Hach CO2 Test Kit

VaughnH

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Jan 24, 2005
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Tom, is this CO2 test kit accurate for our purposes?
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==|
This is the first time I have heard of such a simple procedure for measuring CO2. It is a bit pricey, but if it is accurate for the typical aquarium water we use, with various carbonate sources in it, various sources of acidity, etc. it would be well worth it. I downloaded the PDF of the instructions from the site, and for medium ranges of CO2 it says:
1. Fill the mixing bottle to the 15 ml mark with the water sample.
2. Add one droop of Phenolphthalein Indicator Solution to the sample.
3. Add the Sodium Hydroxide Solution drop by drop. Count each drop
as it is added. Swirl the bottle to mix after each drop is added.
Continue adding drops until a light pink color forms and persists for 30 sec.
4. Each drop of Sodium Hydroxide Solution used equals 2 mg/liter CO2.
 

quenton

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Mar 14, 2006
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Re: Hach CO2 Test Kit

Lamotte also has an expensive CO2 test. Here is a note that they provided to someone (via another web site), which talks about accuracy of CO2 testing.

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

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.

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

Not sure what the "nomographic" method is, but since it obviously involves pH and KH, I assume it just means the old CO2 table (a formula exists too), which tends to be inaccurate because our pH and KH tests are inaccurate.
 

VaughnH

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Jan 24, 2005
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Re: Hach CO2 Test Kit

Thank you Quenton. Well, that shoots down another good idea! And, once again it proves if something seems too good to be true it probably isn't true. But, you saved me over $40 trying this out myself.
 

quenton

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Mar 14, 2006
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Re: Hach CO2 Test Kit

I know tom pushes using the pH range test.

The one where you take a water sample (few oz's) and leave it out for 24h.
Then test that and the tank, the 24h aged water should be around 1.0 to 1.1 higher than the one in the tank.

One downside of this one is that for same ranges and some test kits, you end up using different reagents for each of the tests, which gets one back into test accuracy again. If they can both be in the same test-kit range I suspect you are likely to get a more accurate result.