16 HYDROGEN ION CONCENTRATION 



electrolytes, including also the salts of weak acids and bases, such 

 as ammonium acetate and ammonium carbonate.^" In case of 

 the acids the common cause for this behavior is explicable on the 

 basis of the atomic model. The center of the minute H+-ion is 

 able to attain closer proximity than any of the larger ions to the 

 center of electrostatic force of the particles to which it is attracted. 

 The force of attraction increasing greatly with diminishing distance, 

 the H+-ion is held more firmly by the anions than any other cation. 

 This conception does not appear to be capable of ready application 

 to the bases. The OH~-ion is spatially more diffuse than the H+- 

 ion. It cannot be in any sense asserted, for instance, that the 

 OH~-ion is held more firmly by cations than the Cl~-ion. The 

 NH+-ion does not bind the OH~-ion any more firmly than it does 

 the Cl~-ion; i.e., the molecular union NH4OII occurs just as spar- 

 ingly as NH4CI does. Not a single weak base exists in which, 

 analogously to the weak acids, its basic nature is due to its slightly 

 dissociable OH-groups. The countless organic compounds con- 

 taining OH-groups are, without any exception, ^^ non basic. The 

 N-free organic compounds which exhibit a basic character usually 

 hold, by a double bond, an 0-atom capable of forming an oxonium 

 ion, in a way analogous to the formation of NH4+-ions from NH2 

 groups. The usual explanation of the basic nature of NH3 and of 

 every amine is as follows : 



The reaction NH3+ + H2O -^ NH4+ + OH" is assumed. The 

 existence of the molecular entity NII4OH is problematic, and there 

 is nothing compelling us to accept its existence; it is only accepted 

 in order to justify the origin of the NH4+ ion. With at least equal 

 justification its origin may be explained by the reaction; NH3 + 

 H+ -^ NH4+, or, stating it in words, NH3 binds an H+-ion and thus 

 becomes an NH4+ ion. Accordingly, a base may be defined as a 

 molecular species, electro-neutral in itself, which on uniting with or 

 binding an H'^-ion becomes a positive io7i. In complete analogy with 

 the above an acid is defined as a molecular species, electro-neutral in 

 itself, which by splitting off an H-ion becomes a negative ion. 



The strong alkalies do not apparently conform to this definition 

 of a base. But this view is only superficial. The dissociation of 



1" That portioa of these salts which is not hydrohjzed is strongly dissociated. 

 '1 Except for the quartenary ammonium bases which are strong electrolytes 

 and which behave in the same way as NaOH, as detailed below. 



