Questions about the CO2 concentration of water at atmospheric equilibrium - The Planted Tank Forum
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post #1 of 3 (permalink) Old 10-29-2015, 07:41 PM Thread Starter
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Questions about the CO2 concentration of water at atmospheric equilibrium

It's been my understanding that when injecting co2, a 1.0 drop in pH should equal roughly 30 ppm of CO2, and this is because the concentration at equilibrium with the surrounding air should be around 3 ppm. Looking at a pH/KH/CO2 chart, using 3 ppm as the starting point and following to the left at any KH, a pH difference of 1.0 point always equals 30 ppm.

Well... I've heard 3 ppm, I've also heard it's likely closer to 2 ppm, I've also heard it's anywhere from 2-4 ppm. Where did these values come from? Knowing the atmospheric co2 concentration, I assume we can calculate co2 concentration in aqueous solution at atmospheric equilibrium using Henry's law. Is this correct? Has anyone done this?

Another question I have is how much does temperature affect the co2 concentration at atmospheric equilibrium?
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post #2 of 3 (permalink) Old 10-29-2015, 08:22 PM
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My understanding has always been that the chart is a guideline for a nonexistent ideal situation.

This is an excerpt from a 2008 article in AquaScapingWorld on why this is the case:

There are too many variables in aquarium water to use the pH/KH/CO2 chart/calculator. The CO2 pH/KH relationship is a good guideline to get a generally reading of C02 levels, but it not exactly perfect. It assumes tested aquarium water has perfectly set levels of KH and pH which don’t vary. In reality that’s nearly impossible to have such water. There are phosphates, tannins, organic acids and bases that come from fish waste, decaying plant matter, and nitrifying bacteria that will affect the rising and falling of pH on a daily basis. KH readings can be misread due to dissolving minerals from stones, shells in the substrate, and other hardscape materials. As a result, if you rely on a strict value of KH and pH alone, you may find a false CO2 measurement which, in most instances, indicates that you have more CO2 than you actually have in your aquarium.

AquaScaping World Magazine - Understanding the pH/KH Relationship
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post #3 of 3 (permalink) Old 10-30-2015, 03:07 AM
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Several years ago I did some testing to try to find out what the equilibrium ppm of CO2 in water open to the air was. I measured CO2 ppm levels from about 1 to about 3, depending on how long I left the water exposed to the air. Given that it is very hard to measure pH more accurately than about +/- .2, my readings really covered a possibly wider range. But, it has been accepted that the "correct" number to use is 3 ppm of CO2, based, as I recall, on a theoretical calculation, not measurements. So, I accept that number as the best we have. A possible complication for an aquarium in a home is that the CO2 concentration in the air inside the home can be a bit higher than the average concentration in the outdoor air.

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