Dissolved Oxygen Analysis Procedure

Equipment and Techniques

Dissolved oxygen analyses are performed with an SIO/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. Thiosulfate is dispensed by a buret driver (either Titronic or Dosimat) fitted with a 1.0 mL buret. The ODF method uses a whole-bottle modified-Winkler titration following the technique of Carpenter [Carp65] with modifications by Culberson et al. [Culb91], but with higher concentrations of potassium iodate standard (0.012N) and thiosulfate solution (55 gm/l). Standard KIO3 solutions prepared ashore are run before, during and after every set of samples. Reagent/distilled water blanks are also determined during this time to account for presence of oxidizing or reducing agents.

Sampling for Shore Based Analysis

Samples should be collected for dissolved oxygen analysis 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.  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.

Data Processing

The samples are analyzed within 1-2 days of delivery to ODF.  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 umol/kg using the sample drawing temperature.

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 standard iodate solution.

Standards

Liquid potassium iodate standards are prepared and 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 70°C. The standard is supplied by Alfa Aesar (lot B05N35) 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.