Equilibrium CO2 in water

VaughnH

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If I take a sample of water, let it sit out in the open for a day or so, how much CO2 will be in the water - the CO2 in equilibrium with the air? I ask because I calculated a chart of ppm CO2 vs. the change in PH that you see by measuring CO2 in the tank water and in the same water allowed to reach equilibrium with the air. I assumed the answer was 3 ppm, but I can't find the actual number anywhere.

CO2chart-1.jpg
 

PeterGwee

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Re: Equilibrium CO2 in water

Should be about 2-4ppm for a safe range. Temperature, altitude and etc might play some part I guess.

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Tom Barr

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Re: Equilibrium CO2 in water

NA, but interesting:

http://links.jstor.org/sici?sici=0028-646X(198506)100:2<141:PBFAIA>2.0.CO;2-C&size=LARGE

http://links.jstor.org/sici?sici=0028-646X(198012)86:4<393:TROABF>2.0.CO;2-7&size=LARGE

Try looking up henry's law and convert to ppm.
You should get about 3-4ppm, the variation is due to temp differences(cold water holds more gas, warm water holds less) and measurement errors.

That's one of those fun simple questions that is really tough to track down without knowning the chem equation.

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Tom Barr
 

VaughnH

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Re: Equilibrium CO2 in water

It's a fun simple question, but a real bear to get data on. I have been experimenting on the kitchen counter to see if I can come up with a good number for ppm of CO2 if a sample sits there for a few hours open to the air. I finished today, a bit wiser and better informed. (This effort is only because I got teed off at "measuring" 100+ppm CO2 in my tank, and knowing it wasn't even close to being right, then George Booth jogged my conscience by wondering why we would torture our fish to find out the right CO2 level. Now, I think I have a good method of making a reasonable guess at the CO2 level, so I don't sound like an idiot saying I run 170 ppm all the time.)

Method:
Use “purified water”, from grocery store, which is RO water filtered through a carbon filter. Add only sodium bicarbonate to adjust carbonate hardness. Pour water into a large shallow glass bowl and add baking soda, stir for a few seconds and wait for 2 to 12 hours to test KH and PH using AP test kits. Room temperature was about 72 degrees F for all tests.

Initially water was KH=2, from adding about 1/32 tsp baking soda to about .8 gallon of water. Then a small glass of water was removed and diluted about 50-50 with more water from the grocery store bottle, to get KH=1.5. Later that was diluted with about a third more grocery store water to get KH =1.0. The water in the bowl then had another 1/32 tsp of baking soda added to get KH=3.5. So, the range of KH tested was 1.0, 1.5, 2.0, 3.5.

Problems:
Getting an accurate reading of PH was nearly impossible, since all of the PH’s were in the range of 6.9 to 7.6 where the color differences are very hard to be sure about. KH was fairly accurate, since using 10 ml of water instead of 5 ml, doubles the accuracy from about +/- .5 to +/-.25.

Results:
KH=1.0 CO2=3.9 ppm
KH=1.5 CO2=4.5, 4.5 ppm
KH=2.0 CO2=2.4, 3.8 ppm
KH=3.5 CO2=4.2, 4.2 ppm

Assuming all results are equally trustworthy, the average is 3.9 ppm.
Omit the 2.4 reading, and the average is 4.2 ppm.
So, I am going to assume that the equilibrium CO2 concentration for water exposed to air at about 72F for 2 to 12 hours is 4 ppm.

This changes the chart I posted before, but given the accuracy most of us have in measuring PH, the difference isn’t great. Here is the new chart, which I will use until someone finds and posts a better way to do this:

CO2.jpg