Another CO2 Chart to try - The Planted Tank Forum
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post #1 of 18 (permalink) Old 03-08-2006, 10:05 PM Thread Starter
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Another CO2 Chart to try

We have had many discussions about the inaccuracy of the KH/PH readings to measure CO2. One suggestion was to check the PH/KH of a tank sample after it sits out overnight, coming to equilibrium with the atmospheric CO2. That led me to look more at the CO2 equation, which led to reducing that equation to one involving only the PH of the tank vs the water left out for atmospheric equilibrium. I made a chart of those results.


Are there any comments on this?

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post #2 of 18 (permalink) Old 03-09-2006, 01:01 AM
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KH plays a big part in how much pH changes due to co2. In higher KH environments an incremental pH change signifies a much larger co2 change, and vice versa.

What KH is that chart produced at?

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post #3 of 18 (permalink) Old 03-09-2006, 01:21 AM Thread Starter
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When you do it this way the KH cancels out, so it plays no part at all. Also, the other substances contributing to PH also cancel out.

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post #4 of 18 (permalink) Old 03-09-2006, 01:30 AM
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Maybe I am missing the idea behind it; but I don't see how KH "cancels" out... sitting out over night would have no effect to the initial or later KH value.

ex.
1) KH = 3 (constant);
pH = 7.5; reads: co2 = 3.00ppm (atmospheric)
pH = 7.0 (.5 change); reads: co2 = 9.00ppm

2) KH = 7 (new constant)
pH = 7.5; reads: co2 = 7.00ppm
pH = 7.0 (.5 change, as above); reads co2 = 21.00ppm

the pH change stayed the same, while the co2 change was quite a bit more.

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post #5 of 18 (permalink) Old 03-09-2006, 01:44 AM
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You cannot add an acid to the water (carbonic acid) and know the levels of it through measuring pH without accounting for the bases that cancel it out. The chart is flawed. If someone were using well water in their tanks with a very high KH then a 1 point shift in pH would be likely more than enough to kill their fish.
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post #6 of 18 (permalink) Old 03-09-2006, 01:55 AM Thread Starter
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CO2=3*KH*10exp(7-PH)
KH=CO2/{3*10exp(7-PH)
Assume equilibrium CO2 = 3ppm
KH=3/{3*10exp([email protected]) = 1/10exp([email protected]) = 10exp([email protected])

CO2=3*10exp([email protected])*10exp(7-PH) = 3*10exp{([email protected])+(7-PH)
CO2=3*10exp([email protected]) = 3*10exp(change in PH)

See? KH drops out, and since it is the change in PH that is used, any other PH raising or dropping component in the water also cancels out.

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post #7 of 18 (permalink) Old 03-09-2006, 02:34 AM
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Your math is wrong. Either that or everyone else is wrong.

Look at Chuck Gadds' calculator.

A 1 point drop in pH equals a 10X difference in the CO2.

Example.

kH of 3, starting pH of 6.5, ending pH of 7.5, starting CO2 of 28.5, ending CO2 of 2.8

kH of 5, pH 6.5 to 7.5, CO2 47.4 to 4.7

First case has a difference of 25.7 ppm and second case has difference of 42.7
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post #8 of 18 (permalink) Old 03-09-2006, 04:26 PM Thread Starter
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The math isn't wrong, but the assumption that water will reach 3 ppm of CO2 in equilibrium with the air may be wrong. I'm still looking for data on that. Google hasn't been helpful yet.

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post #9 of 18 (permalink) Old 03-09-2006, 05:25 PM
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Quote:
Originally Posted by Rex Grigg
kH of 5, pH 6.5 to 7.5, CO2 47.4 to 4.7
But if atmospheric equilibrium is 3 or so, that would bring the ph of the example to 7.7. That would give you a drop of 1.2. According to the chart, you would have 48ppm co2. I'm not good with the math, but I don't see any problem with this chart.
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post #10 of 18 (permalink) Old 03-09-2006, 05:36 PM
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Hoppy,

Your effort is appreciated. But, it only takes a cursory look at the known CO2/KH chart to see that your graph is flawed. What would be feasible is to make a chart that shows change in pH with respect to the ratio or fraction of CO2 increase/decrease.

The KH can NOT cancel out because KH is NOT a constant in every tank. "KH" is not a alpha description for a constant number (like pi, gravity, speed-of-light). Your algebra has gotten the best of you.


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post #11 of 18 (permalink) Old 03-09-2006, 06:11 PM
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The formula is consistent with the chart if you assume the outgassed sample always has a CO2 ppm of 3, and that the kH remains constant during the outgassing process.

Reading the chart:

kH 10.0
pH @ 3 ppm CO2 = 8.0
drop the pH by 1.0, and the chart reads 30 ppm.

kH 4.0
pH @ 3 ppm CO2 = 7.6
drop the pH by 1.0, and the chart reads 30 ppm.


In fact... find any matching CO2 ppm values on the chart at DIFFERENT kH values. Traverse the chart left or right, and for the same steps in pH, the CO2 ppm values coincide.
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post #12 of 18 (permalink) Old 03-09-2006, 07:09 PM Thread Starter
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I did that CO2 measurement this morning, having removed a sample last night to sit overnight to reach atmospheric equilibrium. The results puzzle me: I got Ph=7.6 and KH=7 for the overnight sample
Ph=6.6 and KH=5 for the tank sample this morning.
How can the KH change like that????? Same water, same source of the water, only about 12 hours between taking the water from the tank.

Also, I'm hearing that the ppm of CO2 for water at equilibrium with the air can range from .5 to 4, depending on who knows what. Why can't this be straight forward easy???

Hoppy
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post #13 of 18 (permalink) Old 03-09-2006, 07:19 PM
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This chart may/may not be correct, but ways to measure/gauge CO2 levels other than the pH/kH relationship are necessary. If you use Aqua Soil then you can't use the pH/kH relationship due to the water chemistry altering properties of the substrate. Right now I'm just judging the levels by the reaction of the plants and fish. Hopefully more technical methods surface, good job looking for one Hoppy.

-Craig
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post #14 of 18 (permalink) Old 03-09-2006, 07:48 PM
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Just for info (Hanns-J Krause CO2 Test)

Hanns-J Krause CO2 Test
Author Handbook Aquaria Technology

Krause recommends a different method to measure CO2 content. That method is insensitive to the presence of unusual buffers
(peat, acid, alcali etc)

Krause describes a method in his book on aquarium water that is supposed to work with any kind of water. Not absolute pH is the key, but the change of pH by two units is used to determine correct CO2 concentration.

Step 1:Take a sample of your water and aerate it for some time until all CO2 is removed. In that case the concentration of CO2 is in equilibrium with the surrounding air (around 0.5 to 0.6ppm).

—>Measure pH of the water (=X).

Step 2: Next exhale through a pipe into the water sample for 2 or 3 minutes. The concentration of CO2 in the water will assume 60ppm.

—>Measure pH of the water (=Y).


The optimum CO2 concentration of 10-20ppm is at the pH value about 2/3 of the difference between X and Y:

optimum pH = X +.67*(Y-X).

This will work even with buffered water, although the change in pH might be small and only detectable with an electronic pH meter.
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post #15 of 18 (permalink) Old 03-09-2006, 08:06 PM
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Kh = (mg/l[HCO3-/1,22]) x 1/17,86 = °dH

Its fairly simple:

CO3'2-' + CO2 + H2O <-> 2HCO3'-'

Le Châteliers:

Any system in stable chemical equilibrium, subjected to the influence of an external cause which tends to change either its temperature or its condensation (pressure, concentration, number of molecules in unit volume), either as a whole or in some of its parts, can only undergo such internal modifications as would, if produced alone, bring about a change of temperature or of condensation of opposite sign to that resulting from the external cause.

A good example of the law is the Haber Process Chemical Equation, a reaction that Le Chatelier worked on but subsequently abandoned. The project was continued by Haber and Claude who successfully produced Ammonia on a commercial scale using this process.


N2 (g) + 3H2 (g) <===> 2NH3 (g) + heat

Increasing H2 in system shifts equilibrium to right
Decrease NH3 to shifts equilibrium to the right
Increase Heat to shifts equilibrium to the left



bla bla bla bla...

Meaning - That whatever you do (or something/one else do) you can never have a stable Kh - NEVER!

Conclusion - CO2 are dependant on HCO3'-'

Since our Ca'2+' reacts with CO3'2-', its dependant on CO2 too (Se graph)

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