42 AMPHOTERIC COLLOIDS. I 



true for gelatin previously treated with NaOH. When such gelatin 

 is subsequently treated with a salt with univalent cation its vis- 

 cosity, osmotic pressure, swelling, etc., are increased, while when it 

 is treated with a salt with bivalent cation this increase does not 

 occur and its previous osmotic pressure, etc., are diminished. In 

 all these cases the gelatin migrates as an anion, and the compounds 

 formed are metal gelatinates where the metal is that of the alkali or 

 salt used. 



On the other hand, if gelatin has first been treated with an acid, 

 e.g. HCl, gelatin chloride is formed which shows an increase in osmotic 

 pressure, swelling, viscosity, and alcohol number. That such a gela- 

 tin (which is freed from the original acid by washing) reacts only with 

 the anion of neutral salts is proved by the fact that a solution of a 

 salt of the type Ca(N03)2 (univalent anion, bivalent cation) influences 

 gelatin chloride quantitatively and qualitatively like a salt of the type 

 NaNOa (univalent anion, univalent cation) of half the molecular con- 

 centration; e.g., m/128 Ca(N03)2 acts like m/64 NaNOa.^ This is 

 only possible if the anions of the neutral salt react with the gelatin 

 chloride, while the cations have no effect, since otherwise the depress- 

 ing effect of Ca mentioned before should be noticeable, which is not: the 

 case. If gelatin chloride freed from the excess of acid by washing is 

 treated with a sufficiently high concentration of a neutral salt with 

 univalent anion the osmotic pressure, swelUng, viscosity, etc., of the 

 gelatin increase after the salt is washed away. If, however, the anion 

 of the salt used is bivalent, e.g. sulfate, oxalate, succinate, it causes 

 less increase or an actual decrease of the osmotic pressure, viscosity, 

 alcohol number, and swelling of the gelatin. 



Hence by making use of the depressing effects of bivalent ions and 

 the opposite effect of the Univalent ions we can determine at which pH 

 the gelatin begins to combine with the cation and ceases to combine 

 with the anion of a neutral salt. Our experiments show that this 

 critical point lies for gelatin at a hydrogen ion concentration 2.10~* 

 (pH = 4.7), i.e. the isoelectric point of gelatin, and not at the point 

 of neutrality. 



* All these effects were measured after the excess of the electrolyte was washed 

 away (see footnote 1 ) . 



