670 REVERSAL BY TRIVALENT CATIONS 



film into a protein-acid salt the reversal in the sign of charge of the 

 membrane would be easily intelhgible since the protein-acid salt 

 dissociates into a positive protein cation forming the surface film 

 of the membrane, and an anion which goes into solution. 



A difiiculty arises, however, if we try to explain the reversal of the 

 sign of charge of the membrane by trivalent cations on the alkaline 

 side of the isoelectric point of the protein. In this case the protein 

 film consists of La or Ce proteinates which are practically insoluble. 

 Practically no dissociation into a negative protein anion and a trivalent 

 cation would be supposed to occur and it seems not possible to state 

 why an insoluble La proteinate should assume a positive charge 

 when in contact with water. As long as this fact is not explained it 

 remains doubtful whether the tentative explanation just given for the 

 acid reversal is correct or complete. It also remains to be explained 

 why the collodion membrane not treated with a protein always 

 assumes a positive charge regardless of the hydrogen ion concentration 

 or the concentration of trivalent cations. It might be argued that 

 the collodion membrane differs from proteins in not being an ampho- 

 teric electrolyte and in not being able to combine with trivalent 

 cations. Another possibility must, however, be considered; namely, 

 that the hydrogen ions and trivalent cations influence the surface 

 electrical potential of the solution and that the combined effects of 

 these ions on the surface electrical potential of the membrane and 

 of the liquid determine the phenomena described in our papers. 



SUMMARY. 



1. Trivalent cations cause a collodion membrane covered with a 

 protein film to be charged positively while they do not produce such 

 an effect on collodion membranes not possessing a protein film. The 

 same had been found for the reversal of the sign of charge of the 

 membrane by acid. 



2. This reversal in the sign of charge of the membrane by trivalent 

 cations occurs on the alkahne side of the isoelectric point of the protein 

 used; while the reversal by acid occurs on the acid side of the isoelectric 

 point. 



