884 Professional VA with MME Pro - Minor Maintenance

Introduction

This guide walks through the complete minor maintenance procedure for the MME Pro electrode. The accompanying video demonstrates how to refill the mercury reservoir, replace the capillary and needle, adjust the needle valve, and verify proper electrode operation before returning the electrode to service.

Before beginning, ensure you have all required tools and replacement parts available, including a collection tray, electrode stand, MME key, gray mounting ring, new needle, new capillary, mercury, syringe with needle, and a mercury catcher for collecting any spilled mercury. Perform all maintenance over the collection tray and follow the videos in sequence to safely complete the procedure.

Video

Topics Covered

  • During the "minor" maintenance of the MME pro, the wear parts needle and capillary are replaced, and the mercury reservoir is refilled.
  • The following items are required: a collection tray, electrode stand, the MME key, a gray mounting ring, a new needle, a new capillary, new mercury, a syringe with needle, and the "Mercury Catcher" for collecting any spilled mercury droplets.
  • Refilling the mercury reservoir (0:07)
  • The following tasks must be carried out over the plastic tray!
    • Remove the electrode from the measuring stand. First, ensure that the electrode is switched off, then unscrew the nitrogen connections and remove the electrode cable.
    • Slowly pour mercury into the electrode. It should be filled to the "max. fill" mark (max. 6 mL) at most.
    • Using the key, tighten the screw cap until it stops. The opening of the mercury reservoir will then be securely closed.
    • Small amounts of spilled mercury can be collected using the mercury catcher (part number 6.2406.000).
  • Replacing the capillary (02:41)
  • The following work must be carried out over the plastic tray! In addition to the MME pro, the following are required to change the capillary: drip tray, electrode stand, new capillary, the MME key, and scissors.
    • 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 retaining screw and pull the capillary out.
    • Do not touch the nInsert the capillary into the MME pro and, using gentle twisting motions, push it down through the seal until it stops. Then, loosely tighten the retaining screw by hand. To secure the capillary, place the key on top and use one finger to turn the retaining screw clockwise until the key falls off. Do not re-engage the key.
    • 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.
  • Replacing the needle (04:23)
  • The following work must be carried out over the plastic tray! 
  • In addition to the MME pro, the following are required to change the needle: drip tray, electrode stand, grey mounting ring, new needle, the MME key, and scissors.
    • Turn the needle holder counterclockwise until it can no longer be turned out, and then carefully pull it out using the grey mounting 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 needle holder with the new needle into the MME pro and turn it clockwise until the black sealing ring is just visible. This prevents mercury from leaking out.
    • Do not turn the needle holder in too far. The needle valve will be adjusted later on the measuring stand.
  • Adjusting the needle valve (05:42)
    • 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 "Manual control" window, select the 884 Professional VA and switch to the "Electrodes" tab. Then click the "Open" button under "Valves / Main valve" to open the main valve. 
    • 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 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.
  • Verifying electrode function in DME, SMDE, and HMDE operating modes (09:42)
    • 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 operating mode on the "Electrodes" tab in the "Manual control" window and start drop formation by clicking "Start". 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 operating mode in the "Manual control" window on the "Electrodes" tab and start drop formation by clicking "Start". Drop formation is correct if each strike of the knock-off 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, select the HMDE operating mode in the "Manual control" window on the "Electrodes" tab and start droplet formation by clicking "Start". Droplet formation is correct when the old droplet is broken off and a new one is formed. The droplet should not increase in size over time. Switching the electrode on again will produce a new droplet.

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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