Proper calibration of dissolved oxygen sensors involves both a span and zero point calibration. While air with its 20.95% oxygen concentration works well for span calibration; the zero calibration can be more complex. Hamilton recommends two different standards for proper zero point calibration. The details below apply to both polarographic and optical oxygen sensors.
Zero Point Calibration Using Nitrogen Gas
Nitrogen gas* is perhaps the most common method to achieve a zero point calibration. High purity nitrogen (grade 5.0) with certification of 99.999% or better should be used. Pharmaceutical grade nitrogen (99.5%) should be avoided due to the potential for impurities. Nitrogen gas is commonly supplied in high pressure bottles thus will require a gas regulator and tubing to direct the flow. Stainless tubing or plastic tubing such as polyamide or polyurethane are recommended due to low permeability.
Sensor calibration is best done in the lab. Hamilton makes a Calibration Station (Ref 243575) which is ideal for this purpose however a conventional flowcell can also be used. Nitrogen gas flow should be controlled to approximately 0.5 L/minute to the inlet of the calibration station. The outlet of the calibration station should vent to atmosphere to avoid any back-pressure at the sensor tip. Flow gas past the sensor for at least 5 minutes prior to initiating any zero point calibration. This ensures that any residual oxygen is fully purged from the tubing.
*NOTE: other gases such as Argon or CO2 can also be used. Purity must be verified to ensure they do not contain trace oxygen.
The Hamilton Calibration Station (Ref 243575) provides a easy to use fixture for calibrating sensors with gas in the lab
Zero Point Calibration Using Liquid Standards
If gas calibration is not feasible then a simple liquid solution can also be used. Mix 1 gram sodium sulfite (Na2SO3) with 100 mL of distilled water. If available, add 1 milligram cobalt chloride (COCl2) to the solution. Sodium sulfite is a scavenger thus should remove any dissolved oxygen from the water within 15 minutes. Cobalt chloride is a catalyst for the reaction thus its addition makes the solution good to use within several minutes.
Calibration is simply performed by immersing the DO sensor tip into the liquid standard. check for the presence of any residual air bubbles prior to performing the zero point calibration. Once complete, the solution should be kept in an air-tight container and should be good for additional use within 7 days.