PRECIPITATION REACTIONS OF THE PROTEINS IOQ 



b. Precipitation with Heavy Metals. Proteins form insoluble compounds 

 with the metals when mercuric chloride, lead acetate, copper sulphate, 

 silver nitrate, etc., are added to protein solutions. 



c. Acetic Acid and Potassium Ferrocyanide Test. To about 5 c.c. of egg- 

 white solution in a test-tube add five to ten drops of acetic acid and then 

 potassium ferrocyanide drop by drop until a precipitate forms. This 

 test is very delicate. 



d. Precipitation with the Alkaloidal Reagents. Prepare six tubes contain- 

 ing about 3 c.c. egg-white solution. To the first add picric acid drop by 

 drop until excess of the reagent has been added, noting the changes with 

 care. Repeat the experiment with trichloracetic acid and tannic acid. 

 Acidify the remaining tubes with hydrochloric acid, and repeat the experi- 

 ment with phosphotungstic acid, phosphomolybdic acid, and potassium 

 mercuric iodide. 



e. Heat Coagulation. Take about 10 c.c. of egg-white solution in a test- 

 tube and heat to boiling. Then add a few drops of dilute acetic acid. The 

 protein will be coagulated. The acetic acid should be added after heating, 

 since otherwise acid metaprotein might be formed. The presence of 

 some neutral inorganic salts tends to give a sharper test. The addition of 

 the acid also will dissolve the earthy phosphates which are often precipi- 

 tated from the urine on heating. Proteoses, peptones, the casein of milk, 

 and a few other proteins are not coagulated by heat. 



/. Precipitation by Alcohol. Add some 95 per cent, alcohol to a test-tube 

 containing about 3 c.c. of egg-white solution. The protein is precipitated, 

 and on standing it is coagulated so that it can no longer be dissolved in 

 neutral solvent. 



g. Salting-out Experiments. Add to some diluted blood-serum in a small 

 beaker, crystals of magnesium sulphate until no more of the salt will go 

 into solution. After standing for a few minutes, filter and test the filtrate 

 and residue for protein by some of the precipitation or color reactions given 

 above. It is found that the filtrate still contains some protein. Further, 

 that this protein can be precipitated on adding a few drops of dilute acetic 

 acid. When the blood serum is similarly saturated with ammonium 

 sulphate, there will be no protein found in the filtrate. If to the blood 

 serum an equal volume of saturated ammonium sulphate solution is 

 added, the result will be the same as that already obtained with mag- 

 nesium sulphate. Some proteins then are precipitated on saturating 

 their solutions with magnesium sulphate or by adding an equal volume 

 of saturated ammonium sulphate solution; albumins, globulins, and 

 proteoses, however, are all precipitated by saturation with the more 

 soluble ammonium sulphate. 



