Introduction
This video demonstrates how to replace both the needle and capillary on the multi-mode electrode (MME) used with the 746 VA Trace Analyzer. Before beginning, gather all required materials, including a collection tray, electrode holder, new capillary, new needle, gray retaining ring, coin, scissors, and a Mercury Catcher. Having all tools and replacement parts prepared in advance helps ensure the procedure can be completed safely and efficiently.
During this procedure, the existing capillary and needle are removed and replaced, after which the needle valve is readjusted on the measuring stand to restore proper electrode operation. Because mercury may be present during the replacement process, all work should be performed over a collection tray, and care should be taken to safely contain any spilled mercury. Once the adjustment is complete, the electrode should be tested in DME, SMDE, and HMDE modes to verify proper droplet formation and confirm the MME is ready for reliable operation.
Video
Topics Covered
- The following work must be carried out over the plastic tray!
- In addition to the MME, the following are needed to change the needle and capillary: drip tray, electrode holder, grey retaining ring, new needle, new capillary, a coin and scissors.
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Changing the capillary (0:06)
- To prepare, open the needle valve with 1 to 2 turns counterclockwise, ensuring that the black sealing ring remains visible.
- Check if there is still mercury in the capillary. Gently tap the capillary to move it back into the electrode body. Then hold the cap nut and pull the capillary out.
- 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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Changing the needle (01:47)
- In addition to the MME, the following are needed to change the needle: collection tray, electrode holder, grey retaining ring, new needle, a coin and scissors.
- Unscrew the needle valve screw until the black sealing ring is no longer visible, then carefully pull it out using the grey retaining ring.
- Replace the needle. To do this, cut open the plastic packaging of the new needle with scissors. Insert the needle into the hole in the diaphragm and push it in until the white mark is reached.
- Insert the screw with the new needle into the MME and turn it clockwise until the black sealing ring becomes visible. This prevents any mercury from leaking out.
- Do not overtighten the screw. The needle valve will be adjusted later on the measuring stand.
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Adjusting the needle valve (03:17)
- 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".
- Step 1: Pressurize the electrode (required pressure 1.0 to 1.2 bar). On the "STAND" page, activate the pressure ("Pressure") using the function key "F1". Once a large mercury droplet has formed, add approximately 20 mL of water and a few drops of KCl solution to the measuring vessel.
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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°). - 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, activate the DME electrode type on the "STAND" page using the function key "F4". 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 on the "STAND" page using the "F6" function key. Drop formation is correct if each stroke of the ejection mechanism removes an equally sized droplet 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, activate the HMDE electrode type on the "STAND" page using the function key "F5". Drop formation is correct when the old drop is knocked off and a new drop forms. 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.