757 VA Computrace with MME Pro - Capillary Change

Introduction

Replacing the capillary on the 757 VA Computrace with an MME Pro is a routine maintenance procedure that helps ensure reliable mercury drop formation and accurate voltammetric measurements. This article walks through removing and installing a new capillary, adjusting the needle valve, and verifying proper electrode operation. Following these steps carefully will help protect the electrode from damage and confirm that the DME, SMDE, or HMDE functions correctly before returning the instrument to service.

Before beginning, ensure you have the required replacement capillary, tools, and accessories available. Perform each step in the order shown, including the needle valve adjustment and electrode function check, to properly prepare the MME Pro for operation with the 757 VA Computrace.

Video

Topics Covered

  • 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.
  • Replacing 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 retaining screw and pull the capillary out.
    • Do not touch the needle with your bare fingers during this process.
    • 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.
  • Adjusting the Needle Valve (01:46)
    • 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.
    • 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 (05:25)
    • 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. 

For more detailed training on any of these topics, explore our Metrohm Custom Training options. 

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