BryceM

I agree that the math according to Henry's law gives a value of 0.5 ppm for distilled water. Why then have people always cited 2-3 ppm as the equilibrium value for planted tanks? Does the presense of a carbonate buffer affect how much is able to dissolve? There is a disconnect somewhere - maybe the 2-3 ppm thing is just a myth, but it's been going around for a very long time.

dkfennell

Here is article by Carroll, Slupsky and Mahter comparing empirical results to Henry's Law's prediction on Solubility of CO2 in Water at Low Pressure:

http://www.nist.gov/srd/PDF%20files/jpcrd427.pdf

hoppycalif

Last night I went to my local grocery store and bought a gallon of water sold for steam irons, etc., labeled as RO water. I poured out about 80% of it into a large shallow bowl, added about 1/32 tsp baking soda, and waited for things to stabilize. I measured KH=2, PH = 7.2 to 7.6, getting different results (or different interpretations of the color) for three measurements taken from 2 to 12 hours later. The KH test I did with double the amount of water, so it is pretty accurate, but the PH is very iffy. Today I have mixed a small batch at about KH=1, and added another 1/32 tsp to the bowl to get KH = about 4. I will check PH/KH in about 4 hours. I'm already almost sure the actual equilibrium CO2 is more than .5 ppm and closer to 3 or 4 ppm. Remember, we don't care what the theoretical value is, just what the water reaches after sitting open to the air for 12 hours.

Bert H

Very interesting discussion here folks! From apd: http://fins.actwin.com/aquatic-plant.../msg00103.html where George Booth states that the theoretical equilibrium CO2 concentration is: 0.485ppm.

Interesting set of threads going on in apd as well regarding this topic.

The other thing that can play a role in this is with the kh measurement. It is my understanding that kh measures alkalinity values. If you have alkalinity not derived from carbonates, then your kh values would be artificially higher, thereby giving you artificially higher perceived CO2 values. This would explain why a lot of us (myself included) appear to have CO2 values in the 60's with no apparent fish/shrimp stress in their tanks.

Any chemists care to chime in?

dkfennell

One thing is certain from this discussion. You can't take at face value anything anyone says about CO2 concentration in their tank without finding out how they derived it. This applies not only to fish stress, but also amount of CO2 for plant growth. It would be a big mistake for someone estimating his own CO2 by a method that understates the CO2 concentration following the suggestions of another who overestimates it. There would be a problem (but lesser in severity) the other way too.

I also think guaiac_boy deserves alot of credit for crystalizing this issue. At least he did for me.

Darrell Fennell

MarcosB

Hoppycalif
There is no reason to a water sample in equilibrium with atmosphere has CO2 levels as far as 3 ppm. The equilibrium is related to CO2 levels in atmosphere, which is estimated at 380-390 ppm.
There are some possible situations where the CO2 can be far from the theoretical equilibrium of 0.5 ppm. This includes an ambient with poor external air ventilation, water with high biological activity (with microorganisms).
I do not understand how can you determine the CO2 equilibrium level with this experiment that you are developing.

Bert H
Compounds like phosphates and humic acids (and others) can interfere in the carbonate alkalinity determination. The KH value became higher than the real leading to errors when compared to pH x KH x CO2 charts.
But these compounds can also capable of influence this method based on pH. The pH variation due to the CO2 loss in a water sample that the KH is constituted only by bicarbonate/carbonate is higher than a water sample with the same KH value but with the buffering constituted of only phosphate salts.

BryceM

Hoppy, look at the hornets nest you stirred up.

I think dkfennel has a good point. One person's CO2 reading is not necissarily the same as another person's. I've been running ideas by a friend of mine who graduated as a chemistry major from Dartmouth. He is, without a doubt, one of the 10 smartest people I know. His conclusion is that the relationship between CO2, pH, and acid buffers in aquarium systems are too complicated to model with simple methods. There are just too many variables.

I think that a couple of ideas have merit. One is to add CO2 unitl the fish show distress and back off a bit. This method has drawbacks. I'm pretty convinced you can slowly acclimate fish to conditions that they wouldn't normally do well at. Therefore, the speed at which you drop the pH is important. The other problem is that it's not always easy to get consistently accurate pH readings, even with an electronic meter.

The other idea is to degas a water sample and plan on dropping the pH about 1.0 units. I'm not sure that I know 'exactly' what ppm of CO2 this gives me, but it seems to work and is probably a good place for a newbie to start. I do think it removes certain innacuracies inherent to the traditional KH/CO2/pH chart.

People's estimation of their CO2 is probably about as accurate as their estimation of light reaching their plants. It doesn't take much checking with light meter to figure out that watts/gallon is an absurd unit of light. It's just the most convenient one we have.

hoppycalif

I don't think any of us would say that we are capable of measuring the CO2 in our water accurately enough for any scientific purpose. What we do need is a way to estimate that number accurate enough so we can compare CO2 levels among ourselves, so we can have a good starting point when we start injecting CO2 into our tank, and so various waters we use will all give the same result within a reasonable error band for the same amount of CO2. I think that is all we have ever wanted. And, that is what I found I didn't have, with my "170 ppm" reading using the Chuck Gadd chart or the many equivalent charts. So, I hope we can all view this effort as just another, possibly more accurate way to find out how much CO2 we have in our water. I finished my experiments trying to determine the CO2 ppm in water that sits on my kitchen counter for a few hours exposed to the air.

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:

RTR

hoppy - don't forget altitude above sea level. Barometric pressure does matter. George Booth is way up in the Rockies and makes sure to mention that fact when he is talking CO2 in his tanks versus other's tanks.

hoppycalif

Since the ppm of CO2 this chart gives you is directly proportional to the "equilibrium" value for ppm of CO2 when a sample sits for a few hours, he, or anyone else could just buy a gallon of RO water and repeat the test to get their own value, then multiply the numbers in the chart by their value divided by 4. And, I agree that barometric pressure would have to be a factor, but I have no idea how much it would change it, especially considering the fact that it is hard to accurately measure PH in any case.