CH. II.] PROTEIN PRECIPITANTS. 37 



15. Acidify the solution with dilute hydrochloric acid and add 

 a few drops of potassio-mercuric iodide (Briicke's reagent). A 

 white precipitate is formed. 



NOTE. Briicke's reagent is prepared by dissolving 50 grms. of potassium 

 iodide in 500 cc. of distilled water, saturating with mercuric iodide (120 grms.), 

 and making up to i litre. 



1 6. Acidify a few cc. of the solution with dilute hydrochloric 

 acid and add a few cc. of a 2 per cent, solution of phosphotungstic 

 acid. A white precipitate is produced. 



17. To a few cc. of the solution add a drop or two of a freshly 

 prepared 25 per cent, solution of metaphosphoric acid. A white 

 precipitate is produced. 



NOTE. Metaphosphoric acid (HPO 3 ) is used by Folin for removing pro- 

 teins from blood and urine in certain quantitative methods. The solution 

 must be freshly prepared, as on standing it slowly passes over into ortho- 

 phosphoric acid (H 3 PO 4 ), which has no precipitating action on proteins. 



18. To a few cc. of the solution add a small amount (a large 

 " knife point ") of sulphosalicylic acid, or a drop or two of a strong 

 (20 per cent.) solution. A white precipitate is obtained. 



NOTE. The reagent is of considerable value for the detection of albumin 

 in urine. (See Ex. 371.) 



It can be prepared by dissolving 13 grm. salicylic acid in 20 grms. H 2 SO 4 

 by warming, and, after cooling, adding 67 cc. of water. 



(4.) The proteins are precipitated by strong alcohol. 

 The albumins and globulins are rapidly changed by alcohol 

 at room temperature into modifications that are insoluble 

 in water, salt solutions, dilute alkalies or acids. That is, 

 they are coagulated. 



The proteoses and peptones, the phosphoproteins, and 

 gelatin are precipitated by alcohol, but the precipitate 

 redissolves in water or dilute alkalies. 



19. Place about 4 cc. of serum in a test-tube and cool to 

 o C. by means of a freezing mixture. Fill the tube with strong 

 alcohol that has previously been cooled to about 8 C., and mix. A 

 white precipitate of the proteins is formed. Filter at once and 

 treat the precipitate with water. It dissolves. 



