JACQUES LOEB 91 



ished. The relative proportion of gelatin salt and non-ionogenic 

 gelatin depends therefore upon the hydrogen ion concentration of 

 the solution and increases in a characteristic way with this concen- 

 tration. This hydrogen ion concentration of the solution is, of course, 

 not identical with the concentration of the acid added to the solution, 

 since part of this acid is in combination with the gelatin forming the 

 gelatin salt. Since practically only that fraction of the 1 per cent 

 gelatin solution which is transformed into a gelatin salt produces an 

 osmotic pressure it is obvious that the osmotic pressure of a 1 per 

 cent solution of gelatin must change in a definite way with the hy- 

 drogen ion concentration of the solution. If we use different acids 

 we find that different quantities of acid are required to bring a 1 per 

 cent gelatin solution to the same pH. 



When the hydrogen ion concentration is below the critical value 

 2 X 10~^ (or to use Sorensen's logarithmic symbol, when pH is > 4.7) 

 a part of the non-ionogenic gelatin is transformed into metal gelatinate 

 and this part is the greater the more the hydrogen ion concentration 

 falls below 2 X 10~^. There is again a definite equilibrium between 

 hydrogen ion concentration, gelatin salt, and non-ionogenic gelatin. 



Hence if we wish to compare the osmotic behavior of different 

 gelatin salts we must see that the solutions have not only the same 

 concentration of gelatin but also the same hydrogen ion concentration.^ 



The influence of solutions of gelatin on the electrification of water 

 is the same as that of any other electrolyte and follows the two rules 

 mentioned above. In solutions of metal gelatinates with monovalent 

 or bivalent cations, water migrates through a collodion membrane 

 as if the molecules of water were positively charged; in solutions of 

 gelatin acid salts water migrates as if its particles were negatively 

 charged. The turning point for the sense of migration seems to 

 be near (or identical with) the isoelectric point of gelatin; i.e., 

 pH = 4.7. 



5 This latter point has been overlooked by the majority of colloid chemists 

 who refuse to admit the chemical nature of the equiUbrium between colloid and 

 crystalloid, and who consequently ignore the role of the hydrogen ion concentra- 

 tion of the solution. Instead they compare the effect of the addition of equal 

 quantities of different acids, overlooking the fact that they are thus comparing 

 solutions in which the proportion of non-ionogenic gelatin and gelatin salt is 

 different in the case of each acid. 



