92 ELECTRIFICATION OF WATER AND OSMOTIC PRESSURE 



It is perhaps not without interest that the behavior of gelatin is 

 paralleled by the behavior of amphoteric electrolytes of a crystalloid 

 character; e.g., aluminium salts. Thus aluminium chloride exists 

 only when the hydrogen ion concentration exceeds a certain critical 

 value which seems to He near that of the point of neutrality. When 

 the solution becomes alkahne metal aluminates are formed. Na 

 aluminate as well as AICI3 is very soluble. At the isoelectric point 

 neither salt can exist and the insoluble A1(0H)3 is formed." The 

 insoluble A1(0H)3 has no osmotic pressure (or does not attract water) 

 while solutions of both AICI3 as well as NaA102 attract water power- 

 fully. In the presence of AICI3 water molecules are apparently 

 negatively electrified and in the presence of NaA102 water shows 

 positive electrification. The turning point for the sense of migration 

 of water molecules seems to he near or at the isoelectric point of 

 aluminium; namely, pH about 7.0. It would be very important if 

 we could measure the permanent osmotic pressure of aluminium salts 

 in collodion bags, but this is impossible since aluminium salts (with 

 the exception of the insoluble A1(0H)3) diffuse through collodion 

 membranes. It is, however, possible to determine the influence of 

 different aluminium salts upon the rate of diffusion of water through 

 a collodion membrane and it is found that this influence obeys the 

 two rules. 



We have mentioned this analogy between an amphoteric crystalloid, 

 AICI3 and NaA102 on the one hand, and an amphoteric colloid, 

 gelatin chloride and sodium gelatinate, on the other to show that 

 the fact of diffusibihty or non-diffusibility through a collodion mem- 

 brane does not force us to assume that the gelatin salts form no true 

 solutions. 



///. Osmotic Pressure of Different Metal Gelatinates . 



Only the metal gelatinates with monovalent and bivalent cations 

 need to be considered since the gelatin salts with trivalent cation 

 seem to be insoluble. Different metal gelatinates were prepared 



' The sparingly soluble A1(0H)3 may be called a colloid since it does not diffuse 

 through parchment paper. Sodium aluminate and AICI3 are crystalloids since 

 they diffuse rapidly through such a membrane. 



