CO2 reaction with water and bicarbonates?

[danielt]

Hi everyone!

I'll say it from the start that I'm not knowledgeable with chemistry but I'll call for anyone's help in making me understand this simple reaction.

I'm interested in finding out what reactions are occurring when CO2 is dissolved in water. The scope is for my hobby which is aquariums.

In planted aquariums we dissolve CO2 as a macro nutrient for the aquatic plants. The side effect of doing this is that the pH of the water drops. To prevent it dropping to dangerous levels we often put bicarbonates in the form of baking soda.

What I noticed is that a high enough level of bicarbonates might also reduce the concentration of CO2 in the water if that's correct.

I'm assuming this as the fish do not show signs of CO2 poisoning as it was the case when low levels of bicarbonates were in the water. I'm using reverse osmosis water in my aquariums so the pH is not buffered unless I put bicarbonates in.

So far I managed to understand that CO2 + H2O <=> H2CO3 <=> H+ + HCO3- <=> CO3 + 2H+

Also, H2CO3 is a weak acid, in water it should quickly dissociate to H+ + HCO3- (if my understanding of the above equilibrium is correct) making the pH go down. If we add more bicarbonates into the water it will shift the equilibrium H2CO3 <<=> H+ + HCO3- thus buffering the pH. Lack of bicarbonates will shift the equilibrium H+ + hCO3- <=>> CO3 + 2H+ plummeting the pH.

Am i talking none sense? Or at a "moron" level I got things right?

Many thanks for your help!

 

[Borek]

So far looks OK to me.

 

[danielt]

Thanks for the confirmation.

This raises another question. If my understanding is correct. A low bicarbonates and carbonates content in the water will make more CO2 available to the plants thus making a poisonous environment for wildlife?

There are certain species of plants that can obtain their Carbon from bicarbonates but all of them prefer CO2 instead of other Carbon sources.

Per the previous equilibrium reactions it means that as long as I dissolve CO2 into water it will be present in a greater amount if there are less or no bicarbonates present. pH buffering is not taken into consideration. Some pH "buffering" is provided as a result of increased photosynthesis. I'm just interested from a wildlife safety point-of-view.

Once again, thank you for your input!

 

[Borek]

This raises another question. If my understanding is correct. A low bicarbonates and carbonates content in the water will make more CO2 available to the plants thus making a poisonous environment for wildlife?

More or less. You can't have everything at the same time

 

[danielt]

Yeah, this hobby is filled with compromises.

Thank you for your help. This explains some of my suspicions of what was going on in my tank. In the past I managed to kill some of my fish with a lower dose of CO2 than I'm dissolving now. This confused me until I did some research. Several search results pointed to this forum and here I am, receiving better answers than what I found on aquatics related sites :D

 

[Solcielo L]

Sorry to necropost, but OP, you're totally screwing it up. CO2 remains CO2 in water, regardless of how much carbonates are in the water. Carbonates can buffer carbonic acid, which is what forms when CO2 dissolves in water. It has no effect on CO2 concentration, just pH since it's binding to the H+. This is why the pH increases after adding carbonates, but it does nothing to lower CO2 concentration. As CO2 concentration goes down, so does carbonic acid.

As for the safety of fish, it's about respiration. CO2 is transported from the gills (high concentration) to the water (low concentration.) If there's high concentration of CO2 in the water, the CO2 from the fish will not be expired. This causes the blood to become highly acidic and with no way to get rid of it, it suffocates.

 

[Borek]

CO2 remains CO2 in water

carbonic acid, which is what forms when CO2 dissolves in water

Do you see that you contradict yourself?

Concentration of carbonic acid/free CO₂ (which is what can inhibit fish respiration) depends on pH. It works both ways - adding CO₂ lowers pH, but making pH higher converts CO₂ into HCO₃^- (and CO₃^2-), these forms don't interfere directly with the CO₂ exchange.