# Carbonic acid



## bigmatt (9 Mar 2011)

Here's one for people WAY cleverer than me...(that isn't such a small group, now i come to think about it...)
So, i've previously added CO2 to my filter INTAKE, using the filter as a CO2 reactor and achieving total dissolution of the added CO2 - benefits seen in terms of plant growth, pearling etc so definitely advantageous.  As CO2 combines with water to form carbonic acid (H20 + CO2 -> H2CO3) would adding carbonic acid to the water column have a similar effect as adding CO2, or is it an increased pCO2 that increases carbon availability tothe plants?  
This won't change anyone's practice, as i'm pretty sure the best way to generate carbonic acid is to dissolve CO2 into the water column using a pressurised system...I just thought it was interesting and if anyone knew the answer (ceg4048 i'm looking at you...  )
Cheers folks!
Matt


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## vauxhallmark (9 Mar 2011)

Carbon dioxide dissolved in water _is_ carbonic acid.

So, yes, if you could find a way to add liquid carbonic acid to your tank in a controllable and automatic fashion then you wouldn't need to use CO2 gas.

But I think you actually answered your question at the end of your first post   .

Mark


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## ceg4048 (10 Mar 2011)

Actually, to be 100% accurate that is not quite the case. CO2 dissolved in water is still CO2. A very small fraction of that dissolved CO2 then reacts with the water to create Carbonic acid, which is Carbonic acid (H2CO3) and is NOT CO2.

This is a bit of a non-starter because at 25 degrees C, if you have 30ppm dissolved CO2 then the amount of Carbonic acid in the water will be something like 0.05ppm. I think people still don't get this point. Think about it; 29.95ppm CO2 and only 0.05ppm Carbonic acid.

That small amount of Carbonic acid, that 0.05ppm H2CO3 is enough to turn distilled 4dkH water + Bromothymol blue reagent a nice lime green colour. This is the amount of acid that all the KH/pH/CO2 tables are based on.

If you add more KH to the water then the reagent stays blue and that's why many people become hysterical about high KH water because they think that not enough CO2 is being dissolved in the water because the small amount of acid is being buffered by the huge amounts of carbonate and bicarbonate in the water. They don't realize that regardless of the KH you still have 29.95ppm of CO2 dissolved in the water but that the 0.05ppm of Carbonic acid is being neutralized by the carbonate/bicarbonate.

Even If you could somehow magically have a bottle of Carbonic acid, you would have to dump an entire boatload of it into the water to drive the equilibrium equation in the opposite direction so that a significant amount of CO2 would be formed by the imbalance.

H2CO3 -> H2O + CO2

So the addition of a pure Carbonic acid solution is not really practical because in that solution, at normal atmospheric pressures it would immediately revert to a solution low in Carbonic acid and high in dissolved CO2, which basically describes a bottle of soda-pop/fizzy drink or in our case a bottle of fizzy mineral water.

Hope this makes sense...

Cheers,


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## plantbrain (10 Mar 2011)

I think it's about 400:1 CO2: H2CO3
For marine water, I keep forgetting due to the diffetrent KH's involved for freshwater.

CO2 is a weak acid and does not dissociate well. Stronger acids like say HNO3, nitric acid.......or HCL........do.


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## bigmatt (10 Mar 2011)

Cheers folks - that answers it very nicely thankyou! 
Matt


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