co2 readings - trust LaMotte testkit over pH/KH cross? - The Planted Tank Forum
 
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post #1 of 5 (permalink) Old 05-04-2005, 03:00 AM Thread Starter
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co2 readings - trust LaMotte testkit over pH/KH cross?

my pH/KH crosspoint on the co2 chart gives me 33ppm co2, but the LaMotte testkit gives me a 18ppm co2. In my 180g (with no co2 injection), the Lamotte testkit shows 25ppm co2.

I am using a pinpoint pH monitor to give me my pH reading.

Not sure what reading to trust. Average them maybe?

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post #2 of 5 (permalink) Old 05-04-2005, 03:16 AM
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It turns out the LaMotte CO2 test is simply a pH test. LaMotte themselves say the kH/pH chart is more reliable.

Code:
_______ E-Mail from LaMotte Technical Services ______________________

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.
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post #3 of 5 (permalink) Old 05-04-2005, 03:27 AM Thread Starter
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quite interesting. thanks for that post!

think the acids used in buffers causes an irregular reading in that LaMotte testkit as well?

It seems odd that I have such a high co2 reading in a tank that I have no form of co2 injection, yet I use a neutral buffer in it to keep a 7.0 pH.

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post #4 of 5 (permalink) Old 05-04-2005, 11:51 AM
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Quote:
It seems odd that I have such a high co2 reading in a tank that I have no form of co2 injection, yet I use a neutral buffer in it to keep a 7.0 pH.
Ok, if you are not injecting CO2 then you would not have high CO2 levels. Any type of buffer that is NOT carbonate based is going to skew the pH/kH/CO2 table.

TANSTAASFL applies here.
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post #5 of 5 (permalink) Old 05-04-2005, 02:22 PM
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Quote:
Originally Posted by Spar
It seems odd that I have such a high co2 reading in a tank that I have no form of co2 injection, yet I use a neutral buffer in it to keep a 7.0 pH.
As Rex mentioned, there's two kinds of pH buffering products. Carbonate based, and phosphate based. The CO2 chart assumes nothing but carbonate based buffers in the water. In any system, there is *some* degree of non carbonate buffering, so the chart reading is a "best guess," but if for example you use RO/DI water and nothing but carbonate based buffering, the better the "guess" becomes.

Baking Soda is a bicarbonate based buffer (sodium bicarbonate). Seachem Alkaline Buffer is another carbonate based buffer which is mostly sodium bicarbonate, but also some potassium bicarbonate and magnesium bicarbonate. These buffers raise KH. Seachem Acid Buffer is bisulfate which reacts with the bicarbonate to lower KH and form CO2 (which gases away). These all are compatible with the CO2 chart.

Phosphate based buffers are a bit stronger in their ability to keep the pH stable but they completely throw off the CO2 chart because none of the buffering is coming from KH and CO2. They can also cause higher phosphate levels which can cause algae outbreaks in tanks with high light (which most planted tanks are).
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