32 PHYSIOLOGICAL CHEMISTRY 



given off into the water, and positive charges will remain on 

 the colloid particles. In alkaline solution the protein forms 

 salts, such as the sodium salt. Sodium ions are given off, carry- 

 ing positive charges, and negative charges will remain on the 

 colloid particles. These facts are of great importance in many 

 of the precipitation reactions of the colloids. 



Methods of Precipitating Colloids. Some colloidal solutions 

 will precipitate, the colloid flocking out, merely on standing. 

 Some will precipitate if they are boiled. Some substances are 

 soluble in hot water, but their solutions will solidify on cooling. 

 Some colloids are thrown out of solution by the addition of an 

 electrolyte. The suspensoid colloids are precipitated by adding 

 a very small amount of an electrolyte such as a salt or an acid, 

 but the emulsoid colloids are precipitated much less readily, 

 that is, only by adding much more of the electrolyte. It has 

 been observed that the effective part of the precipitating salt or 

 substance is the ion bearing the opposite charge to that on the 

 colloid. If the precipitating part of the salt is the metal, then 

 in general colloids bearing negative charges will be precipitated. 

 In this connection it has been observed that trivalent metals are 

 better precipitation reagents than divalent metals, and divalent 

 metals in general are better precipitation reagents than mono- 

 valent metals. Thus to precipitate a given colloid from solu- 

 tion a ferric salt is better than a mercuric salt, and a mercuric 

 salt better than a sodium salt. That is, a smaller concentration 

 of ferric chloride than of mercuric chloride is required, etc. But 

 all ions of the same valence do not have equal precipitation 

 powers. They vary according to their solution tension. 



When a colloid is precipitated by an electrolyte the precipi- 

 tate contains some of the precipitating ion, so the precipitate is 

 believed to be a compound of the colloid and the precipitating 

 ion. The precipitation of colloids, however, is undoubtedly de- 

 pendent on other and more complicated factors than the mere 

 formation of salts or similar compounds of the colloids. For 

 further discussion of this subject the student is referred to 

 larger, or more specialized works. 



