Here on the ICU we have very high/precize equipment to measure co2 in Kpa in the air. My question is, is there any relationship between Kpa and ppm or mg/l. I know that Kpa measures pressure and ppm is a concentration. What do i want to do: Fill a bottle (for 3/4) with my tankwater, close the bottle. The co2 will equalize in the trapped air(i assume) from the water. So in the trapped air will be the same amount of co2, that i can measure in Kpa. Is this a usefull method or does it not make any sense.

thanks

Air having X ppm of CO2 doesn't mean it will reach equilibrium with water in contact with it at X ppm of CO2 in the water. It is the partial pressure of the CO2 in the air that affects the concentration of in-contact water, and that is determined by an equation which I don't have right now. If air and water had the same density I assume the ppm in the water and in the air would be the same.

So my thought does not make sense. So i will take a monster of my tankwater (in a closed little bottle) and take it to the laboratorium to determine the total co2. So i'nm sure about targeting the 30-40 ppm. I've a ph controler system and a dc. I've read that this doesn't give any garanty about your ppm.

Hi,


This is something I've learned firsthand along the way . My pH controller is now only monitoring. I previously had it set to keep pH 6.2 - 6.4 thinking this would provide enough CO2. When I finally got my drop checkers going with 4dKH fluid and after algae issues began I really saw how far off I was.
Now when my DC is yellowish/green I note my pH varies 5.5-5.8 (reads 6.8-7.0 lights off/CO2 off). Even with the DC yellowish/green there is still no guaranty that CO2 is adequate, especially after reading CO2 in situ meter measurements, real time data on localized CO2 ppm readings
Co2 is not as easy as I first thought as Hoppy said so well:
Ryan

your ph varies between 5.5 and 5.8 ????, but what is your kh then? If its about 4 your ph controler is far off the road and needs to be set . Mine is 6.4 at the moment with 4 kh. My dc is yellowish/green.

Actually my KH is about 4 according to an API test kit. There lies the problem with the KH-pH-Co2 chart. Clearly I don't have 200-300 ppm CO2 as the chart would indicate. My fish have never been gasping at the surface or shown other signs of stressed behavior.

Many substances can intoduce great error into the KH-pH-CO2 chart such as phosphates tanins, and humic acids. Just having a bunch of wood in your tank -as I do- can easily make the chart unreliable.

It is going to take a lab that has equipment similar to Tom's to be able to test that water sample and tell you how much CO2 is in it. And, that won't be easy either, since the water will quickly lose the CO2 as soon as it is exposed to air. It will be interesting to hear what the lab tells you, so be sure to post their results here.

ok next week i will post it

So,
This morning I took a sample of my tank (just before lights went on), I didn't shut off the co2 last night , so at the time i took the sample my ph was 6.4 (ph controler). I took the sample in a closed bottle(for blood samples) without any free air, so there could be no escaping co2> In the hospital lab they did the required measurements. My Ph was 6.5 (so my ph controler is very accurate) and my co2 was 4.2 mmol/liter. I cant find the translation to mg/l or ppm. My kh is 4. In theory i should have about 30-40 ppm co2 in my tank.
Can anybody make a translation to mg/l or ppm for me ?? thanks.

I found the following:
# Atom Molar Mass (MM) Subtotal Mass Subtotal Mass
(g/mol) (%) (g/mol)
------------------------------------------------------------------------------
1 C 12.01 27.29 12.01
2 O 16.00 72.71 32.00
_________________________________________________
Total Molecular Weight: 44.01

So the molar weight of CO2 is 44grams per mol, so 1 millimol/L is 44 milligrams per liter. 44mg/L is 44ppm.
So lets go back to my sample 4.2 mmol/liter, makes 4x44 ppm = 176ppm of co2!!!!

Is this correct?????