THE REVERSAL OF THE SIGN OF THE CHARGE OF 



COLLODION MEMBRANES BY TRIVALENT 



CATIONS. 



By JACQUES LOEB. 

 {From the Laboratories of The Rockefeller Institute for Medical Research) 



(Received for publication, May 19, 1920.) 



The reversal of the sign of charge of collodion membranes (treated 

 previously with a protein) by hydrogen ions has been discussed in a 

 preceding paper^ and in ihis paper the influence of trivalent cations 

 on the reversal will be considered. We shall omit the discussion of 

 the action of tetravalent cations {e.g. ThCU) since their solutions 

 have so high a concentration of hydrogen ions that this alone suffices 

 to bring about a reversal in the sign of charge. 



We will prove first that collodion membranes which have previously 

 been treated with a protein give the reversal in the sign of charge in 

 the presence of trivalent cations, while collodion membranes not so 

 treated do not show the reversal. When a solution of CaCl2 of not 

 too high a concentration {e.g. below m/8) is separated from pure 

 water by a collodion membrane, which is negatively electrified, the 

 solution shows no attraction for water, while it attracts water power- 

 fully when the membrane is charged positively. The Ca ion acts as 

 if it repelled positively charged water and as if it attracted negatively, 

 charged water. To find out whether or not trivalent cations reverse 

 the sign of charge of the membrane we have to add a low concen- 

 tration of a "neutral" salt with trivalent cation to the solution of 

 CaCl2. Weak solutions of CeCls and LaCls satisfy this condition. 

 The solutions of CaCi2 are for this purpose made up in m/ 1,024 

 solutions of CeCls or LaCla instead of in distilled water; and the 

 distilled water surrounding the collodion bag is also replaced by 



1 Loeb, J., /. Gen. Physiol, 1919-20, ii, 577. 



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