Another CO2 Chart to try - Fertilizing

hoppycalif · 03-08-2006, 01:33 PM

There have been several discussions about the inaccuracy of CO2 measurement using KH and PH. It was suggested that we take a tank sample and let it sit overnight to reach equilibrium with the CO2 in the atmosphere. That was assumed to be about 3 ppm. If the PH increased by about 1.0 the CO2 was assumed to be about 30 ppm. This led me to look at the CO2/PH/KH equation some more and reduce it to a difference between equilibrium PH and tank PH, to give the attached chart. Any comments? If this is mathematically correct, it looks like a more accurate way to measure CO2, without worrying about what the KH is.

Edit: My experimentation indicates that the equilibrium ppm for my kitchen is about 4 ppm. This chart has been revised to reflect that.

BryceM · 03-08-2006, 03:05 PM

I absolutely agree that the method you describe is a better way to look at CO2 concentration since it removes many of the variables that can upset the traditional KH/pH/CO2 chart. Organic acids and non-carbonate buffers are always present to some degree in a real aquarium which invalidate the equation the chart was derived from.

Your chart above should be correct if your assumption of 3 ppm of dissolved CO2 at equilibrium is, in fact, true.

CO2 (ppm) = equilibrium CO2 (ppm) X 10 ^ change in pH

I've been looking all over the web looking for reference values for dissolved aqueous CO2 levels at atmospheric equilibrium and so far I haven't found anything outside of aquarium related sites to verify this number.

For anyone that hasn't been following this discussion over the past two or three months, the basic idea is to set some aquarium water aside in a container with a large surface area, or to bubble air through the sample. If you don't aerate the sample, allow 24-48 hours for the pH to stabilize as it comes to equilibrium. The differenece in pH between the degassed sample and the water in your tank comes from the difference in CO2 levels.

banderbe · 03-08-2006, 05:35 PM

Quote:

Originally Posted by guaiac_boy

I absolutely agree that the method you describe is a better way to look at CO2 concentration since it removes many of the variables that can upset the traditional KH/pH/CO2 chart. Organic acids and non-carbonate buffers are always present to some degree in a real aquarium which invalidate the equation the chart was derived from.

Your chart above should be correct if your assumption of 3 ppm of dissolved CO2 at equilibrium is, in fact, true.

CO2 (ppm) = equilibrium CO2 (ppm) X 10 ^ change in pH

I've been looking all over the web looking for reference values for dissolved aqueous CO2 levels at atmospheric equilibrium and so far I haven't found anything outside of aquarium related sites to verify this number.

For anyone that hasn't been following this discussion over the past two or three months, the basic idea is to set some aquarium water aside in a container with a large surface area, or to bubble air through the sample. If you don't aerate the sample, allow 24-48 hours for the pH to stabilize as it comes to equilibrium. The differenece in pH between the degassed sample and the water in your tank comes from the difference in CO2 levels.

Well now you're assuming we all have access to accurate pH readings. I use an AP pH test kit and a Red Sea pH test kit. Sometimes they agree, sometimes they are off from the other by 0.2.

BryceM · 03-08-2006, 05:47 PM

Quote:

Originally Posted by banderbe

Well now you're assuming we all have access to accurate pH readings. I use an AP pH test kit and a Red Sea pH test kit. Sometimes they agree, sometimes they are off from the other by 0.2.

Yeah, even pH probes can be off by quite a bit - especially if you don't callibrate often or replace them when they wear out. Color comparison test kits can be even harder to get accurate readings out of, not to mention innacuracies in measuring KH.

Like everything else though, two decimal precision is great for labs but a little ridiculous for a hobby. I'm assuming everyone does the best with what they've got. I think even the cheapest pH test kit can distinguish between a 0.4 and 1.0 pH difference. This will at least get you in the ballpark, especially with several readings over time.

Another method that might be the best of all is to slowly crank up the CO2 until the fish look like they're not liking it. Then back off about 0.2 or 0.3 pH units. Go slow and only do it when you're around to watch the fish. As long as the plants are growing and algae isn't, there isn't much point in seeing how far you can push it.

**acbaldwin** · 03-08-2006, 05:41 PM

What about other things that affect your pH reading? Wouldn't they lead to a false CO2 reading?

BryceM · 03-08-2006, 05:50 PM

Quote:

Originally Posted by acbaldwin

What about other things that affect your pH reading? Wouldn't they lead to a false CO2 reading?

Not at all..... That is the beauty of this method. If you take the water right from the tank and let it degas, the only difference between the two samples will be the concentration of CO2. The pH of both will certainly be affected by organic acids, tannins, phosphate buffers, etc, but equally so.

hoppycalif · 03-08-2006, 06:09 PM

This method seems to me to have only one unique disadvantage, and that, of course, is the time it takes to get a reading. But, it does appear to cancel out all of the various other errors we run into by measuring KH and PH.

Today an "old timer" who is well known in aquatic plant circles was upset that we would even consider using fish as a test device - torturing them with ever higher CO2 levels until their distress is obvious to us. That did impress me and started me to thinking harder about how else to measure CO2.

jude_uc · 03-08-2006, 07:57 PM

Good job! That's a very good idea to solve the problem.

-Adam

MatPat · 03-08-2006, 08:21 PM

Quote:

Originally Posted by hoppycalif

There have been several discussions about the inaccuracy of CO2 measurement using KH and PH. It was suggested that we take a tank sample and let it sit overnight to reach equilibrium with the CO2 in the atmosphere. That was assumed to be about 3 ppm. If the PH increased by about 1.0 the CO2 was assumed to be about 30 ppm. This led me to look at the CO2/PH/KH equation some more and reduce it to a difference between equilibrium PH and tank PH, to give the attached chart. Any comments? If this is mathematically correct, it looks like a more accurate way to measure CO2, without worrying about what the KH is.

Here are my test results from a few months ago:

Tap pH - 8.51.
Rested Tap - 7.59.
Tank - 6.16
Tank Aerated - 7.67

That gives me about 95ppm of CO2 according to the chart. I'm thinking this chart may be fairly close. If I add fish without using drip acclimation for several hours, the fish pretty much sink to the bottom of my tank in a matter of minutes.

I will try and run some more tests on my tank water tomorrow if time permits. It would be nice if we could find a way to eliminate one more test kit that new folks will have to purchase!

I can imagine someone being upset by harming the fish, but if caught before they are unconscious, it should be temporary. Once you see them start to get stressed, you can easily back off the CO2 before any damage is done. Even the fish that sank to the bottom of my tank were revived by aerating them in a small volume of water.

**turbomkt** · 03-08-2006, 08:28 PM

One thing to note about pH testers (digital)...

regardless of calibration (accuracy), as long as the precision is fine this will work. Readings can be off by whatever amount, but they'll both be off by the same amount. An inexpensive hand held digital meter should be quite good enough and cheaper than test kits in the long run.