18 HYDROGEN ION CONCENTRATION 



an 0H~ ion rather than by the loss of an H+ ion, as for instance in the 

 formation of the bicarbonate ion: 



CO2 + OH- -* HCO3- 



As will be shown later, in the discussion of the adsorbability of 

 the ions, the H+ ion among the cations and the 0H~ ion among 

 the anions are also unique because of their extraordinarily great 

 adsorbability. Since the 0H~ ion occupies just as an exceptional 

 position among the anions as the H+ ion among the cations, it is 

 well in discussing the question of acid and base formation not to 

 press the H+ ion to the foreground at the expense of the 0H~ ion. 

 Briefly restated: The yielding of an H+ ion is completely equivalent 

 to the addition of an 0H~ ion and reversely. In many cases these 

 two processes are so nearly identical that it is impossible to decide 

 in favor of one or the other mode of reaction. 



The following definitions may now be stated: an acid is a mole- 

 cule, electrically neutral in itself ivhich yields by dissociation H+ 

 ions (or adds to itself 0H~ ions); a base is a molecule which yields 

 0H~ ions (or adds to itself H+ ions); an amphoteric electrolyte or an 

 ampholyte is a substance ivhich unites in itself both of these properties. 



In these definitions special emphasis is designedly laid upon the 

 words "a, molecule electrically neutral in itself." A negatively 

 charged particle may bind H+ ions and thereby neutralize its elec- 

 tric charge, e.g., CH3COO- uniting with H+ to form CH3COOH. 

 Such a molecule which unites with a H+ ion and is electrically 

 neutrahzed is an acid ion or an anion. A molecule, which on unit- 

 ing with a H+ ion acquires a positive charge is a base (NH3 + H+-^ 

 NH4+). A molecule, which binds a 0H~ ion with the resulting 

 neutralization of its charge is a basic ion or a cation (Na+ + 

 OH- = NaOH). A molecule which on binding OH" ions acquires 

 a negative charge is an acid (CO2 + 0H"~— ^HCOs" and SO3 + 

 0H-->HS04-). 



A molecule which already has a positive (or negative) charge 

 which on binding an H+ ion (or 0H-) doubles its charge is a divalent 

 cation (or anion), as illustrated by hydrazine for cations: NH2NH2 

 + H+-^NH2NH3+ and further NH2NH3+ + H+-^NH3NH3++, and 

 by carbonic acid for anions: CO2 + OH" — > HCOs", HCOa" + 0H~ 

 -^C03= + H2O (this last hydrated ion reaction can be stated as 

 HC03--^C03= + H+). As will be seen in the chapter on ionic 



