Diffusion and Residual Currents:

Diffusion Current

In the presence of excess supporting electrolyte the electrical force on the reducible/oxidizable  ions is nullified and therefore, the limiting current is solely a diffusion current.  Ilkovic (1934) examined the various factors which govern the diffusion current and deduced a theoretical equation as

 i_d = 607 n cD^1/2m^2/3 t^{1 /6}     

where i_d    =   average diffusion current in microamperes during the life of the drop

n          =          a number of electrons consumed within the reduction or oxidation of one mole of the electroactive species

D         =          It denote diffusion coefficient of the reducible or oxidizable substance expressed as cm² /second

c          =          denotes concentration of electroactive species in millimoles per dm³

m         =          indicate the rate of flow of mercury from the dropping mercury electrode expressed in mg/ second

t           =          drop time of mercury in seconds

The constant 607 is a combination of natural constants, including the Faraday.  This is a significant equation because it accounts for the linear dependence of i_d upon c keeping all other factors constant.  It is meaningful in quantitative polarographic analysis.

Within practice, a limiting current is not exactly the diffusion current.  The supporting electrolyte which is 50 to 100 times that of electroactive substance contains impurities which may contribute a small current in addition to non-faradaic condenser current. This is called as 'Residual Current'.