264 HYDROGEN ION CONCENTRATION 



dissociates off H-ions, which, in turn, go to form the external stratum 

 of the double layer, while the negative^ charged ions form the inner 

 layer. 



A gelatin ion which had combined with an H-ion (or, starting from 

 the electroneutral hydrated form, which had spht off an OH-ion) 

 can no longer be designated as "a gelatin membrane with a covering 

 layer of H-ions," just as an H-ion bound to an electroneutral mole- 

 cule (H+ + NH3 = NH4+) is no longer designated as an H-ion. 

 Thus it may appear somewhat forced, in adhering to Perrin's con- 

 ception, to state that the charge on a wall or membrane depends only 

 upon a covering layer of H- or OH-ions. But in reahty there is no 

 contradiction in this; it is merely a purely formal difference of ex- 

 pression. If we should decide to accept the NH4+-ion as an NH3 

 molecule overlaid by an H+-ion, then Perrin's interpretation 

 becomes also acceptable for gelatin or any other charged wall. But 

 this mode of expression is barely suitable. In order to state the 

 basic idea underlying Perrin's rule in a form adequate for our present 

 day theory, we may say that: A wall becomes electrically charged, 

 either by binding (adsorbing) H-ions while acquiring the charge; this 

 is equivalent to saying that it dissociates off OH-ions, which then 

 form an ionic layer at a finite distance from the wall; or by binding 

 (adsorbing) OH-ions; which is equivalent to stating that it dissociates 

 off H-ions which aggregate into a layer at a finite distance from the 

 wall. 



By combining this conception with the definitions given above in 

 section 67, this law assumes the following form: 



A solid wall is always positively charged with respect to an aqueous 

 solution, if the substance of the wall is an acidoid. It is always nega- 

 tively charged, if it is a basoid. A wall may be either positively or 

 negatively charged, depending upon the hydrion concentration of the 

 solution, if its substance is an atnpholytoid. Just as in the case of a 

 true ampholyte, zero charge on an ampholytoid is present only at its 

 isoelectric point, the charge varying directly with the hydrion concen- 

 tration. The isoelectric point of the wall, depending upon the chemical 

 nature of its substance, may lie at an acid, neutral or alkaline reaction, 

 and it is represented by the middle point of a more or less wide isoelectric 

 zone. 



In the case of the dissolved ampholyte, the width of this zone de- 

 pends upon the value of the product ka X kb. For the present no 

 corresponding value for an ampholytoid can be defined. 



