22 BACTERIOLOGICAL CHEMISTRY 



them concerned in the particular reaction) the equilibrium 

 will be expressed by : — 



[Oxidant] x [e]'^ _ , r -in _ [Reductant] x h 



[Reductant] ~ ''' ^^' ^^^ ~ [Oxidant] * ' ^ ' 



Obviously the direction in which the reaction will proceed 

 is influenced by the free electrons ; if their number is 

 increased the system will tend to produce more of the 

 reductant ; if they become fewer more oxidant will be 

 formed. Hence if we know the electronic state of the 

 system, we have a measure of its reducing or oxidising 

 power. The electronic state manifests itself in the 

 electrode potentials set up when non-reacting electrodes 

 are introduced into the system, and these potentials can 

 be measured by comparison with standard half- cells. 

 The electrode potentials depend on the transfer of electrons 

 from the solution, in which they are present in concentra- 

 tion [e] , to the electrode which can be regarded as having 

 a constant concentration of [em]. Now the work W 

 required to move an electron from concentration [e] to the 

 metal electrode at concentration [cm] is equal to that 

 required to transfer a charge of 1 f araday ( F) through the 

 potential difference E concerned. That is, 



W = EF = RT log ^ . . . . (5) 



where " log " indicates logarithms to the natural base, 

 R is the gas constant, and T the absolute temperature. 

 Rewriting the equation we get 



^^ = ^ log [e.J - ^ log [e] ... (6) 



But since [cmj is a constant this expression becomes 



liT 

 E ^K-^'-f log [.] .... (7) 



