276 HYDROGEN ION CONCENTRATION 



of the anion used, at least not with collodion menbranes. Parchment 

 membranes showed a distinct decreasing effect in the series: 



NH4>Li>K>Na>Mg>Ba>Ca 



This effect still needs further study. 



Ampholj'toid diaphragms showed, as they did also in respect to the 

 endosmosis of water, a reversal of the effect when a certain well 

 defined H+-concentration was exceeded. This [H+] represents in 

 regard to changes in concentration a point of indifference. The ex- 

 periments were carried out by Bethe and Toropoff especially on 

 chromated gelatin, in which the disturbing swelling phenomena do 

 not occur, as they do with ordinary gelatin. This point of indiffer- 

 ence of concentration displacement was, as expected, different for 

 different salts, and for 0.01 M solutions at the following pH values: 



pH 



NaaSO^ 3.6 



Na2C204 4.0 



NaCl, KCl 4.3 



BaCl. 4.8 



C0(NH3)6C1 7.0 



The important question arises as to the relation of this point of 

 indifference to the isoelectric point. If we assume that the pH of the 

 solution is equal to the isoelectric point of the diaphragm, then the 

 wall of the diaphragm will not adsorb any ions, the potential of the 

 wall is zero, and the current is conducted equally well within the 

 pores as in the free solution. But when the water-combining power 

 and also the mobiUty of the anions are equal to those of the cations 

 in solution (or better, as in the case where the various cations are in 

 the concentrations ki, ko . . . with the mobilities Ui, U2 . . . , 

 and the anions are in the concentrations ai, 0,2 .. . with the 

 mobilities Vi, V2 . . .; and when Uiki + U2k2 = Vi ai + V2a2 . . .), 

 no water will be transported. Otherwise, even at the isoelectric 

 point, some, even if slight, displacement of water will occur. The 

 point of indifference for the motility of the water is not quite equal to the 

 isoelectric point, the latter corresponding more closely to the point of in- 

 difference for concentration change. In fact, Bethe and Toropoff 

 demonstrated that the indifference points for water mobihty and for 

 concentration change do not always coincide. A total coincidence of 

 these two points is possible only when the dissolved electrolyte com- 



