ELECTRODE POTENTIAL EQUATION 



(10) ^--=Eo+^lnpg^j 



For the sake of simplicity, let us consider a systeni at 30°C. and at constant pH, 

 and let two electrons be concerned in the oxidation process, the equation then 

 becomes : — 



(11) B, = Eo + 0-03 log |-^j 



in which log represents ordinary Briggsian logarithms* ; [Ox.] and [Red.] are the 

 concentrations of the oxidised and reduced forms respectively of the substance 

 studied. 



It is evident from this equation that the electrode potential, E^, is dependent 

 on Eq, a constant for the system, and the proportion of oxidised and reduced forms 

 of the substance studied. The greater the proportion of oxidised form the higher 

 will be the potential, and the more reduced the substance the lower, i.e., more 

 negative, will the potential be. The significance of the constant, Eq, is of interest. 

 It will be seen that when [Ox.] = [Red.], i.e., when the system is 50 per cent, oxidised, 

 Ej, = Eq. That is to say Eq is the electrode potential of the 50 per cent, oxidised 

 system. When the value of Eq of a system is known it is possible to calculate the 

 electrode potential at any degree of oxidation or reduction of the system ; and, 

 vice versa, the degree of oxidation can be calculated from the value of the electrode 

 potential. Furthermore Eq is a measure of the oxidation or reduction intensity 

 level of the system and enables oxidising and reducing agents to be graded in their 

 oxidising or reducing eifects. Thus a system of Eq + 0-1 volt will oxidise a system 

 of Eq say —0-1 volt, but, on the other hand, will itself be oxidised by a system of 

 Eq + 0'3 volt. Measurement of oxidation-reduction potentials therefore enables us 

 to study the oxidation-reduction conditions of a system and to grade different 

 systems in order of their oxidising or reducing tendencies. 



It must be emphasised, however, that E^ is a measure of intensity level and not 

 of capacity. In this E^ resembles temperature and pH, and just as temperature and 

 pH give no information as to heat capacity and buffering power respectively, so Ej, 

 is independent of " poising effect," the capacity term in oxidation-reduction systems. 

 This follows from the derivation of E^ which is dependent on the ratio of oxidised 

 and reduced forms of the substance studied and not on their absolute quantities. 

 Thus a 90 per cent, oxidised system will have the same electrode potential no matter 

 whether the total concentration is 0-01 per cent, or 10 per cent., but the poising 

 effect will be 1,000 times greater in the latter case. It is particularly important 

 to bear these facts in mind when dealing with biological systems, many of which 

 have well-defined electrode potentials but are not well poised. 



* Units employed (Clark, 1928). 



Eh and Eoare measured in volts. 

 R = 8315 joules. 

 T = Absolute temperature. 

 F = 96,500 couJombs. 

 In X = 2-303 log x. 

 At 30°C. : 2-303 RT = 0-0601 

 F 





