REDOX SYSTEMS; ELECTRON TRANSFER PROCESSES 



183 



that too much variation could cause a normally proceeding reaction actually 

 to go backwards ! 



A special application of the Nernst Equation is discussed under concen- 

 tration cells. 



+ 1.22 



+ 0.80 



-0.42 



pH = 



pH=7 



Figure 7-6. £ m7 's (center vertical line), and Their Relation to the Corresponding 



P's. (See text and Table 7-4.) 



Balky Redox Reactions 



There are three tricks provided by nature to promote electron exchange in 

 oxidation-reduction reactions. The first is catalysis : providing a surface or a 

 site on which the exchange can rapidly take place. For example, electrons 

 exchange immeasurably slowly between H 2 and H + in solution, but if a sur- 

 face such as finely divided platinum metal is added, electron exchange is 

 rapid, and the potential readily manifested. 



The second trick is the use of an indicator redox system. If one wishes to 

 know the redox potential of a solution in which the electron transfer is slow 

 or sluggish, one can add a very small amount of an entirely foreign redox 

 system, which exchanges electrons rapidly with the system of interest, and 

 which is either itself highly colored or exchanges rapidly at a metal elec- 

 trode. In the first case the depth of color of the resulting solution can be 

 related to the redox potential; and in the second case the potential can be 

 read directly against a reference electrode. Methylene blue, a colored redox 

 dye, is one of a class of dyes commonly used for this purpose, while the addi- 

 tion of a small amount of potassium iodide often will permit direct measure- 

 ment of the redox potential of the solution against some suitable reference 



