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Discussion Starter · #1 ·
I've been thinking about the CO2 chart that we all take for granted. It's produced by the equation:

CO2 (ppm) = 3 x KH(deg) x 10^(7-pH)

Anyone know where this equation came from?

Mathematically & chemically it doesn't make sense. I've read all (ok, most) of the posts out there. My understanding is that the carbonate buffer system has a PK of about 6.3. In English, this means that as the pH approaches 6.3 that the buffering capacity of carbonate increases greatly. Far from 6.3 the buffer almost doesn't exist. In English, the same amount of CO2 drops the pH faster at pH 8.5 than it does at pH 6.3.

I don't think the above formula takes this fact into account. Everything on the chart is linear.

Also, even if the formula is correct, this chart assumes there isn't another source of acid or other buffering system in the water. Since this is never true we're shooting in the dark, at best.

When bacteria process ammonium per the 'cycle': NH4 + 2H2O --> NO2 & 8 H+ . That's a ton of acid! I know the plants get most of it, but some biological processing of Nitrogen does occur.

Any smart people out there? Chemists, etc, etc? Are we all following a formula that someone dreamed up long ago without a sound scientific basis? Is there a good way to measure dissolved CO2 that applies across the board to people with different water, different buffers, and different levels of KH?

Like I've said before, I know just enough of the chemistry to be dangerous.
 

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I do remember reading on the Krib something similar to what you are stating about the chart not being linear. I think this link describes it pretty well: http://www.thekrib.com/Plants/CO2/kh-ph-co2-chart.html

This chart/relationship worked very well for me when I lived in MD and had well water. I guess the alkalinity of my water in MD was mostly carbonate/bicarbonate based. You already know of my issues with my City water in Ohio so I'll leave that one alone :)

Since we both have pH meters and I am assuming fairly accurate KH test kits, maybe with some tutelage from you, we could both run a test like the one Roger Miller describes in the last reply to the above link. I believe they were limited by their pH kits at the time. I am in the process of changing my 30g tank to straight RO/DI water so I should be able to get some fairly accurate pH and KH readings on this tank in the next couple of days.
 

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Discussion Starter · #4 ·
Hmmm. I think the most useful thing to do would be to run a series of experiments where you took a given sample of aquarium water from an established tank, let it degas completely, added varying known quantities of CO2, and monitored the actual pH change over a range of values.

The only difficulty is how to accurately measure a known quantity of CO2. You'd also need assurance that it was completely dissolved.

I spent a little time looking around on the internet about ways to measure dissolved CO2. It turns out that it's not really that easy. The titration method (what we use) is often used in classrooms, but I found many articles backing up my premise that the method is generally inaccurate. The assumptions that must be made result in a CO2 "reading" that is much higher than is actually there. There are some meters that read dissolved CO2 via a membrane covered probe (similar to a pH meter), but they're used in industrial applications and are quite $$$.

I'll keep looking.
 

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Discussion Starter · #5 · (Edited)
An update .... I've been doing a little research. A couple of things caught my attention. When CO2 dissolves in water a small portion is converted into H2CO3 (carbonic acid). The amount that is converted is very small, depends slightly on temperature, and does not vary with pH. Some of this H2CO3 converts to H3O+ and HCO3-. The equation looks something like this:

CO2 + H20 <---> H2CO3 <---> H+ + HCO3-

The double arrows mean the thing can go in either direction. The second equation is dependent on pH. The basic premise is that if you add a product, the equation is driven backwards and if you add a reactant the equation is driven forward.

Ever notice how plants pearl immediately when you add NaHCO3? The equation is driven to the left, actually adding tons of dissolved CO2 to the water - this despite the fact that the pH actually rises.

The thing that makes this reaction so interesting (and a bit hard to figure out) is that when you dissolve CO2 in water you actually create more bicarbonate, the very buffer detected by our KH tests. I think this fact is what allows the KH/CO2/pH relationship to be approximately linear.

Net conclusion: the chemistry behind the formula is correct (as far as I can determine), but the actual application of it should NOT be used without caution.

The presence of other acid in solution makes a big difference. Organic acids that accumulate from the fish and breakdown of plant matter cause us to overestimate the CO2 we measure. This effect can be quite large. For my tank, at KH 4.5 the pH from the chart says I should be at pH 7.6. I'm actually at pH 7.1 after degassing. 0.5 pH units makes a huge difference! In my degassed water at KH 4.5, the chart says I should have 12 ppm CO2. That's clearly not correct. People who don't know better would mistakenly think they have 10-15 ppm of CO2 already. This effect should be greater in a tank with a higher bioload.

The presence of other buffers in solution can also mess things up. Fortunately, the effects of phosphate buffers actually decrease as pH falls much below 7.0. For most of us (PO4 around 2 ppm), the presence of these buffers probably doesn't make a big difference.

Presently, the thing I'm going by is how much I'm depressing the pH by adding CO2. I take a degassed water sample and compare its pH with the pH in the tank with CO2 running. I'm shooting for a 0.8 to 1.0 pH drop. This should correspond to an actual 30 ppm CO2 concentration.

When you read about people who think they're at 70 or 80 ppm in their tank, smile, realize that they're being thrown off by extra acid, and understand that they're probably at about half that level. To be that high I'd need pH levels in the high 5's in my tank (a little low for me).

Now I'm going to rest my brain & see if I can't use this to make the algae go away (which was the whole point :) ).
 

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This is interesting to think about.

It could very well be the reason why I simply cannot grow plants.
 

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guaiac_boy said:
CO3 + H20 <---> H2CO3 <---> H+ + HCO3-
corrected (or extended):

CO2 (gas) <-> CO2 (dissolved) <-> H2CO3 <-> H+ + HCO3- <-> 2H+ + CO3-- (+Ca <-> CaCO3)

(i've skipped all the water molecules in this chail since it is all happening in water)

the first CO2 is dissolved carbon dioxide. the last is carbonate wich can react with calcium to limestone, and the other way limestone can react with water to carbonate wich can react to bicarbonate and consuming hydrogen ions and thereby raising the pH.

i don't know about the chart, if it's only exact in an absolutely clean water, but of course does other acids cause us (or people following it) to put in to little CO2.
 
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