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.

Bert H

Aren't we all coming up with the fact that a drop in pH units of 1.0 from atmospherically equilibrated water equals approx 30ppm of CO2 in your tank? Or am I missing something here?

hoppycalif

In my opinion, based on my testing, a drop of 1.0 pH units equals nearer to 40 ppm of CO2. But, I doubt that any of the methods we use is more accurate than +/- 5 ppm, if that.

BryceM

Since it's a logarithmic scale, a pH drop of 1.0 unit will mean that you have 10x as much acidity (CO2) as you did before. The final concentration depends on the actual starting concentration, which can be a bit difficult to determine. Most people have assumed something in the 2-3 ppm range - where this number comes from is not clear at all since Henry's law predicts much less (but this is for a non-buffered, distilled sample of H2O).

When it gets right down to it, figuring out exact ppm's of CO2 is a little tricky, but from a practical point of view, dropping one's pH by 1.0 unit by adding CO2 is reproducible from one person's setup to another's and it seems to add enough to grow plants, keep algae at bay, and not kill too many fish. That's good enough for me.

One word of caution. Dropping your pH by 0.5 will result in 3.2x the initial level. Dropping it by 1.0 gives 10x initial, and dropping it by 1.5 gives you a whopping 31.6x initial - more than 3x as much as a 1.0 unit drop. Once you get up in the neighborhood of a 1.0 pH unit drop, going any lower equates to a lot more CO2 very quickly.

hoppycalif

The best part about the PH drop of one unit is how easy it is to remember, and it gets you almost exactly where you want to start. From there you can make small changes in bubble rate and adjust to what the fish like best and the plants respond well to. Until the folks at Seachem or Aquarium Pharm. invent a good, easy titration method that works no matter what else is in the water, this may be the best we can do.

Edward

How much more ‘acid’ do we need to lower pH by 1 in water at 0.5 KH versus 5 KH?