WILLIAM D. HARKINS 



171 



On account of their Brownian movement the particles would collide, and in some 

 cases unite, if they were not kept apart by this repulsion. However, some of the par- 

 ticles have very high velocities, so even with high potentials the solution may not be per- 

 manently stable. If the particles collide there may be an actual union. This is ac- 

 companied by a decrease in surface energy. The union of two such particles involves 

 the deorientation of the films between them, and this may require the expenditure of 

 an appreciable quantity of energy. The particles which collide may merely adhere 

 (agglutination), or they may merge and lose their identity completely. 



In 1906 Burton made some interesting experiments on the cataphoretic velocity 

 of colloidal gold particles in a gold sol to which different amounts of an aluminum salt 

 were added (see Table IV). Such experiments indicate: (a) that the positive alumi- 

 num ions are adsorbed at the interfaces between the gold and the water; (b) that this 

 adsorption may be great enough to change the original negative charge on the par- 

 ticles of gold into a positive charge, the number of ions adsorbed increasing rapidly 

 with the concentration of the aluminum soap; (c) that instability of the colloidal solu- 



TABLE IV 



Effect of Aluminum Ions (Al+ + +) upon the Charge of Particles of 

 Colloidal Gold 



tion increases as the charge on the particles (cataphoretic velocity) approaches zero 

 from either the positive or the negative side. The point at which the cataphoretic ve- 

 locity, and therefore the charge on the particle, becomes zero is called the "iso-electric 

 point." 



It is believed that a small concentration of salt is essential for the stability of a 

 colloidal sol (such as mastic). An excess of salt destroys the stability and causes floc- 

 culation. If the salt is of the type of AlCl, and the sol is negative, further addition of 

 the salt produces a positive stable sol, but still further additions of the salt bring about 

 another flocculation. These peculiar phenomena are designated by the technical term 

 ''irregular series." 



EFFECT OF THE VALENCE OF THE ADSORBED ION 



It is impossible to add negative salt ions to a solution without the addition of pos- 

 itive ions, and vice versa. However, it has been found that with negatively charged 

 suspensoids, such as the arsenic trisulphide or the gold sol, the flocculation produced 

 is a function of the valence of the positive ions of the salt, and with positively charged 

 suspensoids it is a function of the valence of the negative ions of the salt. 



Thus with an emulsion of a paraffin oil the value of phi (<^) is reduced from 69 

 millivolts, the value for water, to 55.5 millivolts by the addition of the specified 



