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How can I prepare the calibration solutions cal0 and cal100 for oxygen sensors?
Preparation of calibration solution 0 (oxygen-free water)
- For preparing calibration solution 0 (oxygen-free water) add 1 g sodium sulfite (Na2SO3) and 50 µL cobalt nitrate (Co(NO3)2) standard solution (mass concen- tration (Co) = 1000 mg/L; in nitric acid 0.5 mol/L) to the vessel and label it cal 0.
- Dissolve the salt mixture in 100 mL water. The water becomes oxygen-free due to a chemical reaction of oxygen with Na2SO3. Additional oxygen, diffusing from air into the water, is removed by surplus of Na2SO3. Cobalt is used as catalyst to accelerate and complete the reaction of sulfite with oxygen.
- Close the vessel with a screw top and shake it for approximately one minute to dissolve Na2SO3 and to ensure that the water is oxygen-free.
Keep the vessel closed after calibration with a screw top to minimize oxygen contamination. The shelf life of cal 0 is about 96 hours provided that the vessel is closed with the screw top.
Prepartion of calibration solution 100 (air-saturated water)
- For preparing calibration solution 100 add 100 mL water to a suitable vessel and label it cal 100.
- To obtain air-saturated water, blow air into the water using an air-pump with a glass-frit (airstone, normally used in aquariums), creating a multitude of small air bubbles, while stirring the solution. The bubbling also ensures mixing in the glass so that oxygen gradients do not form in the water.