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9.5.3 Clean the gold cathode
When the gold cathode is contaminated, use the polishing paper (DO504) to lightly wipe the gold cathode
(along the arc of the gold surface); clean the gold surface with a clean tissue or kimwipe afterwards; rinse
off the electrode with distilled water, then shake off excess water; Add some new inner solution into the
membrane cap; screw on the cap. Then perform the saturated oxygen calibation according to 9.2 before
measurement again.
9.5.4 Replace the membrane cap
When the
DO electrode’s response time becomes slower, obvious errors occur in measurement, or when
the sensitive membrane of DO electrode is wrinkled, cracked or damaged at any extent (the well-
functioning membrane surface should look perfectly smooth), it’s time to replace a new membrane cap
(DO503) according to the following steps.
a)
Screw off the membrane cap;
b)
Rinse off the electrode without membrane cap with distilled water and shake off excess water.
c)
Clean on the cathode surface (gold slice) with a clean tissue or kimwipe;
d)
Add new inner solution (DO502) into a new membrane cap (DO503) slowly and do not let any air
bubbles appear. If you found any air bubbles in the cap, carefully flick on the membrane cap to
eliminate them.
e)
Place the membrane cap on the table and put in the electrode vertically, slowly twist in, and then
screw on the cap tightly. The excess inner solution will be squeezed out. Please wipe it off with
tissue and rinse off the electrode in distilled water.
f)
Check if there are any air bubbles in electrolyte (except for the smaller air bubbles), If so, re-
assembly is needed.
g)
Do not touch the sensitive membrane with fingers directly because the sweat and grease will
affect the quality of the membrane.
9.6
Zero Oxygen Calibration
Usually there is no need to do zero oxygen calibration unless you have a high requirement for accuracy
in low oxygen level (<1.0 ppm). To do zero oxygen calibration, follow the steps below:
Prepare 100mL of zero-oxygen water: In a 100 mL beaker, add in 5.0 g anhydrous sodium sulfite
(Na
2
SO
3
); add distilled water to 100 mL, mix well to dissolve. Note that the zero-oxygen water is only
effective within 24 hours.
