Determination of CO2 in combination with an aquasoil


7 Apr 2008
nr Bath
Hi all,
Do we even know that a green drop checker corresponds to 30ppm CO₂ or is that based on the same extrapolation from the dubious 2-3ppm figure?
My guess is extrapolation, and that "30 ppm CO2" doesn't necessarily mean "30 ppm of CO2".

The normal CO2 sensor for in situ measurement is a "Severinghaus probe", so I'll see what I can find out using that as a search term. I think I've got it (from <"The measurement of dissolved and gaseous carbon dioxide concentration">. It looks to have all the answers.
2.1. Analytical determination of CO2in liquids

Unlike other gases such as oxygen, the determination of dissolved CO2 is more difficult due to its chemical reactions with water. This is important for interpreting analytical results as well as for understanding the measurement principle applied e.g. in the Severinghaus electrode. Therefore, the interaction of CO2 with water and the acid–base equilibrium have to be discussed in more detail. The concentration of CO2 is only reasonable with the knowledge of the pH of the analysed medium
This one <"Dissolved CO2 in freshwater systems">, has some figures, that suggest that lakes and rivers typically have between four and eight times more CO2 than they would at equilibrium (0.6 ppm CO2), so we are back somewhere near the "3ppm CO2" figure. The other issue in vegetated water would be that you would get big diel variations in CO2 & O2 levels (back to <"Canford Park again">)
My suggestion above may well be within the realms of hobbyist aquatics. I'm thinking of Seneye with their approach to measuring free ammonia.
I'm pretty sure that isn't going to work, because of the pH issues, when you change the pH you change the amount of CO2. This means that techniques like converting all the NH3 to NH4+ (by acidification), or measuring oxygen levels by reduction aren't going to work for CO2.
You may find this to be of interest:
We use Ion Selective Electrodes for NO3 and NH3/NH4+ measurement (and also for DO measurement). Because we have scanning spectrophotometers, HPLC, GC, and AAS we tend to use them for most other elements. We don't have MS or ICP, which a big analytical lab. would have.

Other than CO2, I'd be reasonably happy that I can get fairly accurate results for a water sample in the lab. but it would be quite time consuming, and I'd need access to all the appropriate reagents and analytical equipment.

cheers Darrel


21 Jan 2015
On the basis that an aquarist may already own a pH meter, the total cost of my proposed method is trivial - less than a tenner!

I was being ultra-cautious here. Too cautious. For many aquarists, they will already have the items I listed. The only thing that is not likely to be in one's aquarium cupboard or toolbox is the very thin silicone membrane. And I even got that free of charge.


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