# Why CO2 is hard to measure



## dennis (Mar 1, 2004)

Here is link a paper explaining why CO2 in aqueous solution is so hard to measure. Apparently the authors' way is easy! 

http://www.plantphysiol.org/cgi/reprint/72/1/212

This reminded me, in a recent lab we measured off-gassing CO2 concentrations from decomposing organic matter by first capturing the produced CO2 in NaOH. The solution and organic material were in a sealed jar. 
CO2 + 2OH <=> CO3 (2-) + H2O not sure where the Na goes there

Next, the whole solution is mixed with BaCl2 because apparently BaCO3 is extreemly insoluable.
Ba(2+) + CO3(2-) -> BaCO3 (amorph)

Lastly we added phenolphthalein indicator and titrated with .5M HCl. The ml of HCl needed to change color is inserted into the equation (9-(ml HCl/2))/2. This gives one the concentration of CO2 oroginally produced in mmol. I have no idea how to convert mmol to our standard ppm.

Some toughts on this... I suppose it would be possible to add the BaCl2 directly to a water sample, skipping the NaOH step as its purpose was to collect the CO2 from the air. The sample could then be titrated with HCl as originally and somehow you would need to subtract any CO3 that originally came from the water's hardness. Accurate measurment here would be critical. pH might be important also as the carbonate to bicarbonate ratio changes at pH lower than 6.5 I believe. Another problem, neither BaCl2 nor HCl are things the average hobbiest would want to play with.

Now, I am sure that method is to "simple" to be accurate or someone would already be using it. I would not mind trying it though...maybe my old professor would help me out. If memory serves, a few of our members here have the knowledge and access to try this out if it seems feasable. Hint, hint HeyPK, SCMurphy or maybe Gomer


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## hoppycalif (Apr 7, 2005)

I haven't heard of any theoretical determination of how much CO2 is optimum for an aquarium. It isn't like KH, GH, pH, temperature, etc, where there is some research and analysis that suggests acceptable ranges. All we have for a range for CO2 is some experimenting with a flawed theoretical equation relating pH, KH and ppm CO2 to each other. But, since that equation just doesn't work accurately for real world aquarium water, the number we use as goals are almost imaginary - 20 -30 ppm? Those numbers may have actually represented real concentrations of half or even double that amount.

So, why do we care how much ppm of CO2 we have in the aquarium? Isn't the better standard one we develop for our individual tank, light, fish, etc. by careful observation of the plants, algae and fish? If so, it doesn't matter how we measure CO2 or even if we measure CO2.


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## Zapins (Jul 28, 2004)

I don't know if it really matters that much what that CO2 content is in the end, but I feel like if we all accumulated an accurate CO2 ppm reading from our tanks then we could really start to eliminate "other variables" that frequently are brought into question when something goes wrong.

For example, many people feel that a lot of problems are simply due to "not enough CO2," Tom Barr was one of these people. I think that if we had a testable, reliable method for measuring the concentration of CO2, whether that method is dennis' or someone else's it would solve a great number of second guessing and time.


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## rrguymon (Jul 22, 2005)

Zapins said:


> For example, many people feel that a lot of problems are simply due to "not enough CO2," Tom Barr was one of these people. I think that if we had a testable, reliable method for measuring the concentration of CO2, whether that method is dennis' or someone else's it would solve a great number of second guessing and time.


Yes I agree. Combine the EI method with an easy and accruate way to inject and measure CO2 and planted tanks would be must easier to keep and I think alot more people would have them. I think a planted aquarium rivals saltwater tanks in beauty.

Rick


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## hoppycalif (Apr 7, 2005)

Zapins said:


> I don't know if it really matters that much what that CO2 content is in the end, but I feel like if we all accumulated an accurate CO2 ppm reading from our tanks then we could really start to eliminate "other variables" that frequently are brought into question when something goes wrong.
> 
> For example, many people feel that a lot of problems are simply due to "not enough CO2," Tom Barr was one of these people. I think that if we had a testable, reliable method for measuring the concentration of CO2, whether that method is dennis' or someone else's it would solve a great number of second guessing and time.


I agree, that a method that works as well and as simply as a KH test would be great. And, it would make most of us a lot more comfortable if we could simply adjust CO2 to a known value and go on to doing other more fun things. Some day, perhaps, the Seachem folks will produce such a simple accurate test.


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## bpimm (Jun 12, 2006)

How about this test kit?

http://www.hach.com/hc/search.product.details.invoker/PackagingCode=143601/NewLinkLabel=Carbon+Dioxide+Test+Kit%2C+Model+CA-23%2C+Drop+Count+Titration%2C+200+tests/PREVIOUS_BREADCRUMB_ID=HC_SEARCH_BROWSE_PARAMETER_RESULT/SESSIONID|BXpReU1UWXhOaVpuZFdWemRFZFpWVmhPTVRFMU13PT1DVFkzTQ==|

How can I name the link to shorten it?

Brian


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## dennis (Mar 1, 2004)

like this:

I use spaces so it is readable, eliminate spaces whan you do it.

[ url = http://yourlink.whatever ] Link (will appear in blue) [ / url ]


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## hoppycalif (Apr 7, 2005)

bpimm said:


> How about this test kit?
> 
> http://www.hach.com/hc/search.product.details.invoker/PackagingCode=143601/NewLinkLabel=Carbon+Dioxide+Test+Kit%2C+Model+CA-23%2C+Drop+Count+Titration%2C+200+tests/PREVIOUS_BREADCRUMB_ID=HC_SEARCH_BROWSE_PARAMETER_RESULT/SESSIONID|BXpReU1UWXhOaVpuZFdWemRFZFpWVmhPTVRFMU13PT1DVFkzTQ==|
> 
> ...


Wow! What a discovery! I haven't seen this test kit advertised anywhere. It looks almost perfect for what we need. It isn't cheap, but it isn't outrageously expensive either. I will ask Tom Barr if he has used this, has any opinion on it, etc. and then if it still looks good I think I will buy one to try it out. I just have to believe there is some unstated limitation on the kit, but I don't know what it would be.


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## bpimm (Jun 12, 2006)

hoppycalif said:


> Wow! What a discovery! I haven't seen this test kit advertised anywhere. It looks almost perfect for what we need. It isn't cheap, but it isn't outrageously expensive either. I will ask Tom Barr if he has used this, has any opinion on it, etc. and then if it still looks good I think I will buy one to try it out. I just have to believe there is some unstated limitation on the kit, but I don't know what it would be.


The quality of HACH test kist is second to none. I have used them for over 10 years and have found their test kits to be very reliable and repeatable. The color wheel kits are very nice as they compensate for the turbidity and color of your water.

I think my brother even had the C02 test kit, I'll have to ask.

Brian


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## Salt (Apr 5, 2005)

FYI, the Lamotte and Hach CO2 test kits are actually pH tests. So it's really no more effective than using a pH meter.

http://en.wikipedia.org/wiki/Phenolphthalein


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## BigBee (Jul 13, 2006)

Hello good people

Maybe I'm outdated a little, but what is the problem with the pH, kH and CO2 relationship?


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## hoppycalif (Apr 7, 2005)

Salt said:


> FYI, the Lamotte and Hach CO2 test kits are actually pH tests. So it's really no more effective than using a pH meter.
> 
> http://en.wikipedia.org/wiki/Phenolphthalein


No, the Hach kit isn't just a pH test, although it does use pH to determiine the end point of the titration. I asked at another forum and got the answer I was looking for. The story about the kit is: (from Hach)
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.


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## hoppycalif (Apr 7, 2005)

BigBee said:


> Hello good people
> 
> Maybe I'm outdated a little, but what is the problem with the pH, kH and CO2 relationship?


The pH, KH, CO2 relationship is based on having no other buffer except bicarbonates, and no other acid except carbonic acid in the water. Our aquarium water typically has some other buffers, and some other acids (tannic acid, for example) in it. So, the net effect is to make this test indicate more CO2 than is actually present. That's why so many people do the test and say they have 75 ppm or more CO2 in their water, even though that would be lethal to the fish if they actually had that much. It causes us to underdose CO2 and have problems with BBA and other algae.

The problem with the 1.0 drop in pH test, where we degas a water sample from the tank and try to add CO2 to get a 1.0 drop from that pH is that that test depends on the amount of CO2 in the degassed sample being about 3 ppm. But, we have found that this outgassing will result in from .5 to 4 ppm, depending on how long we wait for it to outgas, and other variables. So, the 1.0 pH difference, which is a 10X difference in ppm of CO2 in the tank, can mean anything from 5 ppm to 40 ppm in the tank - not at all accurate.


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## BigBee (Jul 13, 2006)

Now I understand your point about the 1pH change.
Even measuring the pH can be innacurate due to the CO2 loss as you shake the test recipient to mix the sample with the reagent (the 'conventional' test kit). I realised this just after I've started using CO2 in my tank and I 'shook up' a sample to pH=8 from below 7 in a matter of a few minutes.
In hope of getting more reliable readings I would suggest using a permanent visual CO2 test, but with the following modification: 
- instead of filling it with aquarium water use RO water, or something prepared from RO water; this way we would have a known kH value.
- instead of the provided reagent we should use normal pH measuring reagent. This would give the advantage of a more accurate pH reading from the test - because the permanent test kits I saw had only a qualiative Too much/Enough/Too less CO2 grading.


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## banderbe (Nov 17, 2005)

I don't pay any attention to my CO2 concentration anymore.

I keep the bubble count around 3 per second and forget about it.


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## Salt (Apr 5, 2005)

hoppycalif said:


> No, the Hach kit isn't just a pH test,


It is entirely based on pH. As you yourself quoted, any non carbonate based acid or base in the water is going to throw off the test, therefore, it really isn't any more reliable than using the kH/pH/CO2 chart.


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## hoppycalif (Apr 7, 2005)

Salt said:


> It is entirely based on pH. As you yourself quoted, any non carbonate based acid or base in the water is going to throw off the test, therefore, it really isn't any more reliable than using the kH/pH/CO2 chart.


I agree. Hach even tells people that it is only a "field test", not an accurate test.


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## bpimm (Jun 12, 2006)

So the Hach is not going to be amy more accurate than the PH/KH method?

oh well it was worth a look.

Brian


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