ELECTRODE POTENTIAL RELATIONSHIPS 



up at the electrode and this potential can be measured by the usual potentiometric 

 methods. It will be shown in the next few pages that a relation can be derived from 

 simple physico-chemical reasoning which shows that the potential at the electrode 

 depends on the proportions present of the oxidised and reduced forms of the substance 

 studied. The more highly oxidised a substance is the higher will be the electrode 

 potential, and the more reduced, the more negative will the potential be. 



Since some readers may find the algebra in the next eight pages unduly wearisome, 

 it is suggested that, in the first instance, the chapter summary may be sufficient intro- 

 duction to the later chapters dealing with practical applications and biological problems. 



It must be emphasised that we are dealing with truly reversible systems wherein 

 the reaction can proceed in one way or the other according to the conditions. Such 

 a reversible system is that of ferrous-ferric ions which we have already considered. 



(1) Fe®© ^ Fe®®® -f e 



This has the form of an ordinary reaction equation and accordingly the ordinary 

 mass action equilibrium equation can be applied, namely : — 



[Fe®®®] [ej _ 

 ^^' [Fe®®] ~" 



where the square brackets indicate the concentration of the reactants when at 

 equilibrium, and " k " is a constant. The exceptional feature of this equation is 

 the significance of the term [eJ which, as it stands, is the concentration of free electrons 

 in the system. Without making any assumptions as to the validity of the existence 

 of such concentration it may be stated that [eJ represents the escaping tendency 

 or " fugacity " of electrons in the system, or some similar electronic function. Its 

 exact definition is unnecessary since it will be eliminated from the equations at an 

 early stage. 



From these equations it follows that oxidation-reduction conditions of the 

 system can be controlled by the movement of electrons ; for if [eJ is increased the 

 reaction will move in the direction of reduction and if [eJ is decreased, oxidation of 

 Fe®® to Fe®®® will occur to balance the equation. Further the oxidation-reduction 

 condition of the system can be observed quantitatively by measurement of the 

 potential at an " unattackable electrode " when electrons tend neither to enter nor 

 leave the system. An " unattackable electrode " is one which when immersed in 

 the system will not participate therein but act merely as an inert conductor of 

 electrons to or from the system when the circuit is suitably arranged. 



Let now such an unattackable electrode be immersed in the ferrous-ferric ion 

 system. The electrode can be looked upon as a reservoir of electrons of fixed 

 concentration [eJ. The significance of the term electronic concentration is dis- 

 cussed above. Since the concentration or escaping tendency of the electrons in the 

 electrode ([eJ) is different from that in the system ([eg]) a potential difference will be 

 set up at the electrode, and this potential difference (E) can be calculated in accordance 

 with thermodynamics. The electrical work done in the passage of a faraday of 

 electricity (F) against this potential difference (E) will be equal to the work done 



