746 VA Trace Analyzer with MME - Major Maintenance

Introduction

Major maintenance of the multi-mode electrode (MME) on the 746 VA Trace Analyzer is a comprehensive procedure that restores the electrode to optimal operating condition by completely disassembling, cleaning, and reassembling the electrode with new wear components. Before beginning, gather all required materials, including a collection tray, electrode holder, coin, gray retaining ring, waste container for mercury, soft cloths, new needle, new capillary, mercury, syringe with needle, and a Mercury Catcher. Having all tools and replacement parts prepared in advance helps ensure the procedure can be completed safely and efficiently without interruption.

During this procedure, the MME is emptied and disassembled, the electrode body is thoroughly cleaned, and the electrode is reassembled with a new capillary, new needle, and fresh mercury. The needle valve is then adjusted on the measuring stand, and the electrode is tested in DME, SMDE, and HMDE modes to verify proper droplet formation and reliable operation. Because mercury is handled throughout the maintenance process, all disassembly and filling steps should be performed over a collection tray, and the Mercury Catcher should be used to safely collect any spilled mercury droplets.

Video

Topics Covered

  • The following items are needed for the "major" maintenance of the MME: collection tray, electrode holder, a coin, grey retaining ring, waste container for mercury, soft cloths, new needle, new capillary, new mercury, syringe with needle, and the mercury catcher for collecting scattered mercury droplets.
  • Emptying and disassembling the electrode (0:06)
    • The following work must be carried out over the plastic tub!
    • To empty and disassemble the MME you will need: a collection tray, electrode holder, a coin, a grey retaining ring, a waste container for mercury and the mercury catcher for collecting any scattered mercury droplets.
    • Remove the electrode from the measuring stand. First, ensure that the electrode is switched off, then unscrew the nitrogen connections and electrode cables.
    • Remove the mercury from the screw using the "Mercury Catcher".
    • Empty the mercury into a clean container. Gently tap the container with the MME to remove any remaining mercury from cavities. If the mercury is still usable, it can be reused later.
    • Small amounts of spilled mercury can be collected using the Mercury Catcher (part number 6.2406.000).
    • Remove the capillary, along with the seal and plastic ring, from the electrode, as well as the plastic insert.
    • Dispose of the capillary.
    • Clean all other parts of any adhering mercury using the "Mercury Catcher".
    • Unscrew the needle valve screw until the black sealing ring is no longer visible, then pull it out using the grey retaining ring.
    • Discard the needle.
    • The capillary and needle cannot be cleaned and reused.
    • Clean all other parts of the electrode with a soft cloth.
  • Cleaning the electrode body (03:17)
    • Wipe the inside of the MME body with a soft cloth. To reach the entire reservoir, a used capillary tube can be used, for example.
    • Clean the contact pin at the rear of the electrode body particularly thoroughly.
  • Reassembling the MME (03:53)
    • Reassemble the multi-mode electrode using the cleaned parts, a new capillary, and a new needle.
    • The capillary is installed "at the bottom" of the electrode body. "At the bottom" is the side where the screw for securing the electrode cable is also located.
    • First, insert the plastic insert (cone facing down), then the seal with the plastic ring into the electrode.
    • Finally, lightly screw the union nut into the thread, but do not tighten it completely.
    • 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.
    • Cut open the plastic packaging of the new needle with scissors. Insert the needle into the hole in the membrane and push it in until the white mark is reached.
    • The needle is installed "at the top" of the electrode body. "At the top" is the side opposite the screw that secures the electrode cable.
    • Insert the screw with the new needle into the MME and turn it clockwise until the black sealing ring becomes visible. This prevents mercury from leaking out.
    • Do not overtighten the screw. The needle valve will be adjusted later on the measuring stand.
  • Refilling the mercury reservoir (05:49)
    • The following steps must be carried out over the plastic tray!
    • Slowly pour mercury into the electrode. It should be filled to no more than 2/3 capacity (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).
  • Adjusting the needle valve (07:34)
    • 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.
    • 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.
  • Verifying electrode function in DME, SMDE, and HMDE operating modes (10:43)
    • 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. 

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