Is there a certain window of time

[waterbear]

Ben's sticky says you need to get PH low to induce carbon dioxide production.

Once carbon dioxide is present, I assume it's no longer measurable as part of the ALK test, and this is why we get the drop, right?

And if it's not gassed off, when PH increases due to aeration, carbon dioxide reverts back to CA, which is measurable, right?

Anyone know at what PH level carbon dioxide re-converts to CA? Waterbear?

Exactly right, except that the incease in pH is due to the CO2 being gassed off...as there is less carbonic acid the pH will rise but the buffer is in equalibrium so things shift as the pH rises..It takes a bit of time and patience to lower TA. pH determines the ratio of carbonic acid to carbonates/bicarbonates in the water.....lower pH means more carbonic acid and less measurabe TA. Higher pH is the reverse. If you remove the carbonic acid by gassing off the CO2 you have lowered the TA because you have less of the total buffer system in the water. As you you lower the carbonic acid the carbonate/bicarbonate try to reach a new equalibrim so they lower. The purpose of a buffer is to stabilze pH so the pH will rise as the equalibrium is reached. The net effect is a lower TA. In most pools the TA buffer will stabililize at a pH of around 8 so the addition of acid to keep the pH in range is still necessary.