Metrohm 757 VA Computrace User Manual

Page 29

3.3 Multi-mode electrode (MME)

757 VA Computrace – Hardware

1 Install multi-mode electrode in 757 VA Computrace Stand

• With the measuring head arm 19 tilted back, slide the empty measuring

vessel 6 into the holder 3 of the 757 VA Computrace Stand and then

lower the measuring head arm 19.

• Carefully insert multi-mode electrode 17 in opening 55 of the measuring

head 20 (during insertion, the tip of the capillary 88 must not touch the

measuring head) and push in carefully as far as it will go. Avoid water

drops touch the tip of the capillary.

2 Connect multi-mode electrode

• Screw FEP tubing 30 for the inert gas supply into connection 72 of the

multi-mode electrode 17.

• Screw FEP tubing 38 for the inert gas supply into connection 73 of the

multi-mode electrode 17.

• Attach electrode cable 16 (WE) to screw connection 89 of the multi-mode

electrode 17: push cable lug under the screw and then tighten screw

firmly.

3 Fill capillary with mercury

• Switch on 757 VA Computrace Stand with mains switch 12 (the 757 VA

Computrace Stand must first be installed properly as described in section

3.2).

• Start the VA Computrace program and click on

MAIN WINDOW /

Utility / Computrace control

to open the

COMPUTRACE CONTROL

window.

Then switch on the inert gas supply to the 757 VA Computrace Stand by
clicking on

DME

. This pressurizes the multi-mode electrode 17 and the

mercury begins to flow slowly out of the capillary.

• Gently tap the MME with your finger (to remove any air bubbles) and

allow the mercury to flow out of the capillary into the empty measuring

vessel for approx. 2 min.

• Fill measuring vessel 6 with 10 mL ultrapure water and add 1 drop KCl

solution (in pure water, mercury drops from the capillary only with diffi-

culty).

• Allow mercury to flow out of the capillary for ca. 2 min while checking the

drop formation: The drop time should be ca. 3 s.

4 Adjusting the sealing needle 76

• Turn slotted screw 75 using a suitable coin slowly in a clockwise direction

until the mercury flow stops.

• Open slotted screw 75 slightly in an anticlockwise direction until the

mercury flow restarts.

• Gently tap the MME with your finger and turn the slotted screw 75 very

slowly clockwise until the mercury flow just stops. (The tapping action is

used to knock off the mercury drops so that it is easier to see whether

mercury continues to flow.)

• Finally, turn slotted screw 75 a quarter of a turn clockwise.