884 Professional VA with MME Pro - Major Maintenance

Introduction

This guide walks through the complete major maintenance procedure for the MME Pro electrode used with the 884 Professional VA. The accompanying video demonstrates how to safely disassemble the electrode, clean the electrode body, replace the needle and capillary, refill the mercury reservoir, reassemble the electrode, adjust the needle valve, and verify proper electrode operation before returning the electrode to service.

Before beginning, ensure you have all required tools, replacement parts, and safety equipment available. Perform all maintenance over the collection tray and follow the videos in sequence to safely complete the procedure. During major maintenance, the needle and capillary must be replaced with new components and should not be cleaned or reused.

Video

Topics Covered

  • The following items are needed for the "major" maintenance of the MME pro: collection tray, electrode stand, the key for the MME, grey mounting ring, waste container for mercury, soft cloths, new needle, new capillary, new mercury, syringe with needle, and the mercury catcher "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!
    • 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.
    • To empty and disassemble the MME pro, you will need: a collection tray, an electrode stand, the key for the MME, a grey mounting ring, a waste container for mercury, and the "Mercury Catcher" for collecting any scattered mercury droplets.
    • Remove the mercury from the screw using the "Mercury Catcher".
    • Empty the mercury into a clean container. Gently tap the container with the MME pro 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 and seal from the electrode. Dispose of the capillary. Clean all other parts of any adhering mercury using the "Mercury Catcher".
    • Turn the needle holder counterclockwise until it can no longer be turned out and then pull it out using the grey mounting 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 (03:19)
    • Wipe the inside of each MME electrode 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 Pro (04:12)
    • Reassemble the multi-mode electrode pro 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 connection for the electrode cable is also located.
    • First, insert the seal into the electrode, then loosely insert the retaining screw for the capillary into the thread, but do not tighten it.
    • Insert 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.
    • 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 electrode cable connection.
    • 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 overtighten the needle holder. The needle valve will be adjusted later on the measuring stand.
    • 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 the "max. fill" mark (max. 6 mL).
    • 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).
  • Refilling the mercury reservoir (06:12)
    • The following tasks must be carried out over the plastic tray!
    • In addition to the MME pro, the following are required for refilling mercury: collection tray, electrode stand, mercury, syringe with needle, the key for the MME, and the mercury catcher for collecting any spilled mercury droplets.
    • 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).
  • Adjusting the needle valve (07:31)
    • The needle valve is adjusted on the measuring stand.
    • 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 (11:30)
    • DME – Dropping Mercury Electrode. The classic dropping mercury electrode.
    • 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. 

Related to

Was this article helpful?