Diffusion current:

The conditions are set such that the diffusion is the main process of mass transfer. This can be done by minimising convection and migration. The limiting current under these conditions is known as the diffusion current (I_d).

The factors affecting the diffusion current were examined by D. Ilkovic and he gave the following equation which is known as the Ilkovic equation.

I_d = 708 nm ^2/3 D ½ t^1/6 c                                                                              ... (1.44)

where I_d is the diffusion current, n is the number of electrons consumed in the reduction of Mⁿ⁺ , D is the diffusion coefficient, m is the rate of flow of mercury, t is the time and c is the concentration of Mⁿ⁺  in the solution and 708 is the combination of constants and conversion factors involved in changing in units taken into account.

Equation 1.43 gives a direct relation between I_d and hence can be applied for quantitative measurements, we could write from equation 1.44,

I_d ∝ c or   I_d = Kc                                                                           ... (1.45)

where K could be evaluated by noting the current with a standard solution of the substance of interest.

The curve shown in Fig.1.6 is called I-E or polarogram or polarographic wave. Two important features of a polarogram are (i) the half wave potential E_{1/ 2}, and (ii) the diffusion current.