Dissolved Oxygen in Surface Water

1. Sample Dissolved Oxygen Measurement

1. At the water collection site, use a calibrated pipette to add 2 mL manganous sulfate to a clear 300-mL BOD bottle filled with the sample water. Be careful not to introduce oxygen into the sample by inserting the pipette tip under the sample surface and carefully dispensing manganous sulfate. This will avoid creating bubbles until the sample is “fixed” and prevents change to the dissolved oxygen concentration.
2. Using the same technique, add 2 mL alkaline iodide-azide reagent.
3. Immediately insert the stopper, tilting the bottle slightly and quickly pushing the stopper in place so no air bubbles are trapped in the bottle.
4. Carefully invert several times (without creating air bubbles) to mix. A floccule (floc) will form from a precipitated aggregation of material with a cloudy appearance (Figure 1).
5. Wait until the floc in the solution has settled. Again, invert the bottle several times and wait until the floc has settled. The sample is now fixed to prevent change in dissolved oxygen content, and can be transported back to the lab and stored for up to 8 h, if needed, in a cool and dark condition.
6. If storing, samples should be sealed using a small amount of deionized water squirted around the stopper, and the stopper should be wrapped in aluminum foil, secured with a rubber band.
7. Pipette 2 mL of concentrated sulfuric acid into the sample by holding the pipette tip just above the sample surface. Invert carefully several times to dissolve the floc (Figure 2).
8. In a glass flask, and using a calibrated pipette, titrate 200 mL of sample water with 0.025 N standardized sodium thiosulfate, swirling and mixing continuously until a pale straw color forms (Figure 3).
9. Add 2 mL of starch indicator solution with a dropper and swirl to mix. Once the starch Indicator is added, the solution will turn blue (Figure 4).
10. Continue the titration, adding one drop at a time until one drop dissipates the blue, causing the colorless endpoint. Be sure to add each drop of titrant carefully and to evenly mix each drop before adding the next. Holding the sample against a white piece of paper can help enhance visualization of the endpoint.
11. The concentration of DO is equivalent to the volume (mL) of titrant used. Each milliliter of sodium thiosulfate added to the water sample equals 1 mg/L dissolved oxygen. 

Figure 1. A sample after the alkaline iodide-azide reagent has been added and mixed, showing floc formation at the top of the sample before settling.

Figure 2. A sample with dissolved floc after addition of sulfuric acid.

Figure 3. A sample after addition of sodium thiosulfate displaying a pale straw color.

Figure 4. A sample showing the blue color after the starch indicator is added and mixed.