How to Measure Dissolved Oxygen in Water

Natural bodies of water contain dissolved oxygen.

When scientists learned how measure dissolved oxygen in water, they discovered that bodies of water breathe---at least the ones with plant life in them do. The dissolved oxygen level in a healthy pond or lake rises and falls rhythmically with the rising and setting of the sun. This occurs because the plants in the water are producing oxygen via photosynthesis when the sun is up, and they stop producing the oxygen when it sets. You can measure the dissolved oxygen in a body of water yourself. Cheap, self-explanatory test kits and meters are available for you to purchase, or you can use the Azide-Winkler titration method if you have the appropriate materials available.

Things You'll Need

• 300-mL BOD bottle and stopper
• Manganese sulfate
• Concentrated sulfuric acid
• Alkali-iodide-azide reagent
• 2-mL pipettes
• 300-mL Erlenmeyer flask
• Calilbrated burette
• Dissolved starch solution

Instructions

1. Fill your 300-mL BOD bottle with the water you are testing. It is very important that you do not introduce bubbles to the bottle as you fill it, because this will add oxygen to the water. The best way to fill it is to half-submerge the opening of the bottle and let the water slowly flow in. As the bottle fills, submerge the opening more fully until it is completely full.

2. Add 2 mL of manganese sulfate to the water sample. Remember to add it in such a way that will not introduce bubbles. A good way to do this is to submerge the tip of the pipette just beneath the surface of the water sample and slowly let the manganese sulfate into it. If any bubbles occur, you'll have to start over.

3. Put 2 mL of alkali-iodide-azide reagent into the water sample just like you added the manganese sulfate. Do not create bubbles.

4. Stopper the bottle and invert it several times to mix it. The formation of an orangish-brown precipitate indicates the presence of oxygen. Stop inverting the bottle to let the precipitate settle, then re-agitate it and let it settle again. If you see bubbles at any point you'll have to start over.

5. Pipette 2 mL of concentrated sulfuric acid into the sample. Restopper the bottle, then mix by inverting. The sulfuric acid will dissolve the precipitate. At this point, you can now store the sample in a cool, dark place for up to 8 hours, if need be. If you see bubbles during this mixing process, you'll have to start over.

6. Carefully transfer 201 mL of the water sample into an Erlenmeyer flask. Fill your calibrated burette with sodium thiosulfate and slowly add it to the water in the Erlenmeyer flask until you achieve a pale straw color. At all times keep track of how much sodium thiosulfate you have added.

7. Pipette 2 mL of starch solution into the sample so that a pale blue color forms. Then continue to titrate with sodium thiosulfate, one drop at a time, until the blue color disappears. Swirl and mix each drop of sodium thiosulfate into the solution before adding the next one.

8. Note the total amount of sodium thiosulfate you have used during the whole experiment when the blue color disappears. For every milliliter of sodium thiosulfate you used, 1 milligram per liter (mg/L) of dissolved oxygen is in the water sample. For instance, if you used a total of 6 mL of sodium thiosulfate, then the water sample has a dissolved oxygen concentration of 6 mg/L.

Tips & Warnings

• Always wear protective goggles, gloves and an apron when working with chemicals.

References

• Washington State Department of Ecology: How to Measure Dissolved Oxygen