Dissolved oxygen v Temperature

[laureldee]

Thanks for your help!

I followed the protocol in the Science Buddies website. I used LaMotte Dissolved Oxygen kit.
I measured dissolved oxygen at temperature for 30, 50, 70, 90, 110°F today. I aerated with an aquarium pump and airstone. I used water from the stream in my backyard.

I am confused by some of my results. Why is the dissolved oxygen getting lower when I aerate the warm water?
With my experiment, I expected at all temperatures with aeration that it would hold more dissolved oxygen compared to the nonaerated sample at the same temperature.

I took a picture of my data.

IMG_7963.jpeg (4.28 MiB) Viewed 25268 times

Thank you!



Moderator Note: https://www.sciencebuddies.org/science- ... emperature

[laureldee]

Hello! My Mom and I are unsure. Did we post our question wrong, is that why we haven’t received a response?
Thanks!

[amyCC]

I am sorry you haven't received a reply. With the holidays, it may be that fewer experts have been available. I hope that this will get expert input soon.

Amy
Science Buddies

[audreyln]

Hello,

Thank you for your patience in a reply over the holidays!

This looks like a fun experiment! I didn't know the answer to this question and had to do some reading myself and learned something new!

The Introduction section of the project guide has an overview of how some water can hold more or less dissolved oxygen. The aerated water is moving and has a rougher surface than the still water. This larger surface area has more contact with the air in the room and will equilibrate with the air in the room more quickly.

Audrey

[laureldee]

Hi Audrey!
Thank you for your reply. I did know that information from the background reading and my research with my mom.
I am confused by some of my results. Why is the dissolved oxygen (DO) getting lower when I aerate the warm water?
I saw that cold water with aeration increased the dissolved oxygen. I expected that aerating all samples would at least increase oxygenation in all my samples. At 70F we saw aerated and nonaerated sample have about same DO. And as it got warmer the aeration sample’s DO dropped below the nonaerated sample.

With my experiment, I expected at all temperatures with aeration that it would hold more dissolved oxygen compared to the nonaerated sample at the same temperature. Shouldn’t aerating increase DO in all samples?

Thank you!
Kate and mom

[Snehaarun]

Thanks for providing all the necessary information about the data/methods.

It looks like you did the project correctly, and your question is totally valid as well. Here’s a possible reasons why this problem might be occurring:

If the water had more organic load (debris/other particles), then more time would have been needed to aerate the sample, especially since it was warm water which already holds less DO (due to higher temperatures decreasing stability). This could mean that the 5-10 minutes was possibly not enough to induce equilibration for the aeration process to work and increase the DO in the water.

Some other topics to consider are:
1. The oxygen values within each temperature are pretty consistent; were these the values taken after doing each trial separately for each experimental condition?
2. If you try repeating the experiment with a longer aeration time, it would be helpful to record the aeration times and water temperatures itself at baseline for each trial, then again at the time they record the final oxygen values for each trial.


Let me know If you have any more questions!
Sneha

[laureldee]

Hello Snehaarun,
Thank you for your thoughtful reply.

Our samples at collection were clear and colorless, so organic load would have been low.

You mentioned some great points in the "topics to consider." We are using the LaMotte Dissolved Oxygen test kit with the modified Winkler titration. We have provided our procedures below. We think it will clarify how and when we measured temp and DO. Please let us know, does having our full procedure help clarify?

Here is our procedure:

1. Collect streamside sample. Record temperature (It was 48F). Perform Dissolved Oxygen (DO) testing streamside and repeat.
2. Collect 4 gallons of stream water in gallon pitchers. Bring water inside for lab testing.
3. For temperature 30°F – Place two 500 mL cups of water in freezer to cool water to desired temperature. Use an ice bath around the cups to keep cool water at the desired temperature during testing when outside of freezer. Verify temperatures before collection. Take first 500mL of water and follow testing procedures for DO, collecting and measuring a new sample for three times. Use second 500 mL of stream water and aerate with aquarium pump and air stone for 10 minutes. Verify temperature before collection. Perform DO testing on three aerated samples. (Simply, We measured temperature, collected the sample, ran the DO test. Repeated two more times.)
4. For temperature 50°F– 1000 mL of stream water was allowed to warm up to 50°F. Verify temperatures before collection. Take 500mL of water and follow testing procedures for DO, repeating it three times. Use another 500 mL of stream water and aerate with aquarium pump and air stone for 10 minutes. Verify temperature before collection. Perform DO testing on three aerated samples.
5. For temperatures 70°F, 90°F and 110°F – Use a sous vide to warm up the water in a water tub. Then verify the temperature and collect three samples. Perform DO testing. Collect 500 mL sample of heated water and pour into a large cup. Immediately following, Place large cup in a water bath of same temperature water to maintain water temperature. Let the sample aerate for 10 minutes. Verify temperature, collect sample, and perform DO testing. Repeat two more times.

Here is the Dissolved Oxygen Procedure:

1. Rinse the water sampling bottle with sample water.
2. Tightly cap the bottle and submerge it to the desired depth.
3. Remove the cap and allow the bottle to fill.
4. Tap the sides of the bottle to dislodge any air bubbles.
5. Replace the cap with the bottle still submerged.
6. Retrieve the bottle and make sure there are no air bubbles trapped inside.
7. Remove cap from bottle, immediately add 8 drops of manganous sulfide solution and add 8 drops of alkaline potassium iodide azide. Replace cap immediately. Hold dropper bottles vertically upside-down, and not at an angle, when dispensing a reagent. Squeeze the bottle gently to dispense the reagent one drop at a time. Count out loud as dispensing each drop.
8. Mix by inverting three times, a precipitate will form.
9. Allow precipitate to settle below the shoulder of the bottle.
10. Remove cap from the bottle and immediately add 8 drops of sulfuric acid.
11. Cap and gently invert the bottle to mix the contents until the precipitate and the reagent have totally dissolved. The solution will be clear yellow to orange, if the sample contains dissolved oxygen. Note: The sample has now been fixed meaning contact to the atmosphere and air will not affect the test results.
12. Fill the titration tube to the 20-milliliter line with fixed sample. Place sample cap on the tube.
13. Depress the plunger of the direct read titrator down completely, insert titrator into the plug in the top of the sodium thiosulfate titrating solution.
14. Invert the bottle and slowly withdraw the plunger until the large ring on the plunger is the opposite the zero line on the scale.
15. Turn bottle upright and remove titrator.
16. Insert the tip of the titrator into opening of the titration tube cap.
17. Slowly depress the plunger to dispense the titrating solution drop by drop until the yellow-brown color changes to a very pale yellow. Gently swirl the tube during the titrations to mix contents. Check the color by holding upo the bottle in front of a white piece of paper, this allows color to be judged more accurately.
18. Carefully remove the titrator and cap. Do not disturb the titrator plunger.
19. Add 8 drops of Starch Indicator Solution into the titration tube. The sample should turn bluish purple.
20. Cap the titration tube. Insert the tip of the titrator into the opening of the titration tube cap.
21. Continue titrating drop by drop until the blue color disappears and the solution becomes colorless. Check the color by holding upo the bottle in front of a white piece of paper, this allows color to be judged more accurately. Note: if the plunger ring reaches the bottom of the line on the scale 10 parts per million before the end point color change occurs refill the titrator and continue the titration include the value of the original amount of reagent dispensed (10 parts per million) when recording the test results.
22. Read the test result directly from the scale where the large ring on the titrator meets the titrator barrel. Record as parts per million (ppm) dissolved oxygen. Each minor division on the titrator scale equals 0.2 PPM. When testing is complete, discard the titrating solution in the titrator. Rinse titrator and titration tube thoroughly.
23. Discard samples into waste jar. Discard down drain.


THANK YOU!
Kate and mom