Dissolved Oxygen Analysis Procedure

ODF designed an automated titration system and LabView software for the determination of dissolved oxygen in seawater that can be used both onshore and at sea. Contact Susan Becker for details and pricing.

Sampling

Samples should be collected soon after the rosette is brought on board. Using a silicone drawing tube, rinse volume-calibrated nominal 125 mL iodine flasks 3 times with minimal agitation then overflow for at least 3 flask volumes.  Record sample drawing temperatures using an electronic resistance temperature detector (used to calculate umol/kg concentrations). Add reagents, first MnCl2 then NaI/NaOH. Add stopper and shake flasks thoroughly. Re-shake after ~20 minutes. Apply a distilled water seal to the rim of each bottle to prevent gas exchange with the atmosphere. Top off water seal as needed and store in the dark until delivery to ODF technicians.

Equipment and Techniques

Dissolved oxygen analyses are performed with an ODF-designed automated oxygen titrator using photometric end-point detection based on the absorption of 365nm wavelength ultra-violet light. The titration of the samples and the data logging are controlled by ODF software compiled in LabView. 

The ODF methods can be found here. The analytical methods used are also compatible with those described in the oxygen section of the GO-SHIP repeat hydrography manual (Langdon, 2010). 

Thiosulfate and potassium iodate are dispensed by a buret driver fitted with a 1.0 mL and 10.0 mL buret, respectively. Standard solutions are run at least every 24 hours. Daily reagent/distilled water blanks are used to account for presence of oxidizing or reducing agents.

Data Processing

Thiosulfate normalities are calculated from each standardization and corrected to 20°C. Thiosulfate volume and normality is then used to calculate resulting dissolved oxygen concentrations in mL/L. This can be converted to µmol/kg using the sample drawing temperature. On shore, the samples are analyzed within 1-2 days of delivery to ODF. 

Volumetric Calibration

Oxygen flask volumes are determined gravimetrically with degassed, deionized water. This is done once before using flasks for the first time and periodically thereafter when a suspect volume is detected. The volumetric flasks used in preparing standards are volume-calibrated by the same method, as is the 10 mL buret used to dispense the standard iodate solution.

Standards

Liquid potassium iodate standards are prepared on-shore, tested in 6 liter batches, and bottled in sterile glass bottles. The normality of the liquid standard is determined by calculation from weight of powder temperature of solution and flask volume at 20°C. The standard is supplied by Alfa Aesar and has a reported purity of 99.4 - 100.4%. All other reagents are "reagent grade" and are tested for levels of oxidizing and reducing impurities prior to use.

References

Dickson, A. "Determination of dissolved oxygen in sea water by Winkler titration," WHP Operations and Methods v.1.01 (1996).

Langdon, C. "Determination of Dissolved Oxygen in Seawater by Winkler Titration Using the Amperometric Technique," IOCCP Report No. 14, GO-SHIP Repeat Hydrography Manual: A Collection of Expert Reports and Guidelines, ICPO Publication Series No.134 v.1 (2010).