SALTING OUT OF PROTEINS 87 



They are used in variable degrees of concentration, 

 the strength of solution necessary for the precipitation 

 of a particular protein being constant for that body. 

 The most efficient of the substances named are ammo- 

 nium and zinc sulphate, either of which, in saturated 

 solutions, will salt out all proteins and their derivatives, 

 except the peptones. 



Certain acids (so-called alkaloidal reagents) precipitate 

 many proteins when used in excess ; those most com- 

 monly employed are : 



Phosphotungstic acid 

 Phosphomolybdic 

 Tannic 



They are extensively used for the precipitation of 

 proteoses and peptones, the latter of which cannot be 

 salted out from their solutions. 



Ex. 45. Break an egg across the edge of a cup; open the crack, 

 and allow the white to escape into a beaker, keeping back the 

 yolk. Transfer the white to a flask ; mix it with about ten 

 times its volume of water, and shake vigorously so as to dis- 

 solve as much as possible of the proteins. Use small quantities 

 of the solution of egg-white obtained to illustrate the colour 

 tests for proteins. 



(a) Biuret test. To i c.c. of the egg-white solution add 5 c.c. 

 of strong caustic potash solution and one or two drops of a 

 i per cent, solution of copper sulphate ; heat slightly, a violet 

 colour is seen. 



(b) Milloris test. Weigh out 10 grams of strong nitric acid 

 (sp. g. 1.4), add to it 10 grams of mercury ; heat, and allow the 

 metal to dissolve ; then dilute the liquid with twice its volume of 

 distilled water ; mix, and after allowing it to stand for a time, 



