[Simonauv]

I have just read on an aquarium forum that someone is using crushed coral (CaCO₃) as a buffer to maintain an acidic pH in an aquarium that is also injected with CO₂. The idea of the CO₂ injection is that it will boost the rate of photosynthesis in the plants (light is also boosted). The overall idea is to get the plants growing fast enough that ions such as nitrates are consumed at a fast rate to prevent growth of lower plants like algae which as less efficient competitors for nutrients than higher plants. Anyway, back to the question... Reading this and thinking about it for a while a few things that I thought I had a handle on started to make less sense. I'm probably just confusing myself here but here's the gist of it.

When CO₂ dissolves in water there is no such thing as aqueous CO₂ is there? It forms H₂CO₃ in an equilibrium reaction system as follows:

CO_2(g)+ H₂0_(l) <-> H₂CO_3(aq)

And adding more CO₂ to the reaction system will drive it to the right making more H₂CO₃ which is why the pH drops because you are just increasing the H⁺ conc. That's right so far isn't it? Why then, do tanks with CO₂ injection show greater plant growth than tanks without it? It doesn't make sense. Where do aquatic plants get their CO₂ for photosynthesis? Now if CaCO₃ is added as a solid as crushed coral, it would not actually dissolve would it because both the carbonic acid and the calcium carbonate have carbonate ions so no replacement will occur. Correct? Or is water acting as an acid in this case and the lower pH increases its solubility in the water? If so it would kind of act like a buffer wouldn't it because CO₃^2- ions would push the Rx back to the left and CO₂ is continually being pumped into the system as well so the forward and backward Rx would be equal until the coral was completely dissolved??? Now I'm just confusing myself... Is anyone able to explain this to me? I am not seeing how the crushed coral can actually act as a buffer to help maintain an acidic pH. To me it looks like it will either have no effect or will just increase pH rather than maintain a constant pH.

TIA

Simon

[Borek]

When CO₂ dissolves in water there is no such thing as aqueous CO₂ is there? It forms H₂CO₃ in an equilibrium reaction system as follows:

CO_2(g)+ H₂0_(l) <-> H₂CO_3(aq)

As you wrote - it is an equilibrium. Thus there is CO₂ present in water.

And adding more CO₂ to the reaction system will drive it to the right making more H₂CO₃ which is why the pH drops because you are just increasing the H⁺ conc. That's right so far isn't it? Why then, do tanks with CO₂ injection show greater plant growth than tanks without it? It doesn't make sense. Where do aquatic plants get their CO₂ for photosynthesis?

From the equilibrium concentration of CO₂, you forgot about that factor thus your reasoning is partially faullty.

Now if CaCO₃ is added as a solid as crushed coral, it would not actually dissolve would it because both the carbonic acid and the calcium carbonate have carbonate ions so no replacement will occur. Correct? Or is water acting as an acid in this case and the lower pH increases its solubility in the water? If so it would kind of act like a buffer wouldn't it because CO₃^2- ions would push the Rx back to the left and CO₂ is continually being pumped into the system as well so the forward and backward Rx would be equal until the coral was completely dissolved??? Now I'm just confusing myself... Is anyone able to explain this to me? I am not seeing how the crushed coral can actually act as a buffer to help maintain an acidic pH. To me it looks like it will either have no effect or will just increase pH rather than maintain a constant pH.

Adding CO₂ lowers pH:

CO₂ + H₂O <-> H⁺ + HCO₃^-

(in fact I could write this reaction as two steps, with H₂CO₃ as intermediary, but it will not change a thing). But in the presence of CaCO₃ situation is a little bit more complicated, as what happens is that part of CaCO₃ dissolves:

CaCO₃ + CO₂ + H₂O -> Ca²⁺ + 2HCO₃^-

There are CO₂ (or H₂CO₃) and HCO₃^- present in the solution. This is an acid and its conjugate base, thus you have a buffer solution, with pH close to 7. CO₃^2- concentration is neglectable in this system.

There is no risk of dissolving all coral as the solubility of CaCO₃ and Ca(HCO₃)₂ is rather low and the solubility of CO₂ is not that large too (less then 2g/L in normal presure and 20 deg C).

[Simonauv]

Thanks Borek - can I ask what form the CO₂ takes in the water? Also should have mentioned that temps are generally 27 - 30 degrees Celcius in these setups.

[Borek]

None special - it is just molecule. However, the quilibrium between CO₂, H₂CO₃ and HCO₃^-/H⁺ is a dynamic one, so every CO₂ molecule changes its state many times per second.

[Simonauv]

Quick question before I put up something else to make sure I've got this...

Is the CO₂ -> H₂CO₃ <-> H₃0⁺ + HCO₃^- equilibrium and the CaCO₃ reaction two separate reactions or are they combined into one reaction (does that make sense? I have written something with them separated but thought I'd better ask if that's right first)?

[Simonauv]

will check tomorrow -need sleep now... (1:19am here)

[Borek]

Depends on the point of view. In fact there are several processes going on:

CO₂ + H₂O <-> H₂CO₃ (1)

H₂CO₃ <-> H⁺ + HCO₃^- (2)

HCO₃^- <-> H⁺ + CO₃^2- (3)

CaCO₃ <-> Ca²⁺ + CO₃^2- (4)

H₂O <-> H⁺ + OH^- (5)

What you observe is a combination of these. Each of these processes has its own equilibrium constant, but you may combine these ractions and calculate new equilibrium constants using ones already known.