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Re: CO2 and O2

> On another forum, I got myself into a discussion that might end with my
> having to eat crow. Well, to keep it brief, can anybody explain to me why
> [CO2] and [O2] would be completely independent of one another in water?

They are NOT in equilibrium, steady state etc. The tank is constantly
changing and giving off O2(hopefully, if your doing it right/well for most
of the time) and CO2 to the air. Getting "in" and getting "out" is tough and
slow. This takes time to happen.
If you heavily aerate and mix the water this in/out rate will increase and
be closer to the ambient air. If not, then this process can take some time.

Folks seem to think everything is like a sterile glass of water with no
biology going on in it and that is at equilibrium with the air. That's not
the real world in an aquarium and especially a plant tank where a high
amount of primary production is happening. It is NOT an "ideal" set up for
equilibrium of gases.
Folks make this assumption ALL the time.
We pump CO2 gas and dissolve it well going far beyond the saturation level.
The plants use all this CO2 and split the water molecule in the first step
of photosynthesis (not using the O2 off of CO2!!) to produce "food" (PGA).
This gives off the O2.

> argued that you can only have a certain amount of dissolved gases in
> solution under atmospheric pressure (constant P, V, and T), and that
> elevating the concentration of CO2 in water WILL displace O2 to some
> degree.

Not much and none I've seen or been able to measure in a planted tank. See
the above. The amount of CO2 and O2 both change throughout the day. If you
let the glass of water come into equilibirum you are correct but the
application on planted CO2 enriched aquariums is not.

> The other person said that one has nothing to do with the other.

Pretty much in this case.

> I 
> seem to have heard the same argument in the past, with his explanation
> being correct, but my intuition says that if you kept pumping a certain gas
> into water, you're not going to maintain (in solution) the same amount of
> "other" gases that are not being pumped in... It just doesn't make sense to
> me.

It bugs more folks that just you:)
Aquatic biologist get this one wrong all the time and professional aquarist
at public aquariums do as well. Don't feel bad. Just get it right. That's
how we learn, we make mistakes. Some more than others(like myself).
CO2 enrichment changes things as does the surface movements, mixing, surface
area and other processes.

 So I need to have it explained to me why you CAN'T "displace" O2 no
> matter what.

I won't go that far but in our case(planted tanks)I'll say yes.

 I mean, if you keep pumping pure nitrogen gas into water at a
> significant rate, MY MIND pictures all other gases being "pushed out" of
> solution. I know the principle of partial pressures only applies to gases,
> but I feel like there was a similar principle for gases dissolved in liquid.

Well then you get to the point where you are aerating the water and this
will speed up the removal of both O2 and CO2 if they are above 100%
saturation points. It will add CO2 and O2 if below 100% saturation.

So folks that aerate at night.........also drive off O2 if it's above 100%
like in my tanks. I don't use heaters so my tanks are cooler (all gases are
more soluble in colder waters) and my plant growth is fast. I get very high
Dissolved O2 levels and CO2 levels at night when the lights go off. 35ppm of
CO2 and 15ppm of O2. I want to keep the O2. That is the primary production
part and helps the bacteria(the microbial loop) really break down any other
waste and oxidizes any thing else. High DO levels also help the substrate
bacteria and the plant's roots.

> My husband just put my chemistry books into the garage and I'm too lazy to
> get them out. If somebody could give me a formula and simple explanation,
> I'd really appreciate it. Thanks. I hate eating crow...

Well just consider what you are applying this to. Primary production uses
CO2 and produces O2. Bacterial loops mainly use O2 and produce CO2.
Tom Barr