18 OXIDATION-REDUCTION POTENTIALS 



on two factors, (a) the proportions present of the oxidised and reduced forms of the 

 substance studied and (6) a constant for the system studied, i.e., Eq. The value 

 of Eq is given by the potential of the system when half oxidised and half reduced ; 

 that is, when the concentration of the oxidant is equal to that of the reductant. 



Hydrogen-ion concentration has definite effects on electrode potentials, so that, 

 strictly speaking, potentials can only be compared directly with others at the same pH. 

 The effect of pH on the electrode potential is not constant but varies both with 

 different systems and at different ranges of pH in the same system. Considerable 

 use has been made of the term rH, taking into account both electrode potential and 

 pH, and assuming a simple, fixed relationship between them. This, however, is 

 fallacious, especially in the case of in completely studied biological systems, so that it is 

 inadvisable to use the term rH which assumes a simple and constant relationship. 

 Particularly in the case of incompletely studied biological systems the use of rH is 

 inadmissible. The relationship between the free energy changes and the electrode 

 potentials of chemical reactions is discussed and the standard electrode potentials of 

 some inorganic systems are tabulated. 



