Metrohm 757 VA Computrace User Manual

Page 33

3.3 Multi-mode electrode (MME)

757 VA Computrace – Hardware

10 Pressurize the MME

• 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 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.

11 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 drop so that it is easier to see whether

mercury continues to flow.)

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

12 Checking the MME for leaks

• Switch on the dropping mercury electrode by selecting

DME

in the

COM-

PUTRACE CONTROL

window and clicking on

. The mercury

drops freely out of the capillary.

• Select

HMDE

and click on

. A single mercury drop is formed.

Knock this off by gently tapping the MME 17 with your finger and check

that the mercury flow has really stopped. Repeat this operation several

times.

• If mercury continues to flow, turn slotted screw 75 still further in a clock-

wise direction and repeat check.

• If it is not possible to stop the mercury flow, both the glass capillary 88

and the sealing needle 76 have to be replaced (see section 3.3.9).