ELECTRODE POTENTIALS 169 



When KCN is added to a solution of a silver salt, a complex AgCN 

 ion is formed and the Ag+-ions disappear from the solution except 

 for an extremely minute residue. This residue can also be estimated 

 by means of the concentration chain. 



50a. Oxidation-Reduction Potentials and the Quinhydrone Elec- 

 trode. 



A. Theory. In recent years, since the appearance of the last 

 German edition of this book oxidation-reduction potentials have 

 assumed considerable importance, a brief outline of the underlying 

 theory will be given here as an addendum only. 



By the term oxidation we understand either: 



1. The addition of oxygen (or of an OH group), or the loss of hydro- 



gen, or, 



2. The gain of a positive charge, or the loss of a negative charge. 

 These four processes are obviously quite different in nature, but 



their consequences may be identical, so that frequently it cannot be 

 discriminated by which one of these four possible ways the end 

 result of oxidation has been achieved. Thus, for example, in the 

 reaction between metallic sodium and water, 



Na + HoO -* NaOH + § H2 -^ Na+ + OH" + § H2 



the oxidation of the sodium may be interpreted either in terms of an 

 addition of oxygen (OH group) to the sodium, or, as an addition of a 

 positive charge to the metallic sodium at the expense of the H+ 

 ions of the dissociated HoO molecules. 



Reduction is exactly the reverse process to that of oxidation and 

 needs no further elucidation. 



There are some types of oxidation and reduction which in practice 

 can be made to occur with any reasonable velocity only by means 

 of an exchange of electric charges. 



If we have in solution simultaneously a substance and its oxidation 

 product, but no other substances which could act either as oxidizing 

 of reducing agents for the original pair of substances, nothing will 

 happen. For example, in a solution containing Fe++ and Fe+++ 

 salts the following reaction is conceivable: 



Fe++ + Fe+++^ Fe+++ + Fe++ 



This reaction, however, will have no discernable consequences, since 

 as much of Fe"*^ disappears as is formed, and the sa"me applies to the 



Fe+++. 



