# Aquasoil Chemistry (Experts Wanted)



## Cédric (Dec 4, 2021)

Hello everyone. In our tanks, the couple carbonic acid/bicarbonates (H2CO3/HCO3) buffers the pH. When using aquasoil, KH can go down to 0 because H+ + HCO3- = H2CO3 (carbonic acid) = CO2 + H20 (H+ come from acidic aquasoil with clay and organic matter). In this case, the bicarbonates disappear (KH=0) and pH go down. My question is : when couple carbonic acid/bicarbonates is neutralized, when water can't buffer the pH anymore (KH=0), how does it work ? What is replacing H2CO3/HCO3 and buffering the pH all the same ?
People answer me : "_Soil buffers the pH_"...hmmm...yes....great thing.... But how ? Is there some organic acid into aquasoil doing the job by creating a new acid/base couple ? 
Thank you if you can help with that.


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## mistergreen (Mar 3, 2007)

CaCO3 in the soil & tap water is a buffer.

something like


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## Cédric (Dec 4, 2021)

mistergreen said:


> CaCO3 in the soil & tap water is a buffer.
> 
> something like
> View attachment 74594


Hi mistergreen. The couple HCO3/CaCO3 can buffer the pH but it works for high pH and marine tanks mainly. With pH<7 I guess we won't find CaCO3 in the tank...only bicarbonates HCO3.


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## mistergreen (Mar 3, 2007)

CaCO3 is everywhere. I wouldn't say it's only in marine tanks.


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## Cédric (Dec 4, 2021)

mistergreen said:


> CaCO3 is everywhere. I wouldn't say it's only in marine tanks.


I'm ready to believe it but you can see below the graphic showing that carbonates start to raise around pH 8. I doubt we can find carbonates and CaCO3 in a water with pH<7. I'm not saying you are wrong... I'm just looking for something that makes sense.


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## Steven F (Aug 1, 2021)

I don't think the bicarbonate and carbonate changes are the reason KH drops in aquarium. Many aquariums have organics in the water and substrates. These organics can react with Calcium carbonate creating a calcium organic salt while the CO2 floats away in the water and eventually outgassed to the atmosphere. 

Also carbonic acid doesn't dominate PH in aquariums. In many aquariums sulfate may dominate PH. So if you have water with calcium sulfate in it a and growing plants the plant will consume the calcium and sulfur but not at equal rates. And as a result sulfate will build up in the water sushing the PH down. If you don't have any KH in the water the PH can fall to less than 6. But if you have KH in the water (most likely from Calcium Carbonate, the sulfate will react with the carbonate creating Calcium sulfate and CO2 (which outgases from the water. ). And once the reactions occurs the PH goes back up.

In my tank the only source of KH is a decorative snail shell. My tank PH is always 7 +/-0.3. If the water PH is 7or higher it doesn't dissolve and has no effect on PH. But it the water gets acidic it reacts with the water and dissolves. bushing the PH back up to 7. As to a PH above 7 my understanding is that calcium bicarbonate converts to Calcium carbonate which then precipitates out of the water and settles in the substrate. and stops having any effect on PH and allows the tank PH to drop..

Also keep in mind that KH test will not detect carbonic acid in pure water. It will only detect carbonate salts like sodium, bicarbonate, calcium bicarbonate, potassium bicarbonate, and magnesium bicarbonate. Also tap water can easily have 30 or more elements dissolved in the water and each will affect PH differently. So while a chart of HCO3 and CO3 verses PH my hep you understand. you need to keep in mind that chemistry in aquariums is very different than in a test tube with only water and carbon dioxide.


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