384 PRACTICAL ORGANIC AND BIO-CHEMISTRY 



C. Emulsoids. 



Silicic acid sol is one of the few examples of an inorganic emulsoid. 

 The organic emulsoids are very various. The types are represented 

 by gelatin and agar, albumin, gum-arabic, cellulose and nitrocellulose 

 solutions. 



Preparation of Emulsoid Sols. 



(a) Silicic Acid. A solution of sodium silicate is treated with excess 

 of hydrochloric acid and dialysed. A clear solution remains in the 

 dialyser. 



(&) Gelatin and Agar. These substances dissolve in hot water. 



(c) Albumin dissolves in cold water. 



(d) Cellulose dissolves in Schweitzer's reagent or zinc chloride solu- 

 tion. 



(e) Nitrocellulose * dissolves in alcohol-ether, acetone, acetic acid, etc. 



General Properties of Emulsoid Sols. 



(1) Concentration. These colloidal solutions can be prepared of 

 various strengths and are not necessarily dilute as suspensoid sols. 



(2) Osmotic Pressure. They have a low osmotic pressure. 



(3) Viscosity. They have a high viscosity. 



(4) Behaviour to Electrolytes. Silicic acid resembles suspensoid sols 

 by being precipitated as a gel with a small quantity of electrolyte. 



The organic sols require larger amounts of electrolytes to precipitate 

 them from solution, thus : 



(a) Sodium chloride is added to soap solution in small quantities 

 at a time and occasionally shaken. The soap is precipitated after a 

 large amount has been added. 



(&) On adding ammonium sulphate to starch solution precipitation 

 of the starch occurs after a considerable quantity has been added if the 

 solution be occasionally shaken so as to dissolve the salt. 



(c) If some egg-white solution be saturated with (i) sodium chloride, 

 (2) magnesium sulphate, by grinding it in a mortar with the salt, a 

 small quantity of globulin is precipitated. 



The same result is obtained by half-saturating the egg-white solu- 

 tion with ammonium sulphate, i.e. by adding an equal volume of 

 saturated ammonium sulphate solution. On saturating the filtrate with 

 finely powdered ammonium sulphate crystals, the egg albumin is pre- 

 cipitated. This method is employed for separating globulins, which 

 are less soluble, from albumins, which are more soluble and are only 

 precipitated from solution by completely saturating with ammonium 

 sulphate (see under proteins). 



1 Cellulose nitrates. 



