49 



and^why^the positive charge increases with increasing acidity and 

 the negative charge \^dth increasing alkahnity. However, Taylor 

 admits that these explanations are not necessarily correct, because 

 they fit the facts and even suggests that adsorption, rather than 

 differential diffusion, is responsible for the charge. 



The great majority of opinion regarding the origin of the charge 

 seems to be included in the following possibilities ; that the charge 

 results from the selective adsorption of ions at the surface of the 

 colloid; that it is due to the ionization of foreign substances incor- 

 porated in the surface of the particles ; or to the ionization of the 

 colloid itself. 



In his recent book, Burton gives an interesting discussion of the 

 subject. In hydiosols of the oxidizable metals, the particles are 

 positively charged, while in hydrosols of the non-oxidizable metals, 

 the particles are negatively charged. Hardy explained this on the 

 assumption that the charge is due to a reaction between the metal 

 and water at the moment of formation of the hydrosol. In the case 

 of the oxidizable metals, ionizable hydroxides are formed; in the 

 case of the non-oxidizable metals, ionizable hydrides. The particles, 

 however, are so comparatively large that the greater portion of the 

 metal compound has the properties of matter in mass and ionization 

 takes place only at the surface of the particles. Burton has confirmed 

 this view by experiments AAdth methj'l and ethyl alcohols, which 

 have easily replaceable OH groups. He was unsuccessful in repeated 

 attempts to prepare alcosols of platinum, silvei', and gold, but 

 succeeded in preparing alcosols of the oxidizable metals. On the 

 other hand, when platinum, gold, and silver wires were sparked under 

 ethyl malonate, which has a replaceable hydrogen, very stable sols 

 were obtained, in which the particles were negatively charged; but 

 sols of the oxidizable metals could not be obtained with ethyl malonate. 

 Burton's own view is that the charges on the particles are due, in the 

 case of the Bredig metal sols, to the ionization of a layer of hydroxide 

 or hydride on the surface of the particles. Duclaux beUeves that 

 the charge always arises from the dissociation of a portion of extraneous 

 substance retained by the particles from the reacting media and he 

 has shoTVTi that there is always a trace of FeClg in Fe(OH)' sols. 



Zsigmondy admits that in special cases the charge is due to the 

 ionization of the coUoid itself, for example, where an ionizable 

 substance has molecules so large as to give it the properties of a 

 coUoid. He objects to the idea of the formation of chemical compounds 

 in all cases on the ground that it would entail " the inclusion in the 

 category of chemical compounds of a large number of badly defined 

 bodies, and load chemistry Avith much useless ballast." He prefers 

 explaining such reactions on the basis of adsorption of ions. The 

 acceptance of Langmuir's explanation of adsorption as a chemical 

 phenomenon would reduce Zsigmondy 's objection to one of terminology. 



Among the more recent papers dealing with colloidal metallic 

 hydroxides may be mentioned that of Pauli and Matula, who confirm 

 Duclaux's beUef concerning colloidal Fe(0H)3. Kimura regards the 

 charge as due to simple ionization of tlae hydroxide, the extent of 

 which is determined by a balancing of the forces causing ionization 



X 11454 D 



