DME( Dropping Mercury Electrode)
The dropping mercury electrode (DME) is a working electrode made of mercury and used
in polarography. It is a polarisable micro-electrodes in which the drop of mercury trickles down
from a resistance-glass capillary (0.05-0.08 mm in diameter and 5-9cm long) . The dropping mercury
electrode has the following advantages
(a) Its surface is reproducible, smooth, and continuously renewed; the current potential curve is
reproducible and eliminates passivity or poisoning effects.
(b) The diffusion current assumes a steady value immediately after each change of applied potential,
and is reproducible.
(c) The large hydrogen over potential on mercury allows possible the deposition of substances
difficult to reduce, e.g. the alkali metal ions, aluminium ion and manganese(II) ion.
(d) The surface area can be calculated from the weight of the drops.
• The dropping mercury electrode is generally applied over the range +0.4 to about - 2.0 volts
with reference to the S.C.E. Above +0.4 volt mercury dissolves and gives an anodic wave; it
begins to oxidise to mercury(I) ion.
• At potentials more negative than about - 1.8 volts with reference to the S.C.E. , visible
hydrogen evolution occurs in acid solutions and the usual supporting electrolytes commence
to discharge.
• The range may be extended to about - 2.6 volts by using supporting electrolytes having higher reduction potentials than the alkali metals; tetra-alkyl ammonium hydroxides or their
salts are satisfactory for this purpose.
WORKING OF DME
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