohol it is rendered insoluble,. trie 

 protein being coagulated. 



(20) Ether. On adding about half a volume of ether to some of the solu- 

 tion and mixing thoroughly by inverting the liquids, a gelatinous solution 

 results, which contains coagulated protein. 



(3) Strong Mineral Acids. 



Hellers Test. Concentrated nitric acid is added to some of the 

 solution by means of a pipette, or by gently pouring down the sides of 

 the tube, so that the acid forms a distinct layer below the solution. 

 At the junction of the two liquids a white ring of coagulated protein 

 is formed. The precipitate does not dissolve in excess of the acid, 

 if the liquids be mixed by shaking. 



C. PRECIPITATION REACTIONS. 

 (i) Solutions of Heavy Metals. 



Mercuric Chloride. If 2 or 3 drops of mercuric chloride solution be 

 added to some of the protein solution, a heavy white precipitate of the 

 mercury compound is formed. This dissolves on adding some saturated 

 sodium chloride solution. The mercury compound is reprecipitated 

 from its solution in sodium chloride on adding a few drops of dilute 

 hydrochloric acid. 



Copper sulphate, added drop by drop, forms a bluish-violet precipi- 

 tate which dissolves in caustic soda, giving a violet solution (biuret 

 reaction). 



Ferric chloride gives a precipitate soluble in excess. 



Lead acetate and basic lead acetate give white precipitates. 



