W. MANSFIELD CLARK 



189 



ELEMENTARY OXIDATION-REDUCTION THEORY 



It has been found that a few oxidation-reduction processes occur in such a manner 

 that states in their equihbria may be defined very accurately by certain electrical 

 measurements. For instance, let a solution of hydrochloric acid be divided as shown 

 in Figure i with a liquid connection made narrow to isolate approximately the parts. 

 In one side place a platinum electrode and let there be one atmosphere pressure of 

 hydrogen. In the other side place a platinum electrode and a mixture of ferric chloride 

 and ferrous chloride. These salts dissociate to furnish the ions Fe+++ and Fe++. If 

 the electrodes are placed in metallic connection, a current of electrons will flow in the 

 connection from the "hydrogen electrode" to the electrode in the iron solution, and 

 this current will be accompanied by the transformation of hydrogen to hydrogen ions 

 on the one side (an oxidation) and the transformation of ferric to ferrous ions on the 

 other side (a reduction). If the electromotive force of this cell is nicely balanced by an 

 external electromotive force, there is at- 

 tained a close approach to the operation of 

 the cell under conditions of maximum 

 work, and there can be applied the ther- 

 modynamic equation for the change in free 

 energy of the cell process. This free energy- 

 change can be factored into two parts: 

 The quantity of electricity determined by 

 the number of chemical equivalents and 

 the intensity factor or electrical potential. 

 The latter, as measured by a potentiometer 

 under the conditions specified above, is a 

 measure of the intensity with which hydro- 

 gen, at one atmosphere pressure, re- 

 strained by the given hydrion concentra- 

 tion, tends to transform the given iron solution in the direction of complete 

 reduction. 



The measurements are of the intensity factor of a free energy-change, and in the 

 present state of our knowledge we must say that it is a matter of good fortune that 

 the iron and several other oxidation-reduction systems are susceptible to study by 

 this method. It furnishes no secure evidence of mechanism, but it may be inferred 

 from facts which we shall not stop to review that the essential process is one of elec- 

 tron exchange. We may imagine that the ferric-ferrous system is exchanging elec- 

 trons not only within the system itself but also with the electrode. With a fixed ratio 

 of ferric and ferrous ions the electrode will receive a characteristic charge. In like 

 manner, many dye systems such as methylene blue-methylene white give stable and 

 characteristic electrode potentials. 



It is therefore a convenience to assume that relative electron-escaping tendencies 

 determine the ability of one system to reduce another. We shall see presently how, 

 in accordance with this orienting assumption, the hydrogen exchange is taken care of. 



For the sake of definiteness let the hydrogen half-cell be that in which the hydro- 

 gen is at one atmosphere and the hydrogen-ion concentration is one normal and let 



