CHAPTER XL 123 



plurivalent or organic ions. But since colloids are readily precipi- 

 tated by such ions, owing to neutralisation of their charges, it may 

 happen that a dye such as Congo red is neutralised and precipitated 

 before it has attached itself to the adsorbing surface by virtue of its 

 charge. 



Since the amount adsorbed is dependent on the magnitude of the 

 electric charge, we have an explanation of the fact that alcohol 

 diminishes the effect of electrolytes. The charge on a surface is 

 proportional to the dielectric constant of the liquid phase in which it 

 is immersed, and the dielectric constant of alcohol is less than that 

 of water. 



It is a well-known fact that colloids of the suspensoid class, such 

 as gold and coagulated egg white, are much more sensitive to the 

 action of electrolytes than are those of the emulsoid class, in which 

 the colloidal particles themselves contain water and differ from the 

 medium in which they are suspended merely by the smaller quantity 

 of water which they contain. Such are gum arabic, starch, colloidal 

 silica, and proteins in general, gelatin, raw egg white, and so on. 

 Faraday showed that colloidal gold could be protected from 

 precipitation by salt if a trace of gelatin was added. This is 

 explained by the adsorption of a coating of gelatin over the gold 

 particles, which are thereby converted into the emulsoid variety so 

 far as their surfaces are concerned. We find similar phenomena in 

 the staining of paper by Congo red. It is protected from the dye 

 even in the presence of salts. But the conditions are made more 

 complex by the possibility of using in the experiment either an 

 electro -positive or electro-negative protein. A trace of acid or 

 alkali respectively has -this effect on proteins, by the production of 

 dissociated salts. Now the former are more powerfully adsorbed 

 by the negative paper than the latter are, while at the same time 

 they reduce, instead of increasing, the electric charge. Actually 

 the latter effect preponderates, so that the presence of electro- 

 positive protein increases the depth of staining. 



Turning now to the basic dyes, we find that the paper is the more 

 deeply stained the lower the concentration of salt present. Accord- 

 ing to Freundlich, it is the colour base that is chiefly adsorbed 

 in this case. As was pointed out above, these dyes are hydro- 

 lytically dissociated, so that free base is present. This free base, 

 being insoluble, is in the colloidal state, and, like colloidal bases in 

 general, has a positive charge, due to electrolytic dissociation of the 

 surface of the particles. See HARDY in Van Bemmelen Gedenkboek, 

 p. 188. Thus, not only are the coloured ions, in this case the 



