78
8.2
The 0% Standard
The 0% calibration standard can be
water mixed with a strong reductant
water flushed with nitrogen gas (N
2
)
nitrogen gas (N
2
)
8.2.1
Water Mixed with a Strong Reductant
Fill an appropriate amount of water into a glass flask (e.g. Duran
flask) with a lid prepared with holes for inserting the oxygen sensor
and a temperature sensor. Add a strong reductant, like sodium
dithionite (Na
2
S
2
O
4
) or sodium sulfite (Na
2
SO
3
) at a concentration
of 30 g L
-1
, creating oxygen-free water by chemical reaction. It is
not recommended to use saline water (e.g. sea water) for this,
because the high salinity of the water might prevent a proper
dissolution of the reductant. Stir the solution until the salt is
completely dissolved and let the solution stand for about 15
minutes. Insert the oxygen sensor and optionally also the
temperature sensor into the flask, and ensure that the sensor tips
are completely immersed into the water. Otherwise, follow the
calibration procedures given in chapter 7.
Do not store the sensors in this solution and rinse them carefully
after the calibration with demineralized water. Especially the
retractable needle-type sensors (item no.
OXR50
,
OXR230
,
OXR430
) need to be rinsed very thoroughly, because salt
crystallization within the needle might damage them irreversibly.
