How to Measure the Dissolved Oxygen Level of Water
Collect a sample., Mix manganese sulfate with the water., Add alkili-iodide-azide to the sample., Fix with sulfuric acid., Titrate 201 mL of sample., Introduce starch solution., Continue to titrate., Understand the results.
Step-by-Step Guide
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Step 1: Collect a sample.
Take a 300 mL sample of water.
This can be from a tap, a stream, a pond, or any other water source.
Collect the sample in a flask with a stoppered top., Use a calibrated pipet to pipet two milliliters of manganese sulfate into the sample.
Put the tip of the pipet just beneath the surface of the water before releasing the contents.
Stopper your bottle and mix the manganese sulfate by inverting the bottle several times slowly.If you drop the contents into the water they will come in contact with the air and this will introduce oxygen into the sample and alter results.
If any air bubbles occur then the sample is contaminated and you will need to start over. , Next, add two milliliters of alkili-iodide-azide to the sample using a calibrated pipet.
The tip of the pipet should be just below the surface of the water before ejecting the contents.
Stopper the bottle and mix the alkili-iodide-azide by inverting the bottle slowly several times.If oxygen is present you will observe the formation of floc.
This is an orange-ish solid that will settle in the bottom of the flask. , Use a pipet to measure two milliliters of concentrated sulfuric acid.
Drop the sulfuric acid into the solution.
Do not place the tip of the pipet into the water.
Mix the acid by inverting the flask several times.
The floc should redissolve.Wear gloves and goggles for safety when working with sulfuring acid.
Avoid contact with skin and eyes.
Do not ingest.
Do not inhale. , Measure exactly 201 mL of sample into a new flask.
Place this sample beneath a graduated pipet filled with sodium thiosulfate.
Add sodium thiosulfate until the sample turns a pale yellow color.Write down the initial amount of sodium thiosulfate in the pipet.
Stir the solution continually as you titrate. , A starch solution will react with the iodine present in the sample to form a blue color.
You only need to add two milliliters of starch solution.
Stir or swirl the solution well.Starch solution can be made by mixing water with corn or potato starch, or you can buy one premade. , Once the sample turns blue, continue titrating with sodium thiosulfate.
Add the titrant slowly, as one drop will make a difference at the end of the titration.
You should stop titrating when the blue color is gone from the sample.Hold the sample up against a white background in order to look for blue color. , When the titration is complete, write down the amount of sodium thiosulfate left in the pipet.
Subtract this amount from the initial amount of sodium thiosulfate to find how many mL of sodium thiosulfate you used to titrate the sample.
The amount of sodium thiosulfate in mL is equal to the dissolved oxygen content in mg/L.For example, if you used 8 mL of sodium thiosulfate, that would correspond the a dissolved oxygen content of 8 mg/L. -
Step 2: Mix manganese sulfate with the water.
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Step 3: Add alkili-iodide-azide to the sample.
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Step 4: Fix with sulfuric acid.
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Step 5: Titrate 201 mL of sample.
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Step 6: Introduce starch solution.
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Step 7: Continue to titrate.
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Step 8: Understand the results.
Detailed Guide
Take a 300 mL sample of water.
This can be from a tap, a stream, a pond, or any other water source.
Collect the sample in a flask with a stoppered top., Use a calibrated pipet to pipet two milliliters of manganese sulfate into the sample.
Put the tip of the pipet just beneath the surface of the water before releasing the contents.
Stopper your bottle and mix the manganese sulfate by inverting the bottle several times slowly.If you drop the contents into the water they will come in contact with the air and this will introduce oxygen into the sample and alter results.
If any air bubbles occur then the sample is contaminated and you will need to start over. , Next, add two milliliters of alkili-iodide-azide to the sample using a calibrated pipet.
The tip of the pipet should be just below the surface of the water before ejecting the contents.
Stopper the bottle and mix the alkili-iodide-azide by inverting the bottle slowly several times.If oxygen is present you will observe the formation of floc.
This is an orange-ish solid that will settle in the bottom of the flask. , Use a pipet to measure two milliliters of concentrated sulfuric acid.
Drop the sulfuric acid into the solution.
Do not place the tip of the pipet into the water.
Mix the acid by inverting the flask several times.
The floc should redissolve.Wear gloves and goggles for safety when working with sulfuring acid.
Avoid contact with skin and eyes.
Do not ingest.
Do not inhale. , Measure exactly 201 mL of sample into a new flask.
Place this sample beneath a graduated pipet filled with sodium thiosulfate.
Add sodium thiosulfate until the sample turns a pale yellow color.Write down the initial amount of sodium thiosulfate in the pipet.
Stir the solution continually as you titrate. , A starch solution will react with the iodine present in the sample to form a blue color.
You only need to add two milliliters of starch solution.
Stir or swirl the solution well.Starch solution can be made by mixing water with corn or potato starch, or you can buy one premade. , Once the sample turns blue, continue titrating with sodium thiosulfate.
Add the titrant slowly, as one drop will make a difference at the end of the titration.
You should stop titrating when the blue color is gone from the sample.Hold the sample up against a white background in order to look for blue color. , When the titration is complete, write down the amount of sodium thiosulfate left in the pipet.
Subtract this amount from the initial amount of sodium thiosulfate to find how many mL of sodium thiosulfate you used to titrate the sample.
The amount of sodium thiosulfate in mL is equal to the dissolved oxygen content in mg/L.For example, if you used 8 mL of sodium thiosulfate, that would correspond the a dissolved oxygen content of 8 mg/L.
About the Author
Patricia Armstrong
Enthusiastic about teaching pet care techniques through clear, step-by-step guides.
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