268 HYDROGEN ION CONCENTRATION 



alkaline solutions that it is impossible to obtain the concentrations 

 of these ions necessary for the reversal of the charge. We shall later 

 discuss in greater detail the profound influence of the trivalent ions 

 upon the sign of the charge, under other experimental conditions. 



It follows from the investigations of other authors that those dia- 

 phragms which do not become positive even in an extremely acid 

 reaction fall into two groups: 1. those which are positively charged 

 by trivalent cations (A1+++, La+++), and 2. those the sign of whose 

 charge cannot even be reversed by such trivalent cations. From the 

 table given on pages 262-3, taken from Gyemant's study, it is seen thai 

 while agar and collodion are not positively charged by A1+++, the 

 charge of kaohn is reversed bj^ this cation. In regard to collodion 

 this is a confirmation of Jacques Loeb's findings.^^ The reversal 

 of the charge on kaohn appears to be easily explicable. Its surface 

 adsorbs A1+++ by exchange for an H-atom of its silicic acid and 

 forms, to a certain extent, a basic sihcate, i.e., an H-atom of the siHcic 

 acid (of the kaoHn) is replaced by an A1+++ ion, which then still has 

 two free valences left, or, because of its dissociated state, this amounts 

 to two free positive charges. In the case of agar or collodion this is 

 not possible, for these substances do not possess a replaceable H-atom, 

 and their negative charge can not be due to the splitting off of H-ions, 

 but rather to the adsorption of H-ions. (This is possibly a case of 

 "apparent adsorption" cf. page 234). Heesch,^^ working in Hober's 

 laboratory, found similar^ that agar membranes could not become 

 positively charged, even by La+++, while he could demonstrate a 

 reversal of the sign of the charge on parchment by La+++ or by other 

 strongly adsorbable organic cations (histone, clupein, Rhodamin S). 



The otherwise irreversible sign of charge of such substances as col- 

 lodion can still be reversed by A1+++ when these diaphragms are 

 covered by an adsorption la^^er of an amphoteric colloid. Thus J. 

 Loeb^i showed that collodion impregnated with a solution of gelatin 

 or albumin can have the sign of its charge reversed by H+ or A1+++, 

 as would be the case for these two colloids taken alone. 



Investigations on electrophoresis show it to be, as was to be ex- 

 pected, an exact reversal of electroendosmosis. The same material 

 which, when used as a diaphragm, propels water towards the cathode, 

 when it is suspended in water, migrates towards the anode. The 

 phenomenon of cataphoresis leads so directly over into the theory^of 



" Heesch, Pfliigers Arch. f. d. ges. Physiol. 190, 210 (1920). 



