Dissolved Oxygen vs. Temperature

[amyC]

[Science Buddies: Questions being posted on behalf of student and parent.]

Project: https://www.sciencebuddies.org/science- ... #procedure

Parent's comments:
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We recently bought a kit from you guys to do my daughter's science project (SBD-5244-KIT_ Dissolved Oxygen Science Buddies Kit), unfortunately after several tries we are not getting any conclusive results. There are some gaps in the instructions that leave some of the steps to interpretations. We need help urgently. We also need better instructions or better yet! a video that describes the steps clearly.

Some of the questions we have are (not neccesarily in this order):
1. How much time is water supposed to be aerated?

2. Are you supposed to use Reagent every time?

3. Is the end result supposed to be blue or clear? The one set of instructions shows it blue and the other shows it gray/clear

4. We could never get rid of the green hue on the end result unless we let the water sit for several minutes without moving, but this was the case everytime we tried regardless of the type of water or its temperature.
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For question #3 above, the following information has been provided to the parent:

In terms of the color, the directions note that the color change will be a "greyish-blue" as shown in Figure 2 of the Procedure. It might help to view the photograph for Figure 2 to evaluate the color. I have pasted it below. (It is also available online: https://www.sciencebuddies.org/science- ... #procedure)



The procedure states: "The reaction is at the end point when the final color changes from brown/green to greyish-blue. Most water samples will not be as vivid a blue as shown in the kit instructions. Instead they will be a greyish-blue color. See Figure 2 for an example."

[donnahardy2]

Hi,

I think you are doing this potentially really great chemistry project from this website.

https://www.sciencebuddies.org/science- ... #procedure

I need to understand what problem you are having. Any water sample that you have aerated will have some dissolved oxygen in it, so there should be something to measure with the Science Buddies test kit. Are are not seeing a brown color in any of the samples? Or, are you not seeing a difference in results with samples at difference temperatures? Please describe exactly what you have done and what the results are so I can understand the problem. Tell me exactly what happened when you added each of the reagents. And, what type of water are your using?

The Science Buddies test kit is based on the Winkler test. An excess of manganese, iodide and hydroxide ions are added to the sample to form a white precipitate. The dissolved oxygen in the sample turns the precipitate brown, which converts the iodide to iodine. The concentration of iodine is proportional to the quantity of thiosulfate required to titrate it.

http://en.wikipedia.org/wiki/Winkler_te ... ved_oxygen

This should be a straightforward titration experiment, but the following conditions could interfere with obtaining good results.

1. The reagents must be added in order and mixed gently with the sample. The chemistry is a sequential reaction, so needs to be done one step at a time in the correct order.

2. What type of water are you using? I suppose that if your local tap water contained any manganese, this would interfere with results. Have you tried using a sample of distilled or deionized water as a control?

3. Are your containers clean and rinsed with plain water? This reaction is very sensitive to reducing or oxidizing reagents, so the presence of residual bleach or detergents could interfere with the results.

If you will provide more details and answer the questions above, I will have more suggestions for you. I'm sure there is a solution to the problem.

Donna Hardy

[amyC]

[Posted on behalf of student/parent working on this project.]

First of all, thank you for your prompt responses.

Now, the problem we are having is that after following the steps exactly as described on the instructions (including the addition of reagent C, as specified in the glossy card), we are getting the same result regardless of the temperature on the water and how careful we are at not letting it be exposed to the air around us.
To begin with, we ARE using tap water. Everything goes well up until we start adding the reagent D drops; we do get the brownish color after adding reagent B and then the water turns a dark greenish blue after adding reagent C, the problem is that regardless of the temperature or how many drops of reagent D we introduce into the mix (we tried way more than 20) the water does NOT lose the dark green/blue color, unless we let it sit for a while until the brown stuff sinks to the bottom, then it is just dark blue. We'd appreciate any suggestions or corrections that you can throw our way, thank you.

[donnahardy2]

Hi,

This is an interesting chemistry problem. Thanks for describing what happened when you added the reagents.

There may be something in your tap water that is interfering with the reaction. Can you do an internet search of your local water company and post the typical composition of the water? Is there a high concentration of carbonates or sulfur-containing compounds? And, what type of chlorine is added to the water? Did you treat the water to remove the chlorine before you did the test? The answers to this inquiry will give me some ideas so I can suggest what to do next to isolate the problem.

Also, can you answer the following questions.

1. Did you gently mix the sample after adding each reagent?
2. Did you test a sample of your tap water that had been heated? It's possible from your description that 20 drops of the thiosulfate is not enough if there is lots of oxygen in your sample.


When is this project due?

Donna Hardy