Dropping Mercury Electrode (DME):

This small polarizable electrode is produced through passing a stream of mercury through a resistance glass capillary having an extremely fine bore of 0.05-0.08 mm internal diameter and 5-9 cm long.  A drops fall at a steady rate of 3-5 sec per drop and is adjusted under a head of 40-60 cm of mercury.  The current oscillates among a near-zero value as drop falls from the capillary to a maximum value as the surface area of a new drop increases.

The dropping mercury electrode has various benefits:

i) Its surface area is reproducible along with a provide capillary

ii) The constant renewal of the electrode surface eliminates passivity or poisoning effects.

iii) The huge hydrogen overpotential on mercury renders possible the deposition of substances difficult to reduce such as aluminium ion and manganese (II) ion (The current-potential curves of these ions are not accessible with platinum micro electrode).

iv) Mercury forms amalgams with several metals.

v)  The diffusion current assumes a steady value instantly and is reproducible.

vi) A surface area of the electrode could be calculated from the weight of the drops.

The dropping mercury electrode thus is meaningful over the range + 0.3 to - 2.0 V vs. SCE.  Above + 0.4 V mercury dissolves and gives an anodic wave because of oxidation to mercury (I) ion.  At potentials more negative than - 1.8 V vs SCE visible hydrogen evolution occurs in acid solutions and at - 2.0 V the supporting electrolytes of alkali salts start to discharge.

Figure: Showing current oscillation with DME.