# CO2 saturation point and pKa



## Rolo (Dec 10, 2003)

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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## danmhippo (Feb 3, 2005)

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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## danmhippo (Feb 3, 2005)

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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## Rolo (Dec 10, 2003)

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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## danmhippo (Feb 3, 2005)

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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## KevinC (May 24, 2004)

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. 

Kevin


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## danmhippo (Feb 3, 2005)

Thanks guys, you are great!

I am saving this page to my hard drive. roud:


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## KevinC (May 24, 2004)

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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