90 ELECTRIFICATION OF WATER AND OSMOTIC PRESSURE 



into m/128 NaCl, and more rapidly into m/128 NaCl than into m/192 

 Na2S04. 



From the theoretical connection between the relative rate of diffu- 

 sion of water from pure water to the solution through a semipermeable 

 membrane discussed before, it follows that the osmotic pressure of 

 a solution should be modified by the nature of the ions it contains 

 in the same sense as the initial rate of diffusion of water is modified. 

 This idea can be put to a test by the choice of electrolytes for which 

 the collodion membrane is strictly semipermeable; e.g., gelatin salts. 

 Gelatin solutions attain in bags of collodion an osmotic pressure 

 which is permanent, provided the hydrogen ion concentration of 

 the solution does not undergo any change during the experiment. 



//. Analogies between Solutions of Gelatin and Aluminium Salts. 



Gelatin is an amphoteric electrolyte which when the hydrogen 

 ion concentration of its solution exceeds the critical value 2 X 10~^ n 

 forms only salts of the type of gelatin chloride, gelatin sulfate, etc., 

 while when its hydrogen ion concentration falls below this value it 

 can form only salts of the form of metal gelatinates; e.g., Na gelatinate, 

 Ca gelatinate, and so on. At the critical hydrogen ion concentration 

 2 X 10~^ N — the isoelectric point for gelatin — it can exist only in 

 the form of pure, i.e. non-ionogenic, gelatin.'* In this condition 

 gelatin is practically insoluble, practically non-ionized, and is practi- 

 cally incapable of producing any osmotic pressure. Both types of 

 gelatin salts, metal gelatinates as well as gelatin chloride, etc., are 

 very soluble, are strongly ionized, and are capable of producing 

 osmotic pressure. The writer's experiments, which are not yet all 

 published, have shown that for each given hydrogen ion concentration 

 there exists a definite equilibrium between non-ionogenic gelatin, 

 gelatin salt, and free acid. If we have 1 per cent solutions of iso- 

 electric gelatin the ionized or salt portion of the gelatin is practically 

 zero. When we add increasing quantities of an acid, e.g. HCl, an 

 increasing portion of the gelatin is transformed into gelatin chloride, 

 while the portion of non-ionogenic gelatin is correspondingly dimin- 



* Loeb, J., J. Gen. Physiol., 1918-19, i, 39, 237, 363, 483, 559. 



