How to Measure Dissolved Oxygen
A water molecule contains an atom of oxygen and also contains dissolved oxygen as a free gas. The concentration of oxygen that is dissolved in a body of water is a general indicator of the organisms that the water can support. Biologists commonly use the Winkler method to determine the oxygen content of a water sample. This test relies on color changes and titration to measure oxygen content. The reagents for the Winkler method are readily available in a single kit.
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Things You'll Need
- 2 ml alkali-iodide-azide
- 2 ml concentrated sulfuric acid
- 2 ml manganese sulfate
- 2 ml starch solution
- 300 ml bottle with a stopper
- Calibrated pipette
- Sodium thiosulfate
Instructions
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1
Fill the bottle to the marked line with the water sample so that it won't contain any air when it's stoppered. Add two ml of manganese sulfate just beneath the surface of the water with the calibrated pipette. It's critical that you not produce any air bubbles, so squeeze the pipette slowly. Likewise, add two ml of alkali-iodide-azide in the same manner.
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2
Stopper the bottle carefully so that it has no air remaining in it. Turn the bottle upside-down a few times to mix the reagents and check for air bubbles. If there are any, discard the sample and begin the procedure again. Otherwise, allow any brownish-orange solids to settle to the bottom of the bottle.
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3
Add two ml sulfuric acid with the pipette held above the surface of the water. Replace the stopper and mix the sulfuric acid so that it dissolves the precipitate, or sediment, by inverting the bottle a few times.
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4
Draw off 201 ml of the water sample and slowly add sodium thiosulfate with the pipette while stirring the sample. Continue adding sodium thiosulfate until the sample becomes a pale straw color, noting the number of ml used. Add two ml of the starch solution and wait for the sample to turn blue.
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5
Resume adding sodium thiosulfate one drop at a time while stirring until the sample becomes clear. The concentration of dissolved oxygen in the sample in milligrams per liter (mg/L) will be the total number of milliliters of sodium thiosulfate added in Steps 4 and 5 to the solution.
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