Even though polarography could be considered just another variation of technique within vol-tammetry,
it differs from other voltammetric methods both because of its unique place in the history of electrochemistry
and in respect to its unique working electrode, the dropping mercury electrode (DME).
The DME consists of a glass capillary through which mercury flows under gravity to form a succession
of mercury drops. Each new drop provides a clean surface at which the redox process takes place, giving
rise to a current increase with increasing area as the drop grows, and then falling when the drop falls.
Figure 37.3 shows a polarogram for a 1 M solution of HCl that is 5 mM in Cd2+. The effect of drop
growth and dislodging can be clearly seen. The potential when the current attains half the value of the
plateau current is called the half-wave potential and is specific to the analyte’s matrix. The plateau current
is proportional to the concentration of analyte. For example, Fig. 37.4 shows a differential pulse
polarogram for the acetyl derivative of chlordiazepoxide. In this case the peak height is proportional to
the analyte concentration.