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Questions about CO2 solubility

Discussion in 'CO2 Enrichment' started by scottward, Oct 4, 2011.

  1. scottward

    scottward Guru Class Expert

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    Carbon Dioxide solubility: 1.45g/L @ 25C.

    So this means that if we could pump CO2 fast enough into our aquariums, it could reach a theoretical maximum concentration of 1450mg/L (1450ppm)?

    We only want about 30ppm though, so it seems this should be 'easy' given the typical temperature for our setups is also around 25C.

    So why isn't it?

    If I understand correctly, CO2 dissolves easily, but also degasses easily as it doesn't 'bond' strongly enough with the solvent?

    What is the CO2 concentration likely to be inside a typical CO2 reactor? I believe Tom has measured CO2 concentration directly from the output of external CO2 reactors and it was only about 100 or so mg/L?

    I was under the impression that the build up gas inside CO2 reactors toward the end of the photoperiod was CO2 that was having trouble dissolving as a result of the rising CO2 levels. But I'm pretty sure that I am wrong here (no surprises there). :rolleyes:

    I think I am wrong that the build up gas would be CO2 because:

    A. If it were CO2 that couldn't dissolve because ambient CO2 levels has risen, the build up would occur much faster; the problem would be significantly worse given the time I believe it takes for my system to reach good CO2 levels (hopefully within the first hour before lights on).

    B. At this water temperature (my tank is about 25C), CO2 is clearly not saturated until it reaches much much higher levels (1450mg/L), so there is no reason why it shouldn't dissolve quickly inside the reactor.

    Scott.
     
  2. Matsyendra

    Matsyendra Junior Poster

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    Perhaps the rate or (speed) of dissolution is quite slow?
     
    #2 Matsyendra, Oct 4, 2011
    Last edited by a moderator: Oct 4, 2011
  3. Biollante

    Biollante Lifetime Charter Member
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    I Have Been Warned Against Butting In On "Guru Level" Discussions...


    Hi Scott,

    I know I should leave this to you Gurus…:eek: This is something I have been struggling to understand…;)


    Far be it from a lowly, most unworthy and unenlightened form of life that I am:eek:, to disagree with a bodhisattva, but the fish lord:) is wrong:(; CO[SUB]2[/SUB] very readily goes into aqueous solution…:D The International Union of Pure and Applied Chemistry (IUPAC) has some provisional recommendation on the definition of hydrogen bonds that may help our understanding.:) More on this, should there be any interest, later. :rolleyes:


    I have always thought, been told, and tried to accept that carbonic acid forms,
    H[SUB]2[/SUB]O + CO[SUB]2[/SUB] H[SUB]2[/SUB]CO[SUB]3[/SUB] that also forms hydrogen bonds and that is why pH lowers and so on.



    This has never been very satisfying and it conflicts with what I observe, as unworthy as the graduate students say my observation are.


    As unworthy as I am some have tried to explain it to my lowly self, that the H[SUB]2[/SUB]CO[SUB]3[/SUB] as carbonic acid is not precise, there are no molecules of H[SUB]2[/SUB]CO[SUB]3[/SUB] in aqueous solution.
    “Carbonic acid” is an equilibrium solution of dissolved CO[SUB]2[/SUB], a bit of H[SUP]+[/SUP] and HCO[SUB]3[/SUB][SUP]-[/SUP] (I am told this is why it is acidic):
    CO[SUB]2(aq)[/SUB] + H[SUB]2[/SUB]O[SUB](I)[/SUB] H[SUP]+[/SUP] + HCO[SUB]3[/SUB]



    As unenlightened as I am, I remember from my 8[SUP]th[/SUP] grade (the best 5 years of my life) education that equilibrium lies to the left, so the CO[SUB]2[/SUB] remains (for the most part) CO[SUB]2(aq)[/SUB].



    So perhaps even one as unworthy as I can find a path to enlightenment.;)


    I have measured inside reactors and generally the internal pressure of the reactor relates to the amount of CO[SUB]2(aq)[/SUB], in my do-it-yourself reactors the dissolved CO[SUB]2[/SUB] runs from 140-180-ppm, measured pressure is 0.124-0.0199-bar.


    I also measure reactor out flow around 100-ppm, though this varies quite a bit and may relate to my measurement or sampling technique.:confused:


    I find it interesting that I find a consistent 65-ppm CO[SUB]2[/SUB] in the downstream corner nearest the reactor. In each case the reactor output is [SUP]2[/SUP]⁄[SUB]3[/SUB] deep about 15 centimeters from the corner, the flow is from that end to the other with a slight gyre.


    Biollante


     
    #3 Biollante, Oct 4, 2011
    Last edited by a moderator: Oct 4, 2011
  4. BigFlusher

    BigFlusher Prolific Poster

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    Biollante

    You wonder why the grad students make fun of you, you big green dummy, maybe you need to go back to the 8th grade.:p

    This equation the equilibrium position is said to be "far to the left" since the reactants form little product.

    The IUPAC reference is available http://media.iupac.org/reports/provisional/abstract11/arunan_310311.html.

    According to this the hydrogen bond, the attraction between water and CO2 constitutes a hydrogen bond. It helps explain, something that isn’t obvious, the reason CO2 is so soluble in water.

    ----sorry about the "big green dummy thing"----
     
  5. Tom Barr

    Tom Barr Founder
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    Compare this to Oxygen gas, which is not particularly souble in water.

    Once you understand both of these 2 gases behavior, then we all have a better understanding on them both in the aquarium.
    CO2, KH, GH are really the tougher things to have hobbyists know and understand.

    This was true today as it was when I first posted in 1995.
     
  6. Tom Barr

    Tom Barr Founder
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    Another thing is the time dependence here.

    In a sealed container, things can take their sweet time, but CO2 dissolving is a function of flow rates, time, surface area flux........for aquariums.

    Fast response time with the CO2 reactor/diffusion method is helpful IME.

    KH also plays a role.
     
  7. scottward

    scottward Guru Class Expert

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    Hi Tom,

    Not sure I follow - the sealed container bit - what difference does this make given that water is flowing through.....?

    Why does KH play a role? Other than preventing a pH crash?

    So if the water coming into the reactor is low in CO2 versus high (what we class as high; say 30ppm) in CO2, this difference of 0-30ppm won't make any noticeable difference in how well the CO2 can dissolve?



    Scott.
     
  8. Matsyendra

    Matsyendra Junior Poster

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    The fish lord is happy to be wrong, because he really doesn't know much at all... and was pondering out loud. Chemistry was never his thing... he got as far as "the equation needs to be equal on both sides" that was about as much as his little brain could handle.. oh that and H He Li Be B C N O F Ne Na Mg Al Si P S Cl Ar K Ca...

    :D
     
  9. scottward

    scottward Guru Class Expert

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    In my case, speaking about myself, Planted aquariums were never his thing might be appropriate. Brain hurts.
     
  10. Biollante

    Biollante Lifetime Charter Member
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    Alone in the wilderness... Path, what path... Hand me the machete...



    Hi,

    To attain greater enlightenment…
    :cool:
    http://www.youtube.com/watch?v=zGM-wSKFBpo
    http://www.youtube.com/watch?v=uJGrwWOWt3Q&feature=related

    Truthfully, in my unenlightened way I have never understood the reason to memorize this stuff.
    :confused:

    Biollante
     
  11. Tom Barr

    Tom Barr Founder
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    http://www.thekrib.com/Plants/CO2/

    You will note that some old references state 100ppm, 35-40ppm etc............

    I've been in this range for close to 2 decades.

    the KH discussion with Paul is good though.

    http://aquaticconcepts.thekrib.com/Co2/co2_faq.htm

    http://www.ca.uky.edu/wkrec/interactionsphetc.pdf

    This last ref above is very good at some of the more elusive questions.
    We use low Alk/low KH water for aquarium plants often times, they do VERY well.
    But we also add and enrich CO2.

    They state that KH above 20ppm will trap cO2 and allow it for use by plants/algae.
    But we add CO2 gas anyways.........so we have far more control there.

    This creates a research/hobby knowledge gap because of the enrichment we do.
     
  12. Paul G

    Paul G Lifetime Charter Member
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    I start with RO/DI water and reconstitute it. I maintain a GH of 5-6 d, around 100 ppm or so. I maintain a kH at 5 d, 90 ppm. These numbers work for all the plant-compatible tropicals I keep, as this is relatively soft water suitable for my tetras and so on, but definitely softer than my local tap.

    With this alkalinity, the pH wants to rise to well over 7.5 with no CO2 injection. With CO2 injection sufficient to hold the pH at 7.0, the kH/pH chart shows I should have CO2 solution of 15 ppm. Since I do not use peat or any other means of supplying acidity, I am confident that this is a pretty accurate estimate.

    This CO2 level is working very well in connection with my lighting scheme in terms of plants thriving, algae kept in check, etc. I use a pressure system with an automatic pH controller. There are two Red Sea powerhead type reactors, one at each end of the tank, about 6 inches below the surface.

    I have never checked the pH of water directly at the reactor discharge. I can't see what use this information could possibly be! The general in-tank circulation I have provided pretty much mixes the water column up more or less immediately, so my pH reading is indicating my CO2 solution in the environment. And that's the info I really need.

    The pH fluctuates within .05, and the cycling of the CO2 valve is nearly periodical. It begins almost immediately as the lights come on in the morning, and stops when the lights go out. The pH does try to drop at night, but the controller turns on the aerator. This drives the CO2 solution back to normal at least, so as to mitigate the pH drop.

    Clearly, I think there's a lot to be said in favor of this automation. It isn't at all necessary to use a pH controller if you get a handle on how to manage your CO2 injection by other means. You do have to make sure your pH probe is working properly, and the controller should be kept calibrated, but it sure is handy!
     
  13. Tom Barr

    Tom Barr Founder
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    If you redo RO/DI, it's fairly close, some KHPO4 asnd some plant biomass will add a little buit to this, driftwood, but not a lot, still, in every case....the pH/KH relationshipo will either be correct or UNDER estimate the amount of CO2. Many think.....they have say 15ppm, when they actually have say 8-10ppm, or many think they have 40ppm and they have 20 ppm. This works best right after a massive water change with RO/DI......but as time passes, without another water change........this can change. KH can also be consumed but plants and by some sediments where denitrification takes place, producing HNO3. This will destroy some KH also.

    I've measured CO2 differences of 2-4x within a 180 Gal tank with 3000 gph flow.
    Within the plant beds, it measured a mere 10-15ppm ranges.......while in the out flow of CO2: 60-70ppm.

    Mid level regions, about 30-40ppm. This was using a CO22 oxyguard meter. The tap I have is about 20ppm of Alkalinity.
    Pretty pure. Still, the pH/KH chart is all over the place, whereas the relative CO2/pH measures and ther CO2 meters are more representative of the standards and references.

    To make a simple reference standard: take a 5 gallon bucket and fill with a few gal of RO/DI and then add enough baking soda to get a specific KH, 17.86 or whatever.......
    Next, take a small powerhead, and add enough CO2 to get a specific CO2 ppm based on the pH/KH chart.

    Take your method from your aquarium and see how well it matches.

    Next move the CO2 from say 10ppm to 50ppm.

    Test the method you use in the tank.

    Allow at least 15 minutes to make sure the reading is stable.

    This should be fairly accurate.

    Take a few readings, say one as soon as the lights come on, then again, 30 min in, then 1 hour and then 2,4 and 8 hours in.
    If you use pH meters for this, it should account for any difference in the reference bucket vs the tank. You can make adjustments to that accordingly.
    I'm afraid I cannot say what a specific ppm CO2 optima really is for any specific tank, my tanks are all different in that regard.

    Some have 70-80ppm. 180 gal
    Others about 55-60ppm 120 Gal
    2 ppm for the non CO2 ADA 60p
    70 gal 45ppm.

    This is what "works" well for these tanks. Other tanks, maybe less, or more..........
    I do not measure aquariums that are doing poorly, their plants are not taking up CO2 at the same rates as a healthy tank, this reading while somewhat informative, is not nearly as informative as a healthy reference tank.
     
  14. bsmith782

    bsmith782 Guru Class Expert

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    For many years I have heard/read that co2 readily escapes from the water from surface agitation and other movement of water causing interaction with air.

    But recently, after really cranking up the co2 in one of my tanks im starting to doubt that co2 leaves the water in our tank that quickly.

    We all know that drop checkers are far from accurate and often have a couple hours of lag time from what it is showing and what actually is in the water.

    Well, my drop checker is still mountain dew yellow after having the co2 off for 16 hours! I even have quite a bit of surface agitation in this tank too.

    So what the real deal here with real tests, Tom?
     
  15. shoggoth43

    shoggoth43 Lifetime Charter Member
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    How clean is your drop checker? If you've ever seen that whitish film at the water surface in your tank, then you probably have that in your drop checker too. I suspect that may be something that will prevent the checker from reacting quickly and may completely prevent the exchange of gases at all within the drop checker. I have no idea if that's true, but it may be a contributing factor and many people clean/move the checker and replace the fluids every few weeks..

    CO2 seems to come out pretty easily. The data Tom's shown on his, way out of my price range, dissolv-O-matic CO2 meter showed something like 45 minutes to an hour to get at least one of his tanks down to static non CO2 injected levels. I think that was the wet/dry vs. cannister dissolved CO2 thread but I'm probably mixing threads up at this point. I don't recall if it was ever determined that the surface skimming causes the outgassing or all the splashing down to the sump through the biotower that did it.

    The "CO2" that people find at the end of the day in the reactor may just be the trace gasses from the tank that aren't CO2 and are less soluble.

    -
    S

     
  16. bsmith782

    bsmith782 Guru Class Expert

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    There isn't much film if any as I have an Eheim 2217 filter on this tank with a spray bar that is pointed at the surface a bit and I get really nice surface movement.
     
  17. pat w

    pat w Member

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    I'm convinced it's the lack of a surface film. After I started using surface skimmers I had to increase the injection rate and my checkers are back to non-injection levels every morning.

    @bsmith - You could help answer this by doing some extra cleaning on your checkers and note any changes. Your tank may be off gassing but your checkers could be blocked - so-to-speak.

    Pat
     
  18. bsmith782

    bsmith782 Guru Class Expert

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    I clean my DC's weekly or every two weeks at the most.
     
  19. Tom Barr

    Tom Barr Founder
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    You need a reference to compare the reading to.

    Something............both pH and KH, not just one or the other, but BOTH.

    My CO2 was 20 ppm in my 180 Gal with the canister filter + 3000 gph of movement.
    After I added the wet/dry sealed?

    Dropped to 2-3 ppm in about 45-60 min.

    Stays there, till the morning.
    That alone was enough to convince me to switch.

    Same tank, same CO2 method, 1 week apart was the only difference.

    I did not expect that.

    If the method was poor, I measured this and it dropped to what it should be with the wet/dry which skims.
    So there's not much doubt there, only the upper ranges and the DI water KH ref bucket test showed similar ranges, within 1-2 ppm. So I'm okay with that. I include an error range of 5+/- ppm which is pretty generous.
    When we get under 30ppm, then each ppm is more meaningful I would argue.

    I have some special thin silicone gas membrane material. I made a KH reference solution for that and rubber grommet and a flat tipped high grade pH probe to make my CO2 probe.
    I am still testing it.

    Should be done sometime this week. This allows a pH meter/probe to be used and has a much faster response time than a DC and gets around the KH issue with the pH meters.

    BTW, the CO2 in the ADA tanks I measure with the CO2 meter, they where all way over 40ppm.
    They where not......15ppm or less.
     
  20. bsmith782

    bsmith782 Guru Class Expert

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    I would venture (totally a guess since I am only using DC's) that my co2 is in the 70-100ppm range. I have it turned up about 2x's the amount that I was using when I gassed/caused all my neons to jump ship (IME, neons/cardinals are much more tolerant to high levels of co2 than any other fish/animals I have had in injected tanks).

    I think it would be a very good experiment to actually test how long it takes co2 to off gas from xppm with different amounts of surface agitation/water-air interaction.

    Send me your co2 meter and I will put it to good use and promise that I will not break it! :D
     
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