Introduction
The capillary is a critical component of the Multi-Mode Electrode (MME), directly affecting mercury drop formation and overall electrode performance. If the capillary becomes damaged, clogged, or worn, it should be replaced to maintain reliable operation and ensure consistent analytical results. After installing a new capillary, the needle valve must be readjusted and the electrode function verified before returning the system to service.
This guide walks through the complete process of replacing the capillary on the MME, including removing the existing capillary, installing a new one, adjusting the needle valve, and testing operation in DME, SMDE, and HMDE modes. Before beginning, gather the required materials, including a collection tray, electrode holder, new capillary, coin, and scissors. Perform the procedure over a collection tray, avoid touching the needle with bare fingers, and follow the recommended adjustment and functional verification steps to ensure the electrode is ready for use.
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 capillary: a drip tray, electrode holder, 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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Adjusting the Needle Valve (01:46)
- 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)!
- 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.
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Step 2 (coarse adjustment): Turn the needle holder clockwise until no more mercury flows out.
It has proven useful 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 rotate the needle holder counterclockwise until a new droplet forms.
Close the capillary again with a small clockwise turn (approx. 10°). -
Step 4: Rotate the needle holder 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 (04:55)
- 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.
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