Ocean Acidification Project

[5thgrade_boy]

I am helping my 5th grader with a science fair project based on the Swimming in Acid, Ocean acidification project. We modified it a bit -- we decreased the pH down to 6.8, rather than 7.5, and are using phenol red as an indicator to see what the pH is, because we don't have access to a pH meter (or time to order one before the fair). The big question is this: we set up our jars today using 6 different types of shells, using ocean water with phenol red as a control, and ocean water with phenol red and vinegar as the acidified ocean water. We added vinegar until the ocean water turned yellow (which is supposed to happen at 6.8 ). I am not concerned if it is less than 6.8, just that it is at least 6.8 or less. So we set up bottles with shark eggs, clam shells, scallops, oysters, moon snails, and crab legs. Here's the thing: most of the bottles' acidified ocean water remained yellow after adding the shells, but with the crab legs, the water turned back to pink to indicate the pH went back to a much more alkaline 8.2. Our question is why? What is the chemistry happening with the crab legs that is re-neutralizing the water and actually making it go back to alkaline? I have a background in chemistry so I would understand it if you get into equations....you don't have to simplify for my sake. Is there something inherent in crab shells that contributes alkalinity so as to spend the acid concentration? It looks like the moon snails are slowly changing pinkish as well. My theory is that it must have something to do with air? Because I left out a sample of the acidified ocean water to open air and that sample has also changed to pink. The crab legs initially floated and released bubbles after the acidified ocean water was added. Moon snails (unbroken) also may entrap air. But what is the reaction with air that causes the pH to go back up? I should add that we went to great effort to make sure the bottles had no headspace, but this was particularly difficult for the moon snails and the crab legs. So does this mean that the CO2 is going out of solution into the air? Should we reopen the bottles and add more vinegar to push the pH back down? If we do, then the pH of those bottles may be way less than the remaining samples, compromising comparability. Perhaps add it, diluted, drop by drop until the pH *just* changes (i.e., until we just see the yellow color change)?

[connief]

Hello there,

That is a very interesting problem, and I'm not sure whether I know exactly why you see that effect with the crab legs and moon snails, and whether that has anything to do with air. Is there a way to do this to ensure that there would be no air for the crab legs and moon snails samples to see if the pH still goes back up? Other than air, it could be that the moon snails produced a by-product as a result of some sort of metabolic pathway, and it is this product that is increasing the pH of the surrounding water. I would suggest also posting your question to the physical sciences thread because there may be some chemists there who could potentially provide some insight as to what could have happened to yield these results. Are there any biochemists in this thread who may also know what is going on?

I'm sorry that I wasn't able to help much, but please let us know if you have any other questions. Hopefully others who see this thread would be able to provide some more insight.

Best,
Connie

[5thgrade_boy]

Thanks Connie, we posted on the Physical Sciences thread as you suggested. Just to give an update: all the types of shells now are showing an *increase* in pH except for the clam shells. After looking at some carbonate chemistry webpages, I do think that maybe the CO2 is possibly leaving solution and escaping to a gas phase. I'm thinking because my ocean water was cold (I kept it in the basement and I live in the NE), when we added the vinegar maybe it took time for the CO2 to come out of solution due to temperature, and perhaps we stopped adding vinegar to get the pH down to 6.8 too soon?

[5thgrade_boy]

Update: Since we didn't hear from anyone here on AAE, and his hypothesis is that shells exposed to acidified ocean water will lose mass, and we are trying to see which ones will lose the most mass, we decided to go ahead and reopen the bottles and add more vinegar. We added just enough so that the color changed from peachish/pinkish (depending on the starting pH) to yellow (6.8ish). I suspect the ocean water buffered against the addition of water, or the shells have started dissolving and the CO2 releases into the headspace and the pH goes basic again. Not sure which, but both are related. We will wait a day or two and update again.

[5thgrade_boy]

Actually, I think I am beginning to understand what is happening:

We added additional vinegar this afternoon, just until we reached a pH of 6.8 (i.e., just until, with a "drop" using a medicine dropper), the solutions turned yellow. In only a few hours, we are back to the colors prior to addition of the additional vinegar. What I think is happening is that for the Swimming in Acid project, although the title suggests that the shells are in acid, they are in fact still in alkaline/neutral ocean water, as they advised using a pH of 7.5. I advised my child to use a pH of 6.8 since the decreased pH will increase the kinetics of the reaction, and we actually might have a chance of measuring a mass difference (I looked in other posts of people who have tried the experiment as is, and they noted no measurable difference between the mass before and after leaving the shells in 7.5 ocean water after about a month. Someone on AAE posted a way to calculate what mass difference you could expect, and it was not possible for her to measure using a scale that only measured to the 0.1 g.) We also only have a balance that measures to the 0.1 g. The only thing is, the person who conducted the experiment before did not measure the pH at the end of the experiment. That is why I thought it would be nice to use phenol red so we wouldn't have to keep opening/closing our sample bottles to find out what was happening.

So back to what we are seeing...I think that 6.8 is, for the shells, hugely different than 7.5 and the calcium carbonate is, in fact, dissolving in the 6.8 acidified ocean water. No matter how careful we are about trying to close the bottles to eliminate any headspace, we are ending up with some headspace, albeit a small amount, in all sample bottles (controls have no headspace). So I hypothesize that as the shells are dissolving, the carbonate is converting to CO2 and is creating a headspace. This drives the pH back up to 8.2 as the CO2 leaves the solution.


CH3COOH +CaCO3  CO2 + Ca(C2H3O2)2
Acetic acid and calcium carbonate react to form carbon dioxide and calcium acetate

The calcium carbonate dissolves in the water, so the mass of the shells in the vinegar should decrease and they should get a little smaller. The carbon dioxide is released as a gas.
The ocean will never become as acidic as acetic acid solution, but the lower pH in this activity allows the effects to be seen over a shorter amount of time. For a longer-term experiment that more accurately models ocean chemistry, we should place shells in soda water (water with CO2 bubbles) for days to months and observe the effects. (It would be fun to add phenol red, too to see when things are happening more precisely.)

But if I am correct, doesn't this beg the question: If CO2 is in fact released into the air as a result of the oceans becoming more acidic, does this mean that the reaction itself could actually be driving itself??? Maybe upon first inspection, but when you consider all other factors going on (temperature, organic matter decomposition, and many other processes going on in the ocean), one must know you can't extrapolate what is occurring in a complex system like the ocean compared to a small bottle with shells in it and nothing else. In any case, the ocean and this process does seem to buffer itself.

Any thoughts?

[connief]

Hello there,

Your reasoning makes sense to me. However, I'm not an expert on this topic so it could be possible that I may be missing something. Does anyone who knows more about this topic have anything to add?

Best,
Connie

[HeatherL]

Hi there,

I agree with Connie that your reasoning looks correct. Nice job interpreting your unexpected results!

Let us know if you have more questions.

Heather