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c02 in Kpa (kilopascal)

Discussion in 'CO2 Enrichment' started by guy tillmans, Aug 18, 2008.

  1. guy tillmans

    guy tillmans Guru Class Expert

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    Here on the ICU we have very high/precize equipment to measure co2 in Kpa in the air. My question is, is there any relationship between Kpa and ppm or mg/l. I know that Kpa measures pressure and ppm is a concentration. What do i want to do: Fill a bottle (for 3/4) with my tankwater, close the bottle. The co2 will equalize in the trapped air(i assume) from the water. So in the trapped air will be the same amount of co2, that i can measure in Kpa. Is this a usefull method or does it not make any sense.

    thanks
     
  2. VaughnH

    VaughnH Lifetime Charter Member
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    Air having X ppm of CO2 doesn't mean it will reach equilibrium with water in contact with it at X ppm of CO2 in the water. It is the partial pressure of the CO2 in the air that affects the concentration of in-contact water, and that is determined by an equation which I don't have right now. If air and water had the same density I assume the ppm in the water and in the air would be the same.
     
  3. guy tillmans

    guy tillmans Guru Class Expert

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    So my thought does not make sense. So i will take a monster of my tankwater (in a closed little bottle) and take it to the laboratorium to determine the total co2. So i'nm sure about targeting the 30-40 ppm. I've a ph controler system and a dc. I've read that this doesn't give any garanty about your ppm.
     
  4. bibbels

    bibbels Prolific Poster

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    Hi,

    This is something I've learned firsthand along the way:D . My pH controller is now only monitoring. I previously had it set to keep pH 6.2 - 6.4 thinking this would provide enough CO2. When I finally got my drop checkers going with 4dKH fluid and after algae issues began I really saw how far off I was.
    Now when my DC is yellowish/green I note my pH varies 5.5-5.8 (reads 6.8-7.0 lights off/CO2 off). Even with the DC yellowish/green there is still no guaranty that CO2 is adequate, especially after reading http://www.barrreport.com/co2-aquatic-plant-fertilization/4601-co2-situ-meter-measurements-real-time-data-localized-co2-ppm-readings.html
    Co2 is not as easy as I first thought as Hoppy said so well:
    Ryan
     
  5. guy tillmans

    guy tillmans Guru Class Expert

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    your ph varies between 5.5 and 5.8 ????, but what is your kh then? If its about 4 your ph controler is far off the road and needs to be set . Mine is 6.4 at the moment with 4 kh. My dc is yellowish/green.
     
  6. bibbels

    bibbels Prolific Poster

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    Actually my KH is about 4 according to an API test kit. There lies the problem with the KH-pH-Co2 chart. Clearly I don't have 200-300 ppm CO2 as the chart would indicate. My fish have never been gasping at the surface or shown other signs of stressed behavior.

    Many substances can intoduce great error into the KH-pH-CO2 chart such as phosphates tanins, and humic acids. Just having a bunch of wood in your tank -as I do- can easily make the chart unreliable.
     
  7. VaughnH

    VaughnH Lifetime Charter Member
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    It is going to take a lab that has equipment similar to Tom's to be able to test that water sample and tell you how much CO2 is in it. And, that won't be easy either, since the water will quickly lose the CO2 as soon as it is exposed to air. It will be interesting to hear what the lab tells you, so be sure to post their results here.
     
  8. guy tillmans

    guy tillmans Guru Class Expert

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    ok next week i will post it
     
  9. guy tillmans

    guy tillmans Guru Class Expert

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    So,
    This morning I took a sample of my tank (just before lights went on), I didn't shut off the co2 last night , so at the time i took the sample my ph was 6.4 (ph controler). I took the sample in a closed bottle(for blood samples) without any free air, so there could be no escaping co2> In the hospital lab they did the required measurements. My Ph was 6.5 (so my ph controler is very accurate) and my co2 was 4.2 mmol/liter. I cant find the translation to mg/l or ppm. My kh is 4. In theory i should have about 30-40 ppm co2 in my tank.
    Can anybody make a translation to mg/l or ppm for me ?? thanks.
     
  10. guy tillmans

    guy tillmans Guru Class Expert

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    I found the following:
    # Atom Molar Mass (MM) Subtotal Mass Subtotal Mass
    (g/mol) (%) (g/mol)
    ------------------------------------------------------------------------------
    1 C 12.01 27.29 12.01
    2 O 16.00 72.71 32.00
    _________________________________________________
    Total Molecular Weight: 44.01

    So the molar weight of CO2 is 44grams per mol, so 1 millimol/L is 44 milligrams per liter. 44mg/L is 44ppm.
    So lets go back to my sample 4.2 mmol/liter, makes 4x44 ppm = 176ppm of co2!!!!

    Is this correct?????
     
  11. jeremy v

    jeremy v Guru Class Expert

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    That's about what I came up with.
    In order to convert from mmol/L to mg/L you just have to multiply your measurement in mmol/L by the molar mass of CO2.

    C- 12.01g/mol
    O- 16.00g/mol

    So 12.01g + 2(16.00g) = 44.01g CO2/mol

    Your measurement of 4.2mmol/L x 44.01g CO2/mol = 184.842mg/l = 184.842ppm, so something is wrong with the measurement unless I messed up with my math (which is possible, haha).

    I will show the whole equation so you can see all the units cancel out and change to achieve this result below. Notice how the two conversions just multiply and then divide by 1000 so you just have to multiply by molar mass and that's it.

    Full equation-

    (44.01g / 1 mol CO2)(1mol CO2/1000 mmol CO2)(4.2 mmol CO2/1L H2O)(1000mg/1g)

    = 184.842 mg/L H2O, it also equals
    = 184.842 ppm CO2 since 1mg/L = 1ppm in water.

    Are you sure the measurement wasn’t 0.42mmol/L instead of 4.2mmol/L? If it was, that would then give you 18.4 mg/L or 18.4 ppm which would sound about perfect based on your parameters, and the fact that the kH vs. CO2 table has a tendency to inflate CO2 concentrations.
     
  12. guy tillmans

    guy tillmans Guru Class Expert

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    Yes i'm sure, this were the results. Ph 6.5, pco2 5.08 Kpa, po2 23.18 Kpa, hco3(activ) 2.9 mmol/l, hco3 (standard) 2.4 mmol/l, Co2 4.1 mmol/l and Base excess -38 mmol/l. The only thing wath is wrong that they measured with a temp of 37 degrees Celcius, but the real temp of the sample must be something about 20 degrees Celcius.
     
  13. guy tillmans

    guy tillmans Guru Class Expert

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    They use the 37 degrees celcius because of the blood temp. of human beings.
    Could the temp. influance the outcome of the measurement??
     
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