CO2 saturation point and pKa - The Planted Tank Forum
 
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post #1 of 9 (permalink) Old 03-03-2005, 03:29 AM Thread Starter
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CO2 saturation point and pKa

Does CO2 has a saturation point? I now know that pearling is the indication that O2 in the water has reached the O2 saturation point. But does CO2 has saturation point as well? What are the indication of CO2 saturation?

I noticed that with my CO2 reactor, If the tank pH is 6.8 and above, water that comes out of the reactor does not have bubbles, which I assume is CO2. However, when I crank CO2 up to get pH to 6.6, a few hours later I see small bubbles shooting out of the CO2 reactor.

With pH of 6.6 and KH of 5, using the equation, my CO2 level should be at 37ppm. Is this near the CO2 saturation level? (assuming there is one).

Also. I've heard CO2's pKa is at 6.4. Does that means that is the lowest pH typical tanks will ever reach with CO2 being the only pH influencing factor?
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post #2 of 9 (permalink) Old 03-03-2005, 04:20 AM
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The reason you see bubbles coming from your reactor doesn't have to do with saturation. I don't know the exact number, but we are not close to saturation since it is into hundreds of ppm CO2. Bubbles are probably coming out because you're building up more gas in the reactor -> so some escape.

Are you talking about pKa of between H2CO3 and HCO3? For what it's worth, it doesn't really apply here. But it depends on temperature and salinity, and IIRC the CO2 charts use a pKa of 6.37.
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post #3 of 9 (permalink) Old 03-03-2005, 04:35 AM Thread Starter
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I was refering to pKa of mixture of CO2 and H2CO3. I know that H2CO3 is a pretty strong acid and its pKa is much much lower, I think it's in the 3.xx range, which, probably won't happen in our tanks as only a fraction of CO2-aq actually forms H2CO3.
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post #4 of 9 (permalink) Old 03-03-2005, 04:39 AM Thread Starter
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With this formula: CO2 (in PPM) = 3 * KH * 10^( 7-pH )

plugging in 6.37 for the pH, and 5 for the KH, CO2 level is 63ppm.

Does that means at KH of 5, CO2 reaches saturation at 64ppm? Is this how pKa works?
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post #5 of 9 (permalink) Old 03-03-2005, 05:11 AM
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I think I understand what you are trying to do. When pH = pKa, this doesn't means CO2 is at saturation, or the lowest pH caused by CO2. A real chemist can explain this way better, but what it practically means here is when pH = pKa (this case pH 6.37), the carbonate buffer system is split 50/50 between two forms: CO2 and H2CO3 (carbonic acid) vs. HCO3 (bicarbonate). pH = pKa is also called the equalivalence point.

BTW where 6.37 came from:
CO2 = 12.838 * KH * 10^(pKa - pH)
CO2 = 12.838 * KH * 10^(6.37-pH)

It is no different from the current CO2 equation:
CO2 = 3 * KH * 10^(7-pH)
This last equation is derived from the 1st only to make the numbers simpler. So really the pKa is 6.37, not 7.
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post #6 of 9 (permalink) Old 03-03-2005, 02:43 PM Thread Starter
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Ah-Ha! I see how the equation works now!

But still wish to understand the interaction of H2CO3, CO2, and HCO3 more in depth.

Hehe, It's confusing for a non-chemist like me to fully understand, but it sure is fun to know it.

Thanks, Rolo.
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post #7 of 9 (permalink) Old 03-04-2005, 03:31 AM
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CO2(gas) <----> CO2(aqueous)
CO2(aqueous) + H2O <----> H2CO3(aqueous)
H2CO3(aq) + H2O <----> H3O+(aq) + HCO3-(aq)
HCO3-(aq) + H2O <----> H3O+(aq) + CO3-2(aq)
CO3-2(aq) + Ca+2(aq) <----> CaCO3(s)

Each of the above is an equilibrium reaction - it can go from left to right or right to left. In addition, LeChatelier's principle says if we change one thing, the entire set of reactions changes (this explains how adding CO2 makes the solution more acidic or adding crushed coral increases HCO3- concentration).

Also, the saturation level for CO2 is dependent on pressure (think soda bottles) and total dissolved solids (more ions = less gas can dissolve).

By adding CO2 artificially, we are actually supersaturating the water - if you define saturation as the maximum amount that can dissolve for a given set of conditions (including temp, pressure, TDS). That is why surface movement has such a dramatic effect - it gives the CO2 an escape route.

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post #8 of 9 (permalink) Old 03-04-2005, 03:40 AM Thread Starter
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Thanks guys, you are great!

I am saving this page to my hard drive.
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post #9 of 9 (permalink) Old 03-04-2005, 02:02 PM
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Also, I need to clarify the issue of pKa WRT carbon dioxide:

Carbonic acid is a diprotic acid, so:

H2CO3(aq) + H2O <----> H3O+(aq) + HCO3-(aq)
HCO3-(aq) + H2O <----> H3O+(aq) + CO3-2(aq)

The first reaction has a pKa of 6.352, while the second has a pKa of 10.329. Notice the bicarbonate ion can act as an acid (2nd reaction) or a base (reverse of 1st reaction) - so a solution made from only the bicarbonate ion (baking soda) will be a good buffer - it can absorb both acids and bases. The pH of such a solution would be midway between the two pKa's, or 8.34 - that number should look familiar to reef/saltwater folks.

In the case of freshwater tanks, our pH is lower - meaning we have a mixture of both bicarbonate ion and carbonic acid. This is the first reaction. If we have an equal mole mixture of the two, then the pH = pKa, or 6.352.

You can calculate the ratio of the two ions for any pH within +/- 1 unit of 6.352 using the Henderson-Hasselbach equation:
pH = pKa + log ([HCO3-]/[H2CO3])

Now the only thing left is to decide how the dissolved CO2 concentration determines the carbonic acid concentration - again, these are equilibrium reactions from my other post.

To sum up then, the pH tells you the ratio of bicarbonate to carbonic acid - or the ratio of kH to CO2 - so if you measure kH, you can use the table or the equation to determine CO2.

Kevin

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72g bowfront planted, CO2, 4x - T5HO, Eheim 2213 and 2217, 2 angels, pristella tetras, blue tetras, betta, albino bristlenose pleco, albino cories. Sword, vals, hygros, ludwigias, java moss and fern, anubias

2g Mac-quarium. Clown gravel, fluorescent plastic plants, and 2 guppies.
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