Figure 6.10
Dropping mercury electrode.that it is essential to control the cell temperature to ± 0.1 K if the highest precision is required.
In practice, a DME has certain advantages over other micro-electrodes:
(1) The surface is continually and reproducibly renewed so that its past history is unimportant.
(2) A reproducible average current is produced instantly on changing the applied potential.
(3) Mercury has a large activation overpotential for hydrogen formation which facilitates the reduction
of many species in acid solution.
Its use is restricted to the determination of reducible or easily oxidized species as at positive applied
potentials greater than 0.4 V with respect to the SCE, mercury dissolves to give an anodic polarographic
wave.
Current Maxima
Polarographic waves often show a peak followed by a sharp fall to the limiting current plateau, the
cause of which is related to streaming of the solution past the mercury drop. Known as a 'current
maximum', it can be eliminated by adding a surfactant such as gelatin or methyl-red to the sample
solution.