582 REVERSAL OF SIGN OF CHARGE 



lodion membrane. When such membranes were used for osmotic 

 experiments it could be shown that the membrane had a positive 

 charge as soon as the hydrogen ion concentration exceeded 10"^ n or 

 a value slightly below 10~^. The point of reversal was therefore near 

 the isoelectric point of gelatin but on the acid side of that point. 



The reader will remember that it had been shown in the preceding 

 papers that when we separate a solution of an electrolyte by a col- 

 lodion membrane from distilled water the water will be attracted by 

 that ion which has the opposite sign of charge and will be repelled by 

 the ion which has the same sign of charge as the watery phase of the 

 double layer, and that both the attractive and repulsive effects increase 

 generally with the valency of the ion.^ In the case of a salt like 

 CaCl2 the repulsive effect of Ca upon positively charged water pre- 

 vails over the attractive effect of CI upon such water and no diffusion 

 of such water into the CaCb solution will occur ; while when the water 

 is negatively charged the attractive effect of Ca prevails over the 

 repulsive effect of the CI ion upon the water and water will diffuse 

 rapidly into the solution. We therefore can use a solution of CaCl2 

 to find out at which hydrogen ion concentration the reversal of sign 

 in the charge of the watery phase occurs. When we separate a m/256 

 solution of CaCl2 from a solution of water by a collodion membrane, 

 water will commence to diffuse into the solution as soon as the water 

 in contact with the membrane assumes a negative charge; while 

 otherwise practically no such diffusion will occur. In these experi- 

 ments the collodion flasks described in the previous experiments were 

 used. They were closed with rubber stoppers perforated by a glass 

 tube with a bore of 2 mm. in diameter serving as a manometer. The 

 collodion bags were dipped into beakers filled with water. The m/256 

 solution of CaCl2 and the water in the beaker into which the bags were 

 dipped were always given the same hydrogen ion concentration; they 

 were rendered acid by the addition of HNO3 and alkaline by the addi- 

 tion of KOH. We can in this way ascertain at which hydrogen ion 

 concentration the water will commence to diffuse into the solution of 

 m/256 CaCl2 and this will give us that hydrogen ion concentration 

 where the reversal of the sign of charge on the membrane occurs. 

 Such a curve is given in Fig. 1. The abscissae are the hydrogen ion 

 concentrations (expressed in terms of pH), the ordinates the rise of 



