# CO2 question for the chemists out there



## carpguy (Feb 3, 2006)

I was researching an alkalinity question and found this little tidbit on Wikipedia:


> In aqueous solution, carbonate exists in three forms. In strongly basic conditions, the carbonate ion predominates, while in weakly basic conditions, the bicarbonate ion is prevalent. In acid conditions, aqueous carbon dioxide, CO2(aq), is the main form, which, with water, H2O, is in equilibrium with carbonic acid - the equilibrium lies strongly towards carbon dioxide. Thus sodium carbonate is basic, sodium bicarbonate is weakly basic, while carbon dioxide itself is a weak acid.


Pardon the befuddlement, but does this mean that if the water is acidic, we're actually getting additional CO2 from the carbonates already in the water?


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## averater (Dec 14, 2004)

if you make the water more acidic (by adding an acid) you'll get more carbon dioxide in the water, but that won't last since it will evaporate to the atmosphere.

one way to countiously add more carbon dioxide to the water would be to solve limestone (CaCO3) with an acid in the aquarium. but i doubt it's an effective method and i think it would be very hard to accomplish. and you'll get a lot of extra calcium.


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## JLudwig (Feb 16, 2004)

carpguy said:


> We're actually getting additional CO2 from the carbonates already in the water?


Yup. You can read up on biogenic decalcification. True aquatics such as Egeria densa and Blyxa japonica are better at this. It's best to prevent it from happening as pH can rocket up on you.


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## carpguy (Feb 3, 2006)

I'm already using that method, I'm just not sure I understand the details of what's going on in there.

I have crushed coral (CaCO3) in my filter to supplement the pitifully inadequate amount that shows up naturally in my tapwater. And I add pressurized CO2, a certain amount of which turns into carbonic acid. The supplemantation drives my pH from the low 7s into the high 6s*. 

So I guess the question is does a certain amount of the CO2 convert to carbonic acid (H2CO3), the CaCO3 simply acting as a buffer once everything is in a state of equilibrium, or is some of the CaCO3 actively being converted to additional CO2.

This isn't really a question with an eye towards changing anything, I'd just like to understand the chemistry better than I do.

*working on a new misting mini-reactor: versions 1 and 2 have not been all that efficient.


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## titan97 (Feb 14, 2005)

Basically, this is how it works:

CO2 gas is dissolved in water to form CO2 aqueous, this reaction goes both directions, and there is a certain equilibrium point of about 3-5ppm dissolved naturally.

CO2(g) <-> CO2(aq)

A very small amount of this dissolved CO2 will combine with water to form carbonic acid. This, too, is an equilibrium equation and goes both ways

CO2(aq) + H2O <-> H2CO3(aq)

The carbonic acid will dissociate into a hydrogen (cat)ion and the bicarbonate (an)ion. Note that the hydrogen cation is what is measured by the pH scale. More H+ gives a lower pH. Again, another equilibrium reaction:

H2CO3(aq) <-> H+ + HCO3-(aq)

The bicarbonate ion dissociates to another hydorgen ion and the carbonate ion:

HCO3-(aq) <-> H+ + CO3--(aq)

So, the complete reaction looks like this(H2O omitted for clarity):

CO2(g) <-> CO3(aq) <-> H2CO3(aq) <-> H+ + HCO2- <-> H+ + CO3--(aq)

When a water sample is exposed to open air, CO2(g) is constantly being dissolved and given off. This dissolved CO2 is constantly being used in the creation of H2CO3, which is also then broken back up into plain CO2. The list goes on.
You CaCO3 may eventually become CO2 gas and dissolved Ca++. It just has a long road to take. Supplimenting CO2 just drives the reaction towards the right, making your water more acidic. However, it does not increase your KH noticably because of the difference in concentrations. For every 1 change in your pH, your KH only changes by about 0.0001 (or less). Most of the CO3-- from the CaCO3 will form HCO3-, where it is the most stable. This can, and does contribute to your KH.

I hope this helps.

-Dustin


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