OXIDATION-REDUCTION POTENTIALS 



involving the taking up of electrons. An academic definition of this sort is isolated 

 in a mental vacuum until it is accepted and absorbed as an integral part of the 

 ordinary point-of-view. By itself such a definition, although possibly of strict 

 accuracy, may appear paradoxical until it is harmonised with every-day laboratory 

 experience. It is therefore a useful exercise to apply the electronic concept of 

 oxidation-reduction reactions to processes not usually regarded electronically. For 

 example, since conversion of metallic silver to silver ion involves loss of an electron 

 it is an oxidation : — 



oxidation 

 Ag ~> Ag® 



In the light of ordinary experience this does not appear an obvious example of 

 oxidation. The silver ion in a silver nitrate solution is not obviously in a higher 

 state of oxidation than metallic silver ; but if the reverse reaction is considered^ — for 

 example, the conversion of an alkaline silver nitrate solution to a metallic mirror of 

 silver by warming with a sugar solution — the process is quite obviously a reduction : — 



reduction 



Ag© ■ > Ag 



Further consideration of similar processes serves only to confirm the logical basis of 

 the electronic concept of oxidation-reduction processes. One further point should 

 perhaps be emphasised here. It is already clear that oxidation is the reverse of 

 reduction (and reduction is the reverse of oxidation), but another fact is that every 

 oxidation is accompanied by a reduction and vice versa. In the case mentioned 

 above of the ferrous-ferric chloride oxidation-reduction system, the ferrous ion gives 

 up an electron and is thereby oxidised to the ferric ion : — 



oxidation 



-pe®® > Fe®©© + e 



but at the same time a chlorine atom takes up that electron and is thereby reduced 

 to a chlorine ion : — 



reduction 

 CI + e > CI© 



Oxidation cannot proceed unless there is a corresponding reductant to take up the 

 electrons liberated, and, conversely, a substance cannot be reduced unless there is a 

 corresponding oxidation to liberate the necessary electrons. 



As we have seen, oxidising agents are substances capable of taking up electrons 

 and reducing agents are those able to part with electrons. The readiness with which 

 substances take up, or part with, electrons determines the intensity level of their 

 oxidising or reducing functions. In order to measure the functions quantitatively 

 it is necessary to find a method of measuring electronic escaping tendency, or " fuga- 

 city " as Clark (1923, 1) has described it. Since oxidation and reduction reactions 

 are, by definition, electronic migrations involving exchanges of electric charges, it 

 becomes clear that the quantitative study of oxidation-reduction processes will be 

 effected by measurements of electric potential differences. 



ELECTRODE POTENTIAL RELATIONSHIPS 



It is found, in fact, that if an " unattackable electrode " (such as platinum metal) 

 be immersed in a reversible oxidation-reduction system, a potential difference is set 



