Metrohm 797 VA Computrace - Manual (Page 28)

2 Installation
797 VA Computrace / Hardware-Manual 8.797.8001EN
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2.3 Multi-Mode Electrode (MME)
The Multi-Mode Electrode combines the most important polarographic and voltam-
metric 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 several drops
are needed (renewal as with the DME).
2.3.1 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 81 flows through the glass capillary 87 forming a drop at its
end. The mercury flow is controlled by the sealing needle 75 which can be raised or
lowered pneumatically. The different types of electrodes (HMDE, DME, SMDE) are im-
plemented by timed opening or closing of the mercury flow using this sealing needle.
The operating characteristics of the MME are illustrated by Fig. 7 and Fig. 8. After
valve V1 (inert gas supply) is opened, the mercury in the reservoir 81 is pressurized. In
the standby mode, a back pressure is built up in the interior of the slotted screw 74
which causes the built-in spring to press the sealing needle 75 onto the capillary
opening of the glass capillary 87 thus preventing the outflow of mercury. Switching
the valve V3 allows the inert gas to escape thus releasing the back pressure. The inert
gas pressure in the mercury reservoir 81 presses the sealing needle 74 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 opening and clos-
ing of valve V4.
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 re-
servoir comes into contact only with inert gas and other inert materials and suffices
for around 200'000 drops.