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THE ELECTRICAL CHARGE ON COLLOIDS. 



By John Arthur Wilson, Chief Chemist, A. F. Gallun <£.• Sons Co., 



Milwaukee. 



The origin of the electrical charge on colloids is stiU a matter of 

 uncertainty, although it is possible that the charges may not always 

 arise from the same cause. It has been common practice for writers 

 to shelve the question by assuming that a sufficiently satisfactory 

 answer is given by Coehn's empirical law that " a substance of higher 

 dielectric constant charges itself positively when it comes in contact 

 with a substance of smaller dielectric constant." Even if the state- 

 ment of this law were true, it would not constitute an explanation 

 since it teUs nothing regarding the manner of bringing about the 

 charging. 



Taylor has suggested the possibihty of the charge arising from 

 the colloid surface being more impermeable to certain ions than to 

 others. He found that a membrane of aluminum hydroxide is formed 

 by the interaction of aluminum salts and ammonia which is permeable 

 to hydrion, but impermeable to hydroxidion, even a large E.M.F. 

 failing to drive hydroxide ions across such a film. Thus, when 

 alumina is suspended in water, the hydrion dissolves in or diffuses 

 into it, leaving the hydroxidion at the surface, and the particles 

 become positively charged. This will also explain the alteration of 

 the charge on albumen by acids and alkalis, if it may be assumed 

 that hydrogen and hydroxide ions are equally soluble (or diffusible) 

 in albumen. In such a case the concentration of either ion in the 

 albumen Avould vary directly as its concentration in the solution 

 This explains why albumen possesses no charge m neutral solution 



