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CO2/pH Data

Discussion in 'Advanced Strategies and Fertilization' started by VaughnH, May 23, 2006.

  1. VaughnH

    VaughnH Lifetime Charter Member
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    I just finished doing some testing to see how long it takes for a tank sample of water, sitting out in the open air, to reach a stable pH, and how much CO2 is in a sample of distilled water with only bicarbonates in it for alkalinity, after it has sat out in the air for a considerable time. Obviously I am trying to convince myself that the two pH measurement method of determining how much CO2 we have in our tanks is reasonably accurate. My data is graphed on the attachment. I didn't succeed in convincing myself that our CO2 measurements are any good.

    The distilled water, which had a KH of 4.5, ended up with only about 1 ppm of CO2 and it was stable at that amount. But, we have assumed that it would have 3 to 4 ppm of CO2. I wouldn't believe that at all, but the KH/pH/CO2 equation for the tank water says it had only about 13 ppm even though the drop in pH from room air condition to tank condition was about 1.0. But, if the room air sample actually did have about 1 ppm of CO2 in it, the 1.0 drop would mean about 10 ppm of CO2. Have I confused everyone, or is my plight clear?
     
  2. yme

    yme Lifetime Charter Member
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    Re: CO2/pH Data

    I thought that there was an equilibrium with the air at a CO2 level of 0.5 mg/l. but only if there is no biological activity that produces CO2.

    greets,

    yme
     
  3. neil1973

    neil1973 Prolific Poster

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    Re: CO2/pH Data

    There must be physical laws that dictate the equilibrium concentration of CO2 in fresh water in relation to temperature and atmospheric pressure. Hopefully someone who understands such things can enlighten us; unfortunately that person is not me!

    I have tried googling for this in the past and have also seen references to 0.5ppm although this was on an aquarium forum thread (can’t remember where I’m afraid) and contained no references or supporting evidence.

    When water from my tank is left for 24h then according to a calibrated ph meter, standard liquid kh test and chucks ph chart I have about 3ppm CO2. This agrees with VaughnH’s original observations. It does seem likely though that in an aquarium there will be acids other than carbonic that may contribute to an over estimate of CO2.

    These days I tend to use careful observation of the fish to set a maximum CO2 level.

    Thanks to VaughnH for taking the time to do these experiments and raise some interesting observations and questions.

    Regards
    Neil
     
  4. vidiots

    vidiots Prolific Poster

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    Re: CO2/pH Data

    I am not much of a chemist, but have read that their are other things in your tank water that will buffer the pH besides just KH. One of them is phosphate. I would imagine that you are adding phosphate and other things to your tank to fertilize your plants. If you did not add those same things to your distilled water it's buffering capacity will be different and change the curve.

    I too have wondered about the accuracy of the pH & KH method of determining CO2. The only other method I have tried is the Red Sea liquid test kit for CO2, and I did not like the measurements it gave me. It told me that the CO2 level in my tank without any CO2 injection was about 30ppm, while the pH & KH method came up with a value of 2ppm.

    In metrology the way a measurement method is validated is to compare the results of different methods using equal conditions for all methods tested. In my case I determined that the pH & KH method was better than the CO2 test kit, because the measurement was much closer to what I would have expected. Generally different methods have different variables that effect their results, and unless those variables are known and accounted for the results can vary signifigantly.

    If I had to guess at what the variables are I would say temperature is a big one because that effects every type of measurement made to some degree. Others would be the amount of actual CO2 in the ambient air, the surface area of the sample exposed to ambient air, the barometric pressure, air drafts above the sample, water currents in the sample, the amount of buffering chemicals in the sample.

    To determine the degree of an effect of a variable on a measurement you would have to repeatedly test changing just one variable at a time.

    Very time consuming process huh? :)
     
  5. quenton

    quenton Guru Class Expert

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    Re: CO2/pH Data

    Here is a note from lamotte posted on another site -- in answer to the question of why the test results seem strange.

    ----------------------------------------------------------------------

    The test method for carbon dioxide is an acid/base titration: meaning that a
    base (sodium hydroxide) is added to the sample until it neutralizes all of
    the acid in the sample and brings the pH of the sample up to 8.3 (shown by
    the color change of phenolphthalein). Carbon dioxide in water is an acid, so
    it is titrated by the base. But, any other acid in the sample will also be
    titrated by the base, including the humic acid in your sample. Also, any
    bases already in the sample will affect the titration. Unfortunately, there
    is no way to accurately calculate out these interferences.

    For this reason the carbon dioxide titration test is considered to be a quick
    field test method, only.

    The nomographic method is more accurate, if the pH and alkalinity are
    measured accurately.
     
  6. pigwiggle

    pigwiggle Junior Poster

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    Re: CO2/pH Data

    I’m not an analytical chemist, but here is what I remember from my thermo classes. There are a few laws that can be used to calculate the concentration of a dissolved gas in water; all depend on temp, the concentration of the gas in the air, atmospheric pressure, and some solvent/gas dependant constant. Henry’s law is the most ubiquitous, but I think there are others that account for more complicated stuff, if I remember correctly.

    Henry’s law only gives the dissolved gas concentration. You need to know all the stuff that is going to bind up the gas in the water (like water and calcium carbonate) and use tabulated equilibrium constants to find the equilibrium concentrations. But then the original calculation is off due to the gas taken up in those reactions. If I remember correctly you can just iterate the whole thing until the concentrations converge.

    But we are not in equilibrium when we are measuring dissolved CO2. We load the water with CO2 and then try to infer the concentration by looking at pH (putatively the carbonic acid concentration) and calcium carbonate concentrations. We neglect other acids and other buffers.

    Here is a good wikipedia page for you.


    http://en.wikipedia.org/wiki/Carbonic_acid
     
  7. VaughnH

    VaughnH Lifetime Charter Member
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    Re: CO2/pH Data

    There are other sources of alkalinity in our tank water besides carbonates or bicarbonates, but they are not normally there in concentrations comparable to the carbonates. For example, at a true KH of 4, there are about 70 ppm of carbonates in the water. We dose phosphates to get less than one tenth of that amount of ppm, nitrates at less than one third of that amount, sulfates (sometimes) at less than a third of that amount, etc. So, at worst, more than half of the alkalinity of our water should be from carbonates. But, the reverse is that our alkalinity may be due almost as much to non-carbonates as to carbonates. So, expecting an accurate reading of true carbonate hardness, ppm of carbonates, may be unreasonable. Our measured KH of 4 may actually represent a true KH of closer to 2, which would mean we have half the ppm of CO2 that we thought we had.

    But, our desired ppm of CO2 is based on nothing more than experience, so maybe we don't care if we actually have only 15 ppm when we think we have 30 ppm? Color me confused!
     
  8. Tom Barr

    Tom Barr Founder
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    Re: CO2/pH Data

    I support such discussion fully.

    I've long held that adding enough CO2 is the crux of many's issues with plants that use CO2.

    I've pretty much gone to doing eye balling of CO2.
    I know Amano does the saem as do many others.

    As you increase the light, the test seem to to have more issues, eyeballs become better at hitting the right amounts.

    Virtually no one suggested more 10-15ppm before I started saying 20-30ppm. Some germans folks were saying that 30-40ppm did well etcIvia communicay Karen Randall), but few English speaking folks would say this.

    pH and KH both have measurement issues, so this is not a bad place to start, then tweak from there.

    How much? Hard to say, the 1.0 pH unit drop method seems fairly decent and better in some respects.

    Lamotte is correct, that was a good response.

    Regards,
    Tom Barr
     
  9. vidiots

    vidiots Prolific Poster

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    Re: CO2/pH Data

    Your charts do seem to match up with what others have said about the time required for the CO2 levels to drop back to equalibrium with atmosphere. Looking at your chart looks like reguardless of whether or not it's pure water or tank water, it'll still take a minimum of 12hrs and possibly up to 18hrs for it to reach this point. I would be curious to know if this process can be accelerated by airating the water in the sample without messing up the desired results too much.

    The calculator I like to use for CO2 is:
    Measuring CO2 levels in a planted tank
     
  10. Tom Barr

    Tom Barr Founder
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    Re: CO2/pH Data

    You might want to make more data points, but unless you have the time etc, it's rough. Make a few more data points, especially where the curve is steepest.

    Regards,
    Tom Barr
     
  11. VaughnH

    VaughnH Lifetime Charter Member
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    Re: CO2/pH Data

    Next time I find the time I will repeat this, starting early in the morning, to get more data points up to about 16 hours, but I refuse to give up a night's sleep for it! I also will do KH vs time to see if water evaporation is pushing the KH up. And, if I really get ambitious (meaning, not likely) I will run some CO2 into a sample of distilled water (wouldn't just blowing my personally generated supply of CO2 suffice?), with a bit of bicarbonate in it, before testing it out in the open air, to see what the pH does vs time for that.
     
  12. vidiots

    vidiots Prolific Poster

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    Re: CO2/pH Data

    Don't know for sure, but I doubt there would be enough CO2 in your breath to signifigantly change the pH. You don't exhale pure CO2, just a slightly higher concentration than you inhale. I think you'd have to suffocate yourself in an air tight container to get the CO2 up high enough in the container to have an effect, which is probably not a good idea unless your competing for the "Darwin Award". :eek:
     
  13. Tom Barr

    Tom Barr Founder
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    Re: CO2/pH Data

    I'd just use a large sample, with a narrower neck, that will address the evaporation to some degree.

    Yes, no sleep and not having more than a few hours in between each test etc is a PITA, but you can see a lot goes on in a just one day when it comes to CO2.

    Both leaving and going into the tank.

    Then you have other things at different time scales such as nuterints like NO3, then even longer scales like Fe etc.

    Regards,
    Tom Barr
     
  14. yme

    yme Lifetime Charter Member
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    Re: CO2/pH Data

    here is my data:

    [​IMG]

    my tap water was at a pH of 6.17.
    for measuring the KH I used a simple titration test that changes from yellow to blue. I doubled the amount of testwater to get a better accuracy. It took five drops to get from yellow to blue, indicating the the KH of the testwater was 2.5
    I noticed that after 9 hours, the testwater was slightly green after the fifth drop. after 12 hours the testwater was clearly green as well as 16 hours later. after 25 hours the water was still yellow after adding the fifth drop and became blue after the sixth drop, indicating that the KH was now 3.

    So, I might have some evaporation problems (I used a long glass) and the KH appears slightly higher after one day but I do not see the nice asymptotic curve.

    Maybe things need to be standardized? e.g. the amount of water, the kind glass/jar that is used...

    greets,

    yme
     
  15. yme

    yme Lifetime Charter Member
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    Re: CO2/pH Data

    additional information: yesterday (after taking the sample) I lowered the pH by 0.5 to 6.10. now I see that half of the shrimps are located just beneath the surface: stress symptoms. This indicates that a pH of 6.15 is as low as I can go.

    I also started looking for articles about CO2 toxicity in fish, but couldn´t find any reliable sources. Is there any scientific evidence that CO2 levels above the 30 mg/l are toxic?

    greets,

    yme
     
  16. quenton

    quenton Guru Class Expert

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    Re: CO2/pH Data

    Your graph hints that the pH may still be rising at 25 hours -- did you take any samples after that?
     
  17. yme

    yme Lifetime Charter Member
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    Re: CO2/pH Data

    I just came home, so I wasn´t able to take more samples during the day. I just now took a sample. the pH was now 6.8, the KH was still 3. if both are correct than I still have 1.4 mg/l CO2 in the water according to the pH/KH/CO2 tabel. Thus the pH will probably rise even more.

    [​IMG]

    greets,

    yme
     
  18. VaughnH

    VaughnH Lifetime Charter Member
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    Re: CO2/pH Data

    This is very interesting! It sure looks like our two-pH test technique doesn't work at all. There doesn't seem to be any time within reason when we can say we know what the ppm of CO2 is, so knowing the drop in pH from an aerated sample to a tank sample won't give an accurate measurement of CO2.

    I wonder if a person blowing bubbles into a water sample "injects" a known ppm of CO2 into the water, with the amount stabilizing within a short time? I doubt it, but if it did, we could do the test by first finding out what that ppm is, then telling folks to blow in a straw in the tank sample for, say 15 minutes, then measure the pH and check the difference from that to the tank. Surely all people exhale about the same concentration of CO2.
     
  19. N_E

    N_E Junior Poster

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    Re: CO2/pH Data

    Like this:

    "By the way, Krause recommends a different method to measure CO2 content. That method is insensitive
    to the presence of unusual buffers.

    Step 1: Take small water sample with some pH indicator. Stick a straw into the water and exhale
    through the straw into the water sample for two or three minutes. This sets the CO2 level at 60ppm.
    Take note of the pH reading.

    Step 2: Take another water sample with some pH indicator and run the hose from an air pump into the
    sampe for a few minutes. This sets the CO2 level at 0.5ppm. Take note of the pH reading.

    Step 3: Measure the pH of the tank water. The CO2 content is proportional to that reading. As an
    example, if you measure pH 6.0 with the first sample, and pH 8.0 with the second sample, a tank pH
    of 7 corresponds to 30ppm CO2, regardless of any buffers that might otherwise confuse a CO2 test or
    distort the charted values."

    From: http://forums.cichlidfish.com/portal/forums/printthread.php?threadid=13043&perpage=16
     
  20. VaughnH

    VaughnH Lifetime Charter Member
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    Re: CO2/pH Data

    This is fascinating to me, but I may be easily facsinated. The 60 ppm number sounds awfully high to me, but I have no data to dispute it. Now I have my next research project mapped out: take some distilled water, add a tiny bit of bicarbonate, blow in it for a fixed time period, measure pH and KH, thus ppm of CO2. Repeat for other blowing times. Plot how fast the resulting pH drops - if it drops in seconds this isn't useful, but if it is minutes, it is. Repeat this using an aquarium sample to see if the blowing will also stabilize a higher CO2 solution. Finally, use the stabilized "blowhard" ppm of CO2 to get the equation and chart for tank CO2 vs the change in pH from the tank to "blowhard". Who else wants to try this?
     
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