746 VA Trace Analyzer with MME Pro - Major Maintenance

Introduction

Major maintenance of the MME Pro is a comprehensive service procedure that restores electrode performance and helps ensure reliable, accurate voltammetric measurements on the 746 VA Trace Analyzer. This article guides you through completely disassembling the electrode, cleaning reusable components, replacing the capillary and needle, refilling the mercury reservoir, reassembling the electrode, adjusting the needle valve, and verifying proper electrode operation.

Before beginning, ensure you have all required replacement parts, tools, and accessories available, including a new capillary, new needle, fresh mercury, the MME key, grey mounting ring, collection tray, electrode stand, waste container for mercury, and Mercury Catcher. Follow the video in sequence to safely complete each maintenance task and confirm proper DME, SMDE, or HMDE functionality before returning the electrode to service.

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
    • 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
    • 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
    • Reassemble the multi-mode electrode pro using the cleaned parts, a new capillary tube, and a new needle.
    • Additionally, the following are needed for filling the mercury: a collection tray, an electrode stand, mercury, a syringe with a needle, the key for the MME, and the mercury catcher to collect any spilled mercury droplets.
    • 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.
  • Refilling the mercury reservoir
    • 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 MME key, 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
    • 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). 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 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
    • 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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