FUNDAMENTAL OXIDATION-REDUCTION EQUATION 29 



the first place to the admirable accounts given by Clark (^^9,^50). 

 Here, it will be sufficient to give the derivation of the parts of the 

 theory most essential for an interpretation of the experimental data 

 recorded in other chapters. 



7.2.1. Fundamental Oxidation-Reduction Equation. In general an oxi- 

 dation-reduction system may be defined by one or other of the equations 

 (where « represents one electron): 



Ox + ne^Red''- (1) 



Oz"+ + ne^Red (2) 



or by any equation expressing conditions intermediate between these. The 

 system contains two components; one, the oxidant (O.v) being capable of 

 reversible reduction to the other, the reductant (Red). The oxidation- 

 reduction process is accompanied by changes in the ionic charge of the com- 

 ponents, the latter being indicated l)y the superscript n+ and n — . To avoid 

 undue complexity, equation 2 is selected as the basis for further treatment. 

 The equilibrium of this system is defined by the equation: 



(o-^^)(^r ^^ (3) 



(Red) 



where parentheses indicate activities. 



Consider also the system defined by equation 4: 



2 H+ + 2 e ^ Ha (4) 



Utilizing the hydrogen pressure, P (iii atmospheres), to express hydrogen 

 concentration, we may write: 



(jqU^ = K„ (5) 



Vp 



defining the equilibrium condition for this system, which is in fact a hydrogen 

 electrode. 



An unattackable electrode (bright platinum or gold) is inserted into the 

 solution containing the first system, the hydrogen electrode is assembled 

 with platinized platinum electrode and hydrogen gas in the usual manner, 

 and the two solutions are connected by means of a potassium chloride bridge 

 or other means of eliminating liquid junction potentials caused by unequal 

 rates of diffusion of ions. Since the "electron activity"* of the two sections 

 of the combined system differs, connection of the two metallic electrodes by 

 an external conducting path results in the production of an electric current 

 from the side of greater electron activity to that of less. If the external 



* The physical significance of the term "electron activity" need not be discussed 

 since it is used here simply as a mathematical convenience and is eliminated from the 

 6nal equations. 



