10 THEORY OF COLLOIDAL BEHAVIOR 



ometrically with acids and alkalies, forming highly dissociable 

 metal proteinates or protein-acid salts, we may define the iso- 

 electric point of a protein as that hydrogen ion concentration 

 in which the protein exists practically in a non-ionogenic (or non- 

 ionized) condition being able to form practically neither metal 

 proteinate nor protein-acid salt. We shall see that this theo- 

 retical result leads to a simple practical method of preparing 

 proteins entirely or practically free from ionogenic impurities. 

 The fact that solutions and suspensions of proteins are least 

 stable at the isoelectric point is then connected with the purely 

 chemical fact that proteins are amphoteric electrolytes which 

 exist at their isoelectric point in the form of practically non- 

 ionizable protein molecules. 



3. THE ADSORPTION THEORY AND THE PRECIPITATION OF 



PROTEINS 



The interest of most investigators of colloidal phenomena was 

 centered on the precipitation of colloids, especially in those cases 

 where the precipitation required low concentrations of electro- 

 lytes. The explanation accepted by the majority of authors is 

 based on the assumption of an adsorption of ions by the colloid. 



Hardy explained his discovery that proteins are most easily 

 flocculated from their solutions at the isoelectric point by the 

 fact that at that point the electrical charges of the protein particles 

 are a minimum, a conclusion derived from his observation that at 

 the isoelectric point proteins do not migrate in an electrical 

 field. He concluded from this that the stability of colloidal 

 solutions is due to the potential difference between each colloidal 

 particle and the surrounding liquid. In this state the charged 

 particles must repel each other with the result that they become 

 evenly distributed through the solvent. When the charge is 

 annihilated or sufficiently diminished "the adhesion or 'idio- 

 attraction' as Graham called it, of the colloid particles for each 

 other makes them cohere where they come together." 1 He 

 originally assumed the positive charge of the particles in the 

 acid solution to be due to a preferential adsorption of H ions 

 and the negative charge in the presence of alkali to the adsorption 



1 WOOD, T. B. and HARDY, W. B., Proc. Roy. Soc., vol. 81, p. 41, 1909. 



