Potential differences and coagulation of collodion i)art¡cles, etc. 477 



high concentrations of salts with trivalent (and probably tetravalent) 

 cations. It is therefore to be expected that only the catión of the salt is 

 responsible for the precipitation, since the catión of a salt has a depres- 

 sing effect upon the negative charge of the particles. 'Jhis is corrobora- 

 ted by the fact that the precipitating efficiency of salts increases rapidly 

 with the valency of the catión. Thus for NaCl, CaCU, and LaCl. the 

 precipitating efficiency measured by the reciprocal valué of the minimal 

 concentration required for precipitation (Column 2, Table II) is as 

 I : 16 : 1024. 



The question has often been raised whether that ion of a salt which 

 has the same sign of charge as the colloidal partióle will not counteract 

 the precipitating action of the other ion. The molecular precipitating 

 concentrations for XaCl, Na^SO^, and Na^Fe(CN)g are Al/2, M/4, and 

 about M/16 respectively. In M;2 XaCl and M/4 Na^SO^ the concentra- 

 tion of cations is practically identical. If the anión had an inhibiting 

 effect on precipitation, the concentration of NaoSO^ required for precipi- 

 tation should be higher Ihan AÍ/4. The precipitating concentration of 

 Na^FeíCX),; is even lower than that to be expected if only the Xa ion 

 acted. 



It foUows from this that the lorce preventing coalescence of the colloi- 

 dal particles into larger aggregates which settle rapidly is the potential 

 difference between the particles and the aqueous solution, and that there 

 is a critical valué for this P. D. which is in the case of collodion particles 

 about 15 millivolts. As soon as the P. D. falls below this valué the 

 collodion particles will coalesce. The depressing and precipitating 

 action of a salt on the negatively charged collodion particles depends 

 practically exclusively on the catión of the salt and increases rapidly 

 with the increase in the valency of the catión. There exists no peptization 

 effect of plurivalent anions. 



