Figure 6.9
Polarographic wave or polarogram.material. In each polarographic wave, the potential at which the diffusion current reaches half the
limiting value is known as the half-wave potential E1/2 and is characteristic of the particular electroactive
species involved. It is therefore useful for qualitative identification.
Diffusion Currents
When the potential applied to a polarographic cell exceeds the decomposition potential of an
electroactive species, its concentration at the surface of the mercury drop is immediately diminished. A
concentration gradient is thereby established and more of that species diffuses from the bulk solution to
the electrode surface (Fick's law of diffusion). The resulting current flow is proportional to the rate of
diffusion which in turn is determined by the concentration gradient, i.e.
where C and Co are the concentrations of the electroactive species in the bulk solution and at the surface
of the DME respectively. By progressively increasing the applied potential, reduction occurs more
rapidly, Co eventually becomes virtually zero, and the concentration gradient reaches a maximum. At
this point, the rate of diffusion, and therefore the current flowing in the cell, reaches a limiting value,
i.e.
Further increases in the applied potential do not increase the current and the cell is said to be completely
polarized or operating under conditions of high concentration overpotential (p. 230). The diffusion
current id is hence directly proportional to the bulk concentration of the electroactive species.