How to Calculate Oxygen Concentration

Healthy lakes have enough oxygen to support the life beneath the waves.

Oxygen concentration is a vital metric when measuring water quality. The organisms that live in lakes and streams require oxygen for life, and water that does not have enough oxygen in it soon loses its plant and animal life. While there are simpler in-the-field methods for measuring oxygen concentration, they are all calibrated on the Azide-Winkler Method.

Things You'll Need

• Biological Oyxgen Demand (BOD) bottle
• Water for testing
• Manganese sulfate
• Calibrated pipette
• Alkali-iodide-azide reagent
• Sulfuric acid
• Sodium thiosulfate
• Starch solution

Instructions

1. Collect your sample water and add 300 mL to a BOD bottle.

2. Drop the end of the pipette below the water's surface and add 2 mL of manganese sulfate. Make sure the end is in the water, so you don't add oxygen to the sample. Repeat the process with 2 ML of alkali-iodide-azide reagent.

3. Put the stopper on the bottle, and turn the bottle over several times. There shouldn't be any air bubbles; if you see any, you have to dump everything out and begin again. You should see a dark orange precipitate forming and settling to the bottom if your sample has oxygen. Once it settles, turn the bottle over a few times and let the precipitate drop out again.

4. Remove the stopper and add 2 mL of sulfuric acid to the sample. This time, though, let the acid drop in from above the surface. Put the stopper back in and turn the bottle over several times. This will break up the precipitate. Now you can store your sample, and it gives valid results for up to eight hours.

5. Pour 201 mL of the sample into a flask. Drip sodium thiosulfate into the flask one drop at a time through the pipette, until the sample is a light beige, and stir after each drop. Record the number of drops it took to get the beige color.

6. Turn the sample blue by adding 2 mL of starch solution through the pipette.

7. Keep adding drops of sodium thiosulfate until the sample is clear. Stir between each drop. When the sample clears, write down the number of drops it took to clear the sample.

8. Add the number of drops from Step 5 to the number of drops from Step 7. That number is the concentration of oxygen, in milligrams per liter.

References

• Microbial Life Educational Resources: The Winkler Method -- Measuring Dissolved Oxygen
• University of Massachusetts: Standard Methods for the Examination of Water and Wastewater
• Hach: USEPA Azide Modification of Winkler Method