IQO 



OXIDATION-REDUCTION POTENTIALS 



too 



1' 



'o Oxidation 



15 SO 



0.50 



0.45 



0.40 



0.35 



0.30 



Eh 



0^5 



O.Z0 



0.15 



0.10 



0.05 



25 50 75 



% Reduction 



Fig. 2. — Relation between percentage reduction and elec- 

 trode potential (Eh) for different oxidation-reduction systems. 



the potential of this half- 

 cell be considered zero. 

 This half-cell will be con- 

 sidered the standard. There 

 may now be substituted for 

 the ferrous-ferric system one 

 or another of several other 

 oxidation-reduction sys- 

 tems. If a half -cell contain- 

 ing each system in turn be 

 joined with the standard 

 hydrogen half-cell and the 

 potentials of the cell at 

 different stages in the re- 

 duction of the system be 

 measured, the results when 

 shown graphically will pro- 

 vide a picture like that of 

 Figure 2. An oxidant in- 

 volving two electrochemical 

 equivalents permol for com- 

 plete reduction will give a 

 curve of the slope of B or 

 C. All the dye systems to 

 be described furnish the 

 two-equivalent slope. A 

 system involving one equiv- 

 alent gives a shape like that 

 of curve A. Any such 

 curve is best characterized 

 by its center point — that of 

 50 per cent reduction or 

 oxidation of the system, the 

 potential of which furnishes 

 a convenient reference 

 point with which to describe 

 quantitatively the relative 

 oxidation or reduction in- 

 tensity of the system. Each 

 system has such a character- 

 istic point under a given set 

 of conditions to be described 

 later. If it be negative to 

 the characteristic potential 

 of another system, the first 

 is reducing with respect to 



