Water CO2 capacity

Discussion in 'Carbon Dioxide (CO2)' started by Themuleous, 12 Aug 2008.

  1. Themuleous

    Themuleous Member

    Messages:
    4,126
    Location:
    Aston, Oxfordshire
    One for all you chemists out there :)

    Does water have a limit to the amount of CO2 it can hold?

    When I set up my new tank, I pumped CO2 into as being a big tank I thought it would need loots. Well the lack of current plant growth and minimal surface water movement I think has meant that it is retaining CO2. Since starting the CO2 I've cut it right back to minimal input so I can get the CO2 level in the tank down to an even 'green' level. Trouble is I think I pumped so much into it in the first place, its taking a while to degas, so the level has been in the yellow range for two days now!

    Thoughts? Just curious really.

    Sam
     
  2. ceg4048

    ceg4048 Expert/Global Moderator Staff Member

    Messages:
    8,952
    Location:
    Chicago, USA
    Hi Sam,
    Yes there are limits based on the atmospheric pressure above the water but the solubility of CO2 in water is very high. It's not clear to me what the problem is though. On the one hand you say that your plants aren't growing but on the other you say that you are injecting a minimum level based on your drop checker. If I had to choose between observing plant growth and dropchecker color to determine the CO2 levels I would choose plant growth and not get mesmerized by the dropchecker color patterns. In a covered tank the CO2 will not degas as quickly since the hood holds the CO2 in the same way that a bottle cap holds the CO2 suspended above the surface of the liquid in a bottle of Coke.

    The problem is that the CO2 distribution in a tank is completely uneven so that a green dropchecker does not guarantee 30 ppm distribution to the plants. Add more until the plants respond, but not so much that you kill your fish and call it good.

    Cheers,
     
  3. Themuleous

    Themuleous Member

    Messages:
    4,126
    Location:
    Aston, Oxfordshire
    Thanks Clive, thought this might be a topic for you :)

    Sorry I dont think I explained myself very well. The plants may well be growing but I only set the tank up on Saturday so the plants haven't been in there very long, and as such there isn't the high level of plant biomass that will eventually be present once the plants grow a bit. This I only say as I assume that more plants = more CO2 uptake. As I dont have lots of plants as yet, CO2 uptake by them is likely to be much lower.

    My surprise was the fact that it seems to be taking days for the excess (i.e. over the 30ppm I'm aiming for) to degas and the CO2 level to drop sufficiently for the DC to turn to green. The DC is a constant yellow at the moment. Which leads me to think I must have put tons of CO2 into the water when I first set it up! Who know 150-200ppm?!?!?!?! :wideyed: :wideyed: :wideyed: :wideyed:

    I'm not overly concerned, just curious. As you say the drop checker is only a guide (I actually have two in there to see if there is a difference depending on location).

    The tank isn't covered though, open top baby all the way!! It certainly makes maintenance a darn sight easier!

    Cheers

    Sam
     
  4. ceg4048

    ceg4048 Expert/Global Moderator Staff Member

    Messages:
    8,952
    Location:
    Chicago, USA
    At a KH of 4 the dropchecker water will turn yellow at a pH somewhere between 6.4 and 6.0 which is about 40 to 75 ppm CO2. This is at the location where the checker is mounted and it doesn't tell you all that much about the surface of the plants as you have surmised. This does not imply "tons". I would be willing to bet that at the substrate, the CO2 concentration level is about one tenth that of the dropchecker.

    Cheers,
     
  5. Themuleous

    Themuleous Member

    Messages:
    4,126
    Location:
    Aston, Oxfordshire
    Well the DC that I have just above the substrate is also yellow! :lol: I'll do a KH/pH test this evening to see roughly what the CO2 level, at least it'll give me an idea of the level. I dont know why I didnt think of this last night! :rolleyes: To reliant on the DC thats my trouble :D

    Sam
     

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