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Re: "Wonder Water", O2 saturation, etc...



George Booth writes:
>I tend to think of it as the
>"equivalent" concentrations in the air and the water are the same and
>CO2 will have no reason to diffuse across the air/water interface.
>
Actually, even at equilibrium (which, BTW,  will _never_ be reached in an
aquarium that contains live plants or fish) CO2 is always diffusing across the
air/water interface.  It's just that at equilibrium the rate of diffusion into
solution equals the rate of diffusion out of solution.  Therefore, there is no
_net_ change in CO2 concentration at equilibrium.

>If there is a higher level of CO2 in the water, either due to bioload
>respiration (typically 3-5 mg/l) or CO2 injection (10-20 mg/l), then
>CO2 will try to diffuse into the region of lower concentration, i.e.,
>from the water to the atmosphere.
>
Again, even in this situation diffusion is always occurring in both directions,
but since the rate of diffusion out of solution is greater there is a net
decrease in the solution concentration.

>In a quiet aquarium, the surface
>layer of water would be close to equilibrium and layers further down
>would have increasingly higher concentrations, with the whole system
>trying to reach a state where all concentrations are equal.
>
Hmm.  Do you have any data to back this up?  Personally I'm not convinced
that a typical aquarium is deep enough to undergo significant stratification.
Also, due to surface tension effects I think the water near the surface may
not be as close to equilibrium as you think.

>With aeration, three things are happening.
>
Only three?<G>  I can think of at least one more: a significant decrease in
surface tension.

>So aeration is not "driving out" CO2 but is simply increasing the rate
>at which the CO2 is able to equilibrate.
>
Amen!  Except for one minor nit which I alluded to earlier.  It might be
better to say "... increasing the rate at which the CO2 approaches
equilibrium".  Since an operational aquarium is not a closed system with
respect to CO2, it will never actually reach equilibrium.  The system may
reach a steady state condition, but since CO2 is always being added and/or
removed it will always deviate from equilibrium.

Bill