270 HYDROGEN ION CONCENTRATION 



for the different substances. Thus charcoal adsorbs H- and OH-ion 

 approximately equally well, and if we write down the two ionic series 

 in such a way that equally adsorbable ions are placed one directly 

 under the other, we obtain the following schematic serial arrange- 

 ment (for charcoal) 



CI I SCN OH 

 Na Ca Al H 



For AI2O3, which adsorbs H-ions" more strongly than OH-ions'^ 

 the arrangement will probably be 



CI I SCN OH 

 Na Ca Al H 



For gelatin which adsorbs OH-ions^^ more strongly than H-ions:*° 



CI I SCN OH 

 Na Ca Al H 



For an ampholytoid adsorbent with even stronger acidic 

 properties: 



CI I SCN OH 

 Na Ca Al H 



For an adsorbent which remains negatively charged even in an 

 extremely acid reaction (collodion) : 



CI I SCN OH 

 Na Ca Al H 



i.e., the cations are displaced so far to the left in the series that their 

 adsorbability may well be regarded as being equal to zero. There 

 is no fundamental difference between a sohd wall, the disperse par- 

 ticle of a colloidal solution and a single molecule in true solution. 

 The process of adsorption for all these cases is a completely equiva- 

 lent one, after it had become established that an NHs-molecule, a 

 glycocoU molecule, a single casein molecule, a molecule superficially 

 located on a colloidal casein aggregate, a molecule on the surface of 

 charcoal, each of these merely binds an H-ion. 



" AI2O3 + 6H+ = 2A1 + + + + 3 H2O. 



38 AI2O3 + 6 OH- = 2 (AIO3) - + 3 H2O. 



"HRNH2 + H+ = (HRNH3)+. 



" HRNH2 + OH- = (RNH2)- + H2O. 



