Introduction
Preparing the Multi-Mode Electrode (MME) for operation is an essential step in ensuring reliable electrode performance and accurate voltammetric measurements on the 797 VA Computrace. This article guides you through installing the capillary, filling the electrode with mercury, adjusting the needle valve, and verifying proper electrode operation before use.
Before beginning, ensure you have all required parts, tools, and accessories available, including a new capillary, mercury, collection tray, electrode holder, syringe with needle, coin, and Mercury Catcher. Follow the videos in sequence to correctly prepare the MME and confirm proper DME, SMDE, or HMDE functionality before starting measurements.
Video
Topics Covered
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Inserting the Capillary (0:06)
- The following items are needed for inserting the capillary: Multi-mode electrode (MME), electrode holder, a new capillary and scissors.
- To prepare, unscrew the union nut, reinsert it lightly into the thread but do not tighten it.
- Do not touch the needle with your bare fingers during this process.
- Insert the capillary into the MME and push it down until it stops. Then tighten the union nut firmly until the capillary is securely fixed and centered.
The application areas of the standard capillary (6.1226.030) and the silanized capillary (6.1226.050) can be found in the corresponding data sheet for the capillaries.
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Adding Mercury (01:22)
- The following steps must be carried out over the plastic tray! In addition to the MME, the following are required for filling the mercury: collection tray, electrode holder, mercury, syringe with needle, a coin, and the mercury catcher to collect any spilled mercury droplets.
- Slowly pour mercury into the electrode. It should be filled to no more than 2/3 full (max. 6 mL).
- The mercury reservoir opening is securely closed once the black sealing ring is clearly visible. No mercury can then leak out.
- The screw must not be fully inserted into the opening.
- Small amounts of spilled mercury can be collected using the mercury catcher (part number 6.2406.000).
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Adjusting the Needle Valve (03:22)
- The needle valve is adjusted in the measuring station.
- Before inserting an electrode with a new capillary, be sure to empty the measuring vessel (any solution entering the vessel can render the capillary unusable)!
- When connecting the nitrogen supply, use the MME connection labeled "HMDE SMDE DME/CC".
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Step 1: Pressurize the electrode (required pressure 1.0 to 1.2 bar).
In the "Computrace control" window, select electrode type HMDE. Do not activate the electrode with "New drop". Once a large mercury droplet has formed, add approximately 20 mL of water and a few drops of KCl solution to the measuring vessel. -
Step 2 (coarse adjustment): Turn the needle valve screw clockwise until no more mercury flows out.
It is advisable to check after each half turn, by observing the formation of new drops, whether the capillary is already sealed by the needle. - Step 3 (fine adjustment): Slowly turn the screw counterclockwise until a new droplet forms. Close the capillary again with a small turn clockwise (approx. 10°).
- A small turn clockwise (approx. 10°) seals the capillary again.
- Step 4: Turn the screw another 90–100° clockwise. The electrode is now ready for use. Droplet formation can be controlled by the software.
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Verifying electrode function in DME, SMDE, and HMDE operating modes (06:30)
- DME – Dropping Mercury Electrode. The classic dropping mercury electrode.
- When the DME electrode type is switched on, the needle valve opens, mercury flows through the capillary and forms a droplet, the actual electrode. Once the droplet reaches a certain size, it falls off. A new droplet then forms. This process repeats itself throughout the entire measurement cycle. In normal operation, the drop-off mechanism ensures that the droplet falls off after a defined time.
- To check the DME function, select the DME electrode type in the "Computrace control" window and activate the electrode with "New drop". Drop formation is correct if the drop falls from the capillary at regular intervals.
- SMDE – Static Mercury Drop Electrode. A dripping mercury electrode, like the DME, but with a static surface, like the HMDE.
- With the SMDE electrode type, the needle valve opens for only a few milliseconds. During this time, mercury flows through the capillary, forming a droplet with a defined surface area. After a short time, this droplet is removed from the capillary by the knock-off mechanism. A new droplet then forms and is knocked off again after a short time. This process is repeated throughout the entire measurement cycle.
- To check the SMDE function, select the SMDE electrode type in the "Computrace control" window and activate the electrode with "New drop". Drop formation is correct if each stroke of the knock-off mechanism removes an equally sized drop from the capillary.
- HMDE – Hanging Mercury Drop Electrode. The hanging mercury drop electrode; a single hanging drop of mercury serves as the electrode.
- When the HMDE electrode type is switched on, any existing mercury droplet is first removed by the knock-off mechanism. The needle valve then opens for a few milliseconds. During this time, mercury flows through the capillary, forming a droplet with a defined surface area, which serves as the electrode for the entire measurement cycle. Only when the electrode is switched on again does a new droplet form.
- To check the function of the HMDE, select the HMDE electrode type in the "Computrace control" window and activate the electrode with "New drop". Drop formation is correct when the old drop is knocked off and a new drop is formed. The drop should not increase in size over time. Reactivating the electrode will produce a new drop.
Additional Support
If you encounter any issues or need additional help, please submit a support ticket through Metrohm Technical Support.
For more detailed training on any of these topics, explore our Metrohm Custom Training options.