Metrohm 757 VA Computrace - Manual (Page 25)

3.3 Multi-mode electrode (MME)
757 VA Computrace – Hardware
21
3.3 Multi-mode electrode (MME)
The multi-mode electrode combines the most important polarographic and volt-
ammetric mercury electrodes in a single construction:
• HMDE Hanging mercury drop electrode
Mercury is forced through a glass capillary until a drop
forms at the capillary tip and the entire voltage sweep
performed on this single stationary drop; in general
with preceding enrichment (stripping voltammetry).
• DME Dropping mercury electrode
The classical electrode, the mercury drops fall from the
glass capillary at a controlled rate.
• SMDE Static mercury drop electrode
The latest electrode, it combines the features of the
DME and the HMDE: during the measurement, the
drop surface is constant and stationary (as with the
HMDE); however, for the complete voltage sweep sev-
eral drops are needed (renewal as with the DME).
3.3.1 Construction and operating characteristics of the MME
The construction of the 6.1246.020 Multi-mode electrode is shown in Fig. 8. The
mercury in the reservoir 82 flows through the glass capillary 88 forming a drop at its
end. The mercury flow is controlled by the sealing needle 76, which can be raised
or lowered pneumatically. The different types of electrodes (HMDE, DME, SMDE)
are implemented by timed opening or closing of the mercury flow using this sealing
needle.
The operating characteristics of the MME are illustrated by Figs. 7 and 8. After valve
V
1
(inert gas supply) is opened, the mercury in the reservoir 82 is pressurized. In
the standby mode, a back pressure is built up in the interior of the slotted screw 75
which causes the built-in spring to press the sealing needle 76 onto the capillary
opening of the glass capillary 88 thus preventing the outflow of mercury. Switching
the valve V
3
allows the inert gas to escape thus releasing the back pressure. The
inert gas pressure in the mercury reservoir 82 presses the sealing needle 76 fixed
to the PTFE membrane of the slotted screw 75 upwards and the mercury can now
flow out. The tapping mechanism of the DME and SMDE is triggered by brief open-
ing and closing of valve V
4
.
The mercury drops formed at the end of the capillary are very small and stable and
thus afford a very good signal/noise ratio. The mercury hermetically sealed in the
reservoir comes into contact only with inert gas and other inert materials and suf-
fices for around 200'000 drops.