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Old 09/25/2017, 08:14 PM   #20
RC Mod
Join Date: Mar 2002
Location: Mountain View, CA, USA
Posts: 87,094
It's very likely that the tank will be close to equilibrium with the air, although photosynthesis can drive the pH up, thus causing a continuing influx of carbon dioxide. The tank is not at equilibrium with the air at that point, although the difference tends to be quite small, well, for my definition of "small". During the night, the pH can drop as photosynthesis stops, but at that point, respiration will drive the pH down at least a bit, leading to outgassing of carbon dioxide. I suspect that most systems only see a small effect, but I don't have any measurements. At some point, you'd need to define very carefully what you mean by "in equilibrium with the air", but in my view, as long as there's net carbon dioxide flux through the water surface, the system is not at equilibrium in that respect.

When the limewater is added, the calcium hydroxide will convert to calcium and carbonate ions, but I don't know how long that process takes. It might be fast enough that we can't easily detect any effect. I'll try to think about this a bit.

As a side note, if the lime is in the form of CaO, the first step is combining with water to form Ca(OH)2.

Jonathan Bertoni

Last edited by bertoni; 09/25/2017 at 09:12 PM.
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