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.