How to Test for Dissolved Oxygen In Water
- 1). Fill the BOD bottle with sample water by dipping it into the stream or lake and allowing it to fill slowly. Check to ensure there are NO air bubbles in the bottle -- air bubbles can alter the test results. Tip the bottle to either side if you see any bubbles to let them escape.
- 2). Add the manganese sulfate using the calibrated pipette. Insert the pipette below the water surface and squeeze it slowly, taking care to ensure no bubbles are introduced into the sample.
- 3). Add the alkali-iodide-azide reagent, using the same technique you used to add the manganese sulfate.
- 4). Carefully stopper the bottle without introducing any air and turn the bottle upside over several times to mix the contents. Check the sample for bubbles. If there are any air bubbles you will need to discard the sample and start over.
- 5). Add the sulfuric acid with a pipette. This time, rather than inserting the pipette below the surface of the water, hold the tip just above the surface instead. Stopper the bottle and turn it over again to mix the contents.
- 6). Pour 201 ml of the sample into the Erlenmeyer flask, using the graduated cylinder to measure out the correct amount. Use a calibrated pipette to add sodium thiosulfate until the solution changes color to pale straw.
- 7). Add 2 ml of starch solution. The sample should turn blue.
- 8). Continue to add more sodium thiosulfate until the solution turns clear.
- 9). Record the total amount of sodium thiosulfate you added in steps 6 and 8 and use it to calculate total oxygen content. For each milliliter of sodium thiosulfate you added, the sample contained 1 mg per liter of dissolved oxygen.