EMULSOIDS 295 



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 hasbeen 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 albumen 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. 



The case of zinc is peculiar, inasmuch as very dilute solu- 

 tions of zinc salts produce irreversible precipitation of egg 

 albumen, whereas strong solutions may either not produce a 

 precipitate, or else cause one already formed to dissolve.* 



The anion also plays an important part in influencing the 

 precipitating power of a given salt. By arranging the various 

 salts of sodium in the order of decreasing precipitating power, 

 the so-called Lyotropic series are obtained as follows : 



Citrate > tartrate > sulphate > acetate > chloride > nitrate 

 > chlorate > bromide > iodide > sulphocyanide. 



* Pauli: " Beitr. z. chem. Phys. and Path.," 1905, 6, 233, 259. 



