salty joe
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If we have company and do a lot of cooking, pH drops in a hurry. When it's nice out, I'll open all the windows and pH seems much slower to rebound. Does seawater have an affinity for CO2 and doesn't want to give it up?
Does CO2 enter water more readily than CO2 leaving water?
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Randy Holmes-Farley
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That's a very complicated question. While I can answer the rate of CO2 coming and going, I think in a case like yours the explanation has more to do with how much CO2 is involved. If you use a gas stove and have people around, you can easily have CO2 many times higher (say, 300-500% higher in the air) than normal outside air. That will drive in CO2 fast just because there is so much.
When you are trying to drive out CO2, you are not generally looking as such a massive difference. If the pH is about 7.8 and the alk is about 7 dKH, you are looking at about double the CO2 level (say, 100% extra in the water) compared to equilibrium with normal air.
Thus, the 500% extra in the air can drive in CO2 in much faster than the 100% extra in the water drives it out.
Back to the actual stated question, CO2 itself equilibrates pretty fast, but most is held as carbonic acid (H2CO3 = CO2 + H2O) and to come out of the water, the H2CO3 needs to turn into CO2 to come out.
That has big implications in a skimmer, since the half time for H2CO3 to become CO2 is much less than a second, but the half time for CO2 to become H2CO3 is much longer, about 17 seconds. Thus, in a single pass in a skimmer, CO2 may well not equilibrate completely due to this difference, and from this alone, CO2 going in can be slower than CO2 coming out, at equal concentrations all around.
This site has more on the kinetics with some nice graphics and highly detailed discussion:
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