62
with an air stone connected to an air pump (available as
commercial equipment for fish aquaria). Alternatively, if no air
pump is available, fill water into the flask leaving >50% air in the
headspace, close it with a lid and shake the flask strongly for about
1 min. Open the lid shortly for ventilating the headspace with fresh
air. Close it again and shake the flask for 1 more minute. Insert the
oxygen sensor and the temperature sensor into the flask and
ensure that the tips of the sensors are immersed in the water and
free of air bubbles. Follow the calibration procedures described in
chapter 5.2.2.
Please consider that streaming air through water may cause
cooling of the water. Ensure a correct temperature
determination!
12.2
The 0% Standard
The 0% calibration standard can be
water mixed with a strong reductant
nitrogen gas (N
2
)
12.2.1
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. seawater) 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, then stop the stirring and let the solution
stand for about 15 minutes. Ensure that there is no headspace and
no air bubbles in the closed flask. Then insert the oxygen and
temperature sensor into the flask and ensure that the sensor tips
