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Measuring Dissolved CO2 in Water

  1. Sep 2, 2014 #1
    Measuring CO2

    I have been searching for an answer to this for quite a while, so I'm hoping somebody here can help me out. I've spent quite a bit of time digging and reading, and I've found plenty of information, but I'm not always sure what to make of it. I am working on making myself a small electronic device to measure the dissolved CO2 in water in ppm by weight.

    I'm going to make a small sealed chamber that will have an opening only to the water (but not to the atmosphere). The CO2 will off gas into the chamber until the concentration of CO2 in chamber becomes at equilibrium with the concentration in the water. I know that some of the CO2 will be converted into carbonic acid, but as I understand it, the majority of it will remain as CO2, and for the precision I'm going for I think I can just ignore that.

    I'm going to place a small CO2 air sensor in the chamber that I will connect to a micro controller. The CO2 sensor reads concentration in ppmv. I'll also put a temperature sensor in there since I'm pretty sure I'll need to correct for temperature.

    The problem I'm having is taking the ppm by volume and equating it to the concentration of CO2 in water, which needs to be reported in ppm by weight. I can't find a formula to do the conversion, and it's been so long since I took physics (I'm 37), so I'm completely unable to figure this out at this point.

    Basically, I'm trying to figure out if it's V ppmv in the chamber, than what's the W ppmw in the water. Anybody have any ideas on how you'd calculate this?
    Last edited: Sep 2, 2014
  2. jcsd
  3. Sep 2, 2014 #2
    Google for Henry's law, and more specific, for Krichevsky-Kasarnovsky Equation, modeling the system CO2-H2O
  4. Sep 2, 2014 #3
    What's funny is that when I read your comment my first thought was "of course I've already read that, why else would I be here?" Except I hadn't read into Krichevsky-Kasarnovsky Equation, so I went off and did that first. The part that was missing for me was the weight in water to volume in air relationship. In my search I found a document on the Krichevsky-Kasarnovsky Equation that explained this clearly. From there I went back to the parts I needed from Henry's law, which made more sense to me, and I was able to figure it out — even correcting for temperature!

    So, thank you for your very simple answer! It was better than just getting a formula, because now I actually understand what the numbers mean.
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