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.