EMULSOIDS 397 



has become negatively charged by the addition of a little 

 alkali ; and similarly negatively charged colloids, such as 

 phosphomolybdic or phosphotungstic acid or certain negative 

 dyes, are only able to precipitate albumen after it has acquired 

 a positive charge by the addition of acid. 



(c) Precipitation by Electrolytes. — The precipitation of 

 emulsoids from their solutions by electrolytes is not to be re- 

 garded as due to the electrical discharge of the disperse phase 

 by the ionic charges, as with suspensoids. The amounts of the 

 salts required for precipitation are considerable, and precipita- 

 tion in this case is more probably due to a redistribution of the 

 solvent between the emulsoid and the salt added. 



Metallic salts, which precipitate emulsoids, can be arranged 

 in three groups as follows : — 



(i) Sodium, potassium, lithium, ammonium, and magnesium 

 salts. 



If not left too long in contact with these salts, the precipi- 

 tated colloid can be redissolved, and the process is, therefore, 

 reversible. 



Practical application has been made of this phenomenon 

 for separating the various types of protein. Thus, for example, 

 if an aqueous solution containing an albumen and a globulin 

 be mixed with an equal volume of saturated ammonium sul- 

 phate solution, the globulin, being insoluble in the resulting 

 half-saturated ammonium sulphate, is precipitated ; after 

 filtering off the globulin, the album.en may be precipitated 

 from the mother liquor by saturating it with ammonium 

 sulphate. 



The precipitated albumen and globulin are chemically un- 

 changed, and can be redissolved if desired, 

 (ii) Calcium, barium, and strontium salts. 

 The process in this case is reversible immediately after 

 precipitation, but after a very short interval it becomes irre- 

 versible. 



(iii) Heavy metal salts, such as those of mercury, copper, 

 lead, or zinc. 



Here the process is irreversible, owing, no doubt, to the 

 formation of definite chemical compounds. 



