Science Buddies: "Ask an Expert"

[Project: https://www.sciencebuddies.org/science- ... #procedure
I'm Trying to Breathe Here! Dissolved Oxygen vs. Temperature]

We've gone through the steps carefully and have looked at other parents' comments.

We're still trying to get any reasonable result on a baseline. We've gone through each step in the appropriate sequence, used distilled water, used the gentle mix process you recommended of inverting the tube, and have read your procedure comments and pictures from the website and the kit.

Essentially, steps 1-3 seem to work -- we get an insoluble precipitate and it looks brown before we add the re-agent. However, the re-agent --despite trying up to 28 drops on multiple tests -- never reaches anything like the color you've shown in your picture. It remains a very robust blue. Because of this, we have not idea if the kit is working and, of course, the results are not readable.

Hi,

It sounds like you are not reaching the endpoint of the titration. I'm sure there is an explanation for your results, but I can't tell what went wrong. Can you confirm exactly what you did after the brown precipitate form.

1. When you added the acid to dissolve the brown MnSO4, did the precipitate dissolve completely?

2, What color was the sample after the sodium thiosulfate step?

3. Did you mix the sample very gently so no more oxygen was added to the sample?

3. Did you process the samples one step at a time or did you stop at any point?

4. What color was the sample after you added the starch solution?

5. When you were titrating with the thiosulfate solution after adding the starch, did you mix the sample very gently after adding each drop of sodium thiosulfate?

6. Did you add any other reagents or a different type of water that may have contained some anions that interfered with the titration?

If you have reagents left, can you try a sample using heated water? This type of sample should contain a very low concentration of oxygen, and there should be no problem in reaching an endpoint.

If you can answer the questions, I might be able to offer some suggestions to solve the problem. Please let me know when your project is due.

Donna Hardy

Hello again,

Have you been able to solve the problem with reading the endpoint of your assay? If not, here is another suggestion:.

The endpoint on this test kit depends on the methylene blue turning from blue to colorless when the sodium thiosulfate reduces all of the methylene blue in the sample and this occurs when all of the iodine in the sample is converted to iodide.

The traditional method for doing the Winkler dissolved oxygen test method used starch solution as an indicator. The end point is a dramatic blue-black color of the starch-iodine complex changing to colorless as the iodine is reduced to iodide.

http://serc.carleton.edu/microbelife/re ... xygen.html

http://cchdo.ucsd.edu/manuals/pdf/91_1/culber2.pdf

So, you might want to repeat the assay, substituting a 1% starch solution for the methylene blue. You can make the starch solution by using any starch, such as corn starch. Try adding 1 tsp cornstarch to 8 ounces of water and boil it or microwave it while stirring frequently until the solution changes from cloudy to clear.

When you titrate with the sodium thiosulfate (reagent D), add this reagent until the solution turns to a pale straw color, Then add 1 ml of the starch solution so a blue color forms and continue the titration until the sample turns clear. This endpoint should be clear and easy to read.

This will not solve the problem, if there is something in your sample that it interfering with the assay, or if you a mixing the sample to much and adding oxygen back into the sample, but it may help with troubleshooting.

Please let me know the answers to the previous questions and if you decide to try using the starch to read the endpoint.

Donna Hardy