36 THE PROTEINS. [CH. II. 



10. Treat 3 cc. of the protein solution with ferric chloride, 

 drop by drop. A precipitate is formed soluble in excess. 



11. Treat 3 cc. of the protein solution with a solution of lead 

 acetate or basic lead acetate. A white precipitate is formed. 



( 3 . ) The proteins are precipitated by the so-called ' ' alka- 

 loidal reagents" These include phosphotungstic, phos- 

 phomolybdic, ferrocyanic, tannic, picric, metaphosphoric, 

 and sulphosalicylic acids, and Briicke's reagent (potassio- 

 mercuric iodide). 



It is possible that the precipitation is due to the adsorp- 

 tion of the complex negative ions by the positively charged 

 colloidal protein. It is suggestive that the substances are 

 only effective in acid solution, in which the proteins are 

 positively charged. The precipitating action of these re- 

 agents on the peptones varies somewhat. As a rule they 

 are not so readily precipitated as the albumins and globu- 

 lins. 



12. Treat 3 cc. of the solution with two or three drops of strong 

 acetic acid and two drops of potassium ferrocyanide. A white 

 precipitate is formed. Boil. The precipitate does not dissolve. 



NOTES. i. Primary proteoses are also precipitated by ferrocyanic acid, 

 but the precipitate produced dissolves on warming and reappears on cooling 

 (Ex. 55). 



2. The precipitate and fluid often become coloured blue-green on boiling. 

 This is due to a decomposition of the hydroferrocyanic acid on boiling it with 

 certain organic substances, such as proteins. 



13. Acidify some of the solution with hydrochloric acid and 

 add a few drops of a freshly prepared solution of tannic acid, or of 

 Almen's reagent. A white or brown precipitate is usually formed. 



NOTE. Almen's reagent consists of 4 gm. of tannic acid in 8 cc. of strong 

 acetic acid and 190 cc. of 50 per cent, alcohol. 



14. Treat 3 cc. with an equal volume of Esbach's solution. A 

 yellowish precipitate is formed. 



NOTE. Esbach's solution is prepared by dissolving 10 grms. of picric 

 acid and 10 grms. of citric acid in water and making the volume up to a litre. 

 It is extensively used for the determination of albumin in urine (Ex. 421). 



