HANGING MERCURY DROP ELECTRODE, HMDE WK2
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Construction
Features
Assembling the electrode
Initial filling the electrode with mercury
Refilling the electrode with mercury
Testing the electrode
References
For further details go to
General description and application
The
hanging mercury drop electrode HMDE WK 2 (named after Prof. Wiktor Kemula) is a
spherical stationary mercury mini electrode with surface area of the order of a
square millimeter [1]. A mercury drop is suspended at the end
of a glass capillary and its size is conveniently adjusted with a micrometer
screw. The electrode HMDE WK 2 can be used as
a working electrode in various electroanalytical techniques for studying
mechanisms and kinetics of electrochemical reactions as well as for
determination of analytes in an electrolyte solution.
Construction
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| Fig. 1 | Figs. 2, 3 | Fig. 4 |
The electrode
HMDE WK 2 features a very reliable mercury sealing system provided by two
bi-conical gaskets (Parts 5 in Figs. 1 and
2). The combination of this system with a
micrometer screw (Part 12 in Fig. 1), used
for mercury proportioning, results in exceptional reproducibility of a mercury
drop surface area. Construction and
design of the electrode HMDE WK 2 are greatly improved over the former electrode HMDE WK 1 [2]. That is, the electrode weight and
dimensions are smaller while the number of mercury drops (available from a
single electrode filling with mercury) is larger. This is because the plunger (Part 11 in
Fig. 1) is
longer and its diameter is larger.
Therefore, a working mercury volume is about six times larger; the
mercury reservoir being only slightly longer. Importantly, the HMDE WK 2 can be
mounted in two different configurations of terminal assemblies (Figs. 2-4). One of them involves a 10/18 or 14/20
PTFE conical sleeve (Part 2 in Figs. 1, 2, and 4A) attached
directly to the translucent intermediate joint (Part 16 in Figs. 1, 2 and 4) that
allows one to house the electrode HMDE WK 2 in a typical commercial electrochemical
cell. The other configuration
involves a duralumin nut (Part 1 in Figs. 3 and
4B). A thermometer type PTFE adjustable
adapter for a standard tapered glass joint 10/18 or 14/20 (Parts 2, 21 and 22 in
Figs. 3
and 4B) is
then used for housing the electrode in a cell. The main body of the electrode HMDE WK 2 is made
of electrochemically colored duralumin.
Both the mercury reservoir and the precision-bore capillary (Part 4 in Figs. 1-4) are made
of borosilicate glass.
Features
| Total length | 240 mm |
| Outer diameter | 14 mm |
| Weight | 52 g |
| Plunger diameter | 1.6 mm |
| Working length of plunger | 45 mm |
| Pitch of thread | 0.35 mm |
| Glass capillary inner diameter | 0.127 mm or 0.178 mm |
| Glass tubing and capillary length | 100 mm (or customized) |
| Volume of working mercury | 96.5 mm3 |
| Reproducibility of the electrode surface area a | 0.60 % |
a
Determined as relative standard deviation of a peak current of the linear
potential scan voltammetry curves for the Cd2+/Cd(Hg)0
electroreduction in 1 mM Cd(NO3)2, 0.5 M KCl for an
electrode area of ca. 2 mm2 (Fig.
7).
Assembling the electrode
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| Fig. 4 | Fig. 5 |
The
electrode delivered to a customer is assembled and filled with mercury. But for changing the capillary (4), the
electrode must be re-assembled. For
that purpose, first, the assembling configuration, depicted in Figs. 4A or
4B, has to be selected. Then, a
new capillary as well as a new lower bi-conical gasket (5) should be
mounted. For that purpose, initially, the plunger
(11) should be raised to its upper position with the scaled revolving sleeve (7)
and both threads of the intermediate joint (16) left loose (Fig. 5a).
Note
1. When changing the capillary, always use
a new lower bi-conical gasket (5).
Next,
move the plunger (11) to its lowest position (Fig. 5b). Subsequently, tighten firmly (Fig. 5c)
the upper (inner), then, lower (outer) thread of the intermediate joint (16).
Initial filling the electrode with mercury
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| Fig. 6 |
The
use of the intermediate joint (16) allows one for initial filling the electrode HMDE WK 2
with mercury by using an "overflow" procedure (Fig. 6). In this procedure, the stopper (18)
should be removed and suction, afforded with, e.g., water aspirator, applied to
the nozzle (18) while the capillary tip immersed in a mercury pool and the
electrode held vertically (Fig. 6a). The electrode can be filled if the
plunger is raised above the lower gasket (5), as viewed through the intermediate
joint, by turning counter clockwise the
scaled sleeve (6). When the
mercury level is raised by suction above the lower gasket (5), move the plunger
below the lower gasket (5) and, then, shut off the suction (Fig. 6b).
Note
2. In order to prevent air entering the
mercury reservoir and, hence, malfunctioning the electrode, first, move the
plunger (11) below the lower gasket (5), i.e., to reach a level of ca. 3.5 main
mark of the scaled sleeve (6) and, then, shut off the suction.
Finally,
replace the stopper (18).
Refilling the electrode with mercury
For
refilling the electrode with mercury (Fig. 6c),
immerse its capillary tip in a mercury pool and turn counter clockwise the
scaled revolving sleeve (7) while holding the electrode vertically.
Note
3. Suction is not needed for refilling the
electrode with mercury. Make sure
that the tip of the plunger (11) remains below the lower gasket (5), i.e., the
scaled revolving sleeve (7) is raised to reach not higher than ca. 3.5 main mark
of the scaled sleeve (6).
Testing the electrode
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| Fig. 7 |
It is recommended to test the electrode under, e.g., cyclic voltammetry conditions, before using it for research purposes, as illustrated in Figure 7. In this figure, 10 consecutive cyclic voltammograms are presented. Each voltammogram was recorded at a fresh mercury drop formed by turning clockwise the scaled revolving sleeve (7) by 180°.
Tap
gently at the cap (23) with your forefinger in order to detach a mercury
drop. After the drop falls, a
mercury column front inside the capillary (4) may raise above the end of the
capillary by a few mm. Therefore,
bring the mercury column front to the end of the capillary with the micrometer
screw before extruding the next drop for the best drop size
reproducibility.
Caution. Mercury is a poison. If you swallow it accidentally, then
contact your physician immediately.
References
1. G³ód,
B. K., Kemula, W., Chem. Anal.
(Warsaw),
1985, 30, 665.
2. Kemula, W., Zawadowska, J., Fresenius Z. Anal. Chem.,
1980, 300, 39.
