ELECTROKINETICS 



373 



of strongly adsorbed aluminum cations on the surface of negative 

 silver particles. The direction (therefore the sign of charge), 

 the rate of migration (therefore the potential), and the stability 

 of colloidal silver particles are determined by the concentration 

 of the aluminum salt. 



* Calculated. 



The adsorption of ions of high valence is the probable cause of 

 reversal of sign in the case of metal particles. This is illustrated 

 by the fact that negative colloidal gold may be made positive 

 by A1+++, Fe+++, La+++, and Th++++ but not (readily) by 

 bivalent metals. 



Reversal of the charge on proteins is due to other properties 

 than ionic adsorption. Proteins have the capacity, when 

 associated with acids or alkalies, to assume a negative charge 

 when on the alkaline side of neutrality and a positive one when 

 on the acid side. This amphoteric property of proteins, as illus- 

 trated by the sign of the electric charge, is due to their capacity 

 to form salts with both acids and bases. In the presence of 

 acids, proteins form salts in which the protein ion is positive; 

 and in the presence of bases, they form salts in which the protein 

 ion is negative. Thus, if A stands for albumin, then A + HCl = 

 A+-C1- (albumin chloride), and A + NaOH = Na+'A" (sodium 

 albuminate). Albumin, therefore, becomes a positively charged 

 ion in an acid solution and a negatively charged ion in basic 

 solution. If, now, a protein particle is in an acid solution, it 

 should, as an ion with a positive charge (A+-C1~), travel to the 

 negative pole of an electric field; this it does. In an alkaline 

 solution, it becomes negatively charged (Na+-A-) and travels 

 to the positive pole. It is, therefore, necessary only to change 

 the acidic or basic condition of a protein solution in order to 



