Co2 Water Chemistry For Dummies (like Me)

surajk

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I've spent a lot of time trying to understand how CO2, Ph and kH all work and I think I finally have convinced myself I understand. I want to write it down in case it helps someone else, or a year later, I forget, or for someone else to tell me I've got it all wrong (which is fine)

1. when we inject CO2 into a tank, some of it outgasses, some CO2 dissolves in water.
2. Some of the dissolved CO2 stays as dissolved CO2, some of it reacts with water to form H2CO3 which is an acid (carbonic acid). The ratio of dissolved CO2 to H2CO3 depends on the Ph of your water. the higher the Ph, the more carbonic acid, the lower the Ph, the more dissolved CO2.
3. Since carbonic acid is an acid, it actually decreases your Ph, so if your tank Ph is 8 (as mine is) as you continue to inject CO2 , more H2CO3 is formed and your Ph goes down to say 7.
4. At ph 7, the ratio of dissolved CO2 to carbonic acid is higher i.e, the ability of the water to hold dissolved CO2 is greater, so now more of the CO2 you're injecting stays as dissolved CO2, not Carbonic acid, so the rate of Ph drop slows.
5. At some particular Ph, you hit an equilibrium, where the CO2 dissolving in the water is just enough to replace the CO2 that's being lost to the surface or being taken up by the plants, and just enough carbonic acid is being created at the same time to match the neutralization by your water's buffering capability. at that point your Ph stops dropping and stays stable.
5. The Ph at which you hit that equilibrium depends on your rate of CO2 injection (bubbles per second), the rate of dissolving of CO2(diffusion method) , the rate of outgassing (surface agitation), the rate of uptake (the number of plants) and the water's buffering capacity (kH), and of course the volume of the tank.
6. The amount of dissolved CO2 you have in the water when your ph becomes stable is a function of your Ph and your Kh at equilibrium. you can consult a chart which tells you how much CO2 is present at a particular Ph and Kh.
7. when you shut off your CO2, the reaction reverses - as CO2 is eaten by plants or by the surface, no new CO2 is injected, but at that Ph, there MUST be that ratio of dissolved CO2 to carbonic acid. So Carbonic acid now starts losing an H2O molecule and turns into dissolved CO2. but when carbonic acid becomes CO2 and water, its no longer an acid, so at the same time, the Ph start to rise.
8. eventually if no more Co2 is added, the water returns to its old equilibrium which is your original Ph. No harm no foul. Fish are fine - this Ph fluctuation does not bother them as long as the Ph does not drop to a level they can't tolerate, or the CO2 does not rise to a point where they suffocate.
9. So its a question of finding and fixing a particular bubble rate that brings your Ph down to a stable point where you are getting a good amount of CO2 that makes both your plants and your fish happy. In my tank (tap Ph 8, 10g, heavily planted) that is 0.5bps to bring Ph down to 7.

please let me know if I got this wrong, and hope this is helpful.
 

Coradee

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‘Whoosh’ that went right over my head
Giving this a bump up, hopefully our Co2 users will be able to answer your questions today
 
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surajk

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Coradee said:
‘Whoosh’ that went right over my head
Giving this a bump up, hopefully our Co2 users will be able to answer your questions today
And I thought I had written this so succintly. : ) this post wasn't burning questions as much as 'is this right'?

I essentially wanted to answer 3 questions that were bugging me when I started with Co2. I like to know how things work and it drives me crazy until I feel I understand them well.
1. does Ph continue to drop as long as you keep injecting CO2 or does it stop at some point?
2. if it does stop, why and where does it stop? what are the processes that keep the Ph stable at a lower value?
3. if the Co2 outgasses when you stop injecting, what happens to the carbonic acid? does it just stay in there? if not how is it removed?

Thanks for the bump.
 

aniroc

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Carbonic acid is a weak acid (has a high pKa). It does not matter how much you inject, it will not reach a pH of 3 or lower. I looked up at the values: for 7200 ppm Carbonic acid, the pH is about 4. That value is theoretically for our hobby because you need cold water under pressure to reach this concentration. Another example is rain water. When is fresh, the pH is 5.6 because of dissolved CO2 (about 300 ppm CO2).

Up to a point, the KH (Carbonate hardness or Alkalinity) keeps the pH stable while you inject CO2. When KH's buffering capacity has been used up, pH starts going down.

CO2 reacts with water to make Carbonic acid. The reaction is reversible. Carbonic acid breaks down into CO2 (gas, leaves the water) and water. A soda drink is an example of CO2 and water. Left in a glass, in few days, there are no more bubbles, becomes flat.
 
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surajk

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Thanks Aniroc! That validates and slightly corrects my understanding in the original post.
 

aniroc

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No problem Surajk! The topic of CO2 in water goes well beyond our hobby, deep into the ocean acidifying process, thoroughly studied issue, science based, well-funded research.
Sure, nobody is injecting CO2 into the oceans but higher CO2 in the air above means higher CO2 in the water and marine's life are used to an incredible stable pH (unlike the freshwater relatives...)
 
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surajk

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aniroc said:
No problem Surajk! The topic of CO2 in water goes well beyond our hobby, deep into the ocean acidifying process, thoroughly studied issue, science based, well-funded research.
Sure, nobody is injecting CO2 into the oceans but higher CO2 in the air above means higher CO2 in the water and marine's life are used to an incredible stable pH (unlike the freshwater relatives...)
Yes, I've been reading about ocean acidification as I've been researching Co2. Actually the whole aquarist experience makes me realise that we live in a giant, but closed, ecosystem that we're consistently fouling. Not a comfortable thought...

I'm going to update an earlier thread on DiY CO2, and would love your comments on that too!
 

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