162 



THEORY OF COLLOIDAL BEHAVIOR 



than the observed P.D. and the 

 cause for this difference is un- 

 known at present. It is, how- 

 ever, a constant difference and 

 has therefore no relation to the 

 nature of the salt used. When 

 we use as a standard for com- 

 parison of the relative depressing 

 effect of a salt the concentration 

 required to depress the observed 

 P.D. to about 10 millivolts, we 

 find that the following concen- 

 trations of the five salts are 

 required for this purpose. 



NaCl about M/64 

 CaCl 2 slightly above M/128 

 BaCl 2 slightly above M/128 

 CeCl 3 between M/256 and M/128 

 Na 2 SO 4 about M/256 



The main fact is that the de- 

 pressing effect of the four salts 

 NaCl, CaCl 2 , BaCl 2 , and CeCl 3 

 is determined by the chlorine ion 

 concentration, and that the val- 

 ency of the cation has no influ- 

 ence. This leaves no doubt that 

 the charge of the micellae is an 

 unequivocal function of the, 

 Donnan equilibrium. 



The depressing action of Na 2 - 

 SO 4 is about four times as great 

 as that of NaCl. 



If the precipitating effect of a 

 salt on the stability of colloidal 

 suspensions is due exclusively to 

 the depressing effect of the salt 

 on the P.D. between micellae and 

 surrounding liquid, only that ion 



