THE ELECTRICAL CHARGES OF MICELLA 163 



should have an effect on the precipitation which has the opposite 

 charge to that of the micellae; and this precipitating effect should 

 increase with the valency of the active ion. The Hardy-Schulze 

 and Linder-Picton rule of precipitation is, therefore, only the 

 consequence of the Donnan equilibrium. 



It is necessary to correct in this place an error which occurs 

 frequently in the colloidal literature, namely, the statement that 

 in the precipitation of colloidal suspensions by neutral salts the 

 colloidal particles are brought to the isoelectric point by the 

 salt. What happens is that by the addition of neutral salts the 

 P.D. between suspended particles and liquid is diminished, and 

 if enough salt is added completely annihiliated. This is due to 

 the fact that as a consequence of the addition of the salt the value 



y in the term log ( 1 + -j , upon which the P.D. depends, increases. 



When, however, the gelatin granules are brought to the iso- 

 electric point of gelatin, i.e., to pH 4.7, through a change in the 

 hydrogen ion concentration, the P.D. between particles and 

 surrounding liquid becomes also zero, but for a different reason, 

 namely, because the gelatin is now no longer ionized at this point. 

 In this case the P.D. becomes zero because z in the term log 



4- -) becomes zero. 



If the theory of the Donnan equilibrium is applied to these 

 phenomena it becomes therefore obvious that the P.D. between 

 colloidal particles and surrounding liquid can become zero in two 



different ways : first, by making the value of z in the term 1 + - 



equal to zero, and this is only possible by bringing the hydrogen 

 ion concentration of the solution to that of the isoelectric point 

 of the protein (which in the case of gelatin is at pH 4.7); and 



second, by making y in the term 1 + - very large, i.e., by increas- 

 ing the concentration of the ions having the opposite sign of 

 charge to that of the colloidal particles, and this can be done at 

 any pH by adding a neutral salt. 



It is therefore entirely wrong to say that the salt causes the 

 precipitation of the suspended particles by bringing them to the 

 isoelectric point or that the isoelectric point of a protein is shifted 



