CO2 pH reference - quick degassing


Community Member
May 8, 2023
Hong Kong
CO2 users may measure the pH drop, and use that as an estimation for the CO2 ppm. The reference level should be the pH of fully outgassed water (in equilibrium with ambient air), and is usually checked on a glass of tank water that has been left open for about 24-36 hrs. That's a long time.

I have a faster method that works in perhaps 10 minutes, that I posted on another forum (deleted, then probably soon forgotten so most can go back to waiting for the traditional 24 hrs), where others tested it successfully. The topic came up in a PM conversation today, and I believe it is good to share here so that it is not forgotten and we can all benefit from it.

I use a glass jar, to which I add just enough tank water to later take a pH measurement with a probe. Less water, more air in the jar, speeds up the process.


I go outdoor with the jar (our homes may have elevated CO2 concentrations), let air in, close the lid and shake violently for a few minutes. Open, new fresh air in, shake again and repeat a few times. I can get to the fully outgassed reference in perhaps 5 minutes, but for others 10 minutes may be a safer target without accidentally dropping the jar.

How to double check if it works, how much time is needed, and if indeed the final degassed pH value has been achieved? Just measure pH, shake another few minutes and measure pH again. If pH is stable and not further increasing, then the first measured pH was already good enough and further shaking or waiting does not give a better value.

I hope this saves us all some time, and we have a good solid reference for outgassed pH as well.
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