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Re: Aquatic Plants Digest V2 #713

David Brooks wrote, May 16:

>I used to use a tetra (I think) bell with a small aerosol of Co2.  The
>idea was to fill the bell with air in the morning and it would dissolve
>throughout the day. after a few days the level of the gas in the bell jar
>did not go down as much during the course of the day and I had to put
>less Co2 in each morning.  Apparently other gases from the tank water
>were replacing the co2 in the bell as the co2 dissolved into the water.
>However, now that I bubble DIY CO2 into the bell, and the flow is
>continuous, any air in the bell would be pushed out as the bell
>overflowed. The air that is trapped in the top few inches of the DIY
>bottle would not last a week or more before dissolving or spilling out of
>the reactor.  So for there to be air in the bell it would have to come
>directly from the yeast reaction.  Is this possible?.  What else does a
>yeast reaction produce apart from alcohol CO2 and water in sufficient
>quantity to ëcompromiseí the CO2?.
>David Brooks
Even assuming that the aerosol puts out 100% CO2, you will still not get
all the gas in the bell to dissolve in the water because, as the CO2
dissolves out of the bell into the water, nitrogen and oxygen are diffusing
from your water into the gas in the bell.  Your water, which is pretty much
in equilibrium with the atmosphere has nitrogen and oxygen dissolved in it.
This immediately begins to diffuse into the CO2 in the bell, and, while it
may start out as 100% CO2, you always wind up with a portion at the top of
the bell that is in equilibrium with the atmosphere.  Each time you add
more CO2 to the bell, more nitrogen and oxygen diffuse in.  It is no big
problem, however.  Nearly all the CO2 will diffuse into your water in 12
hours.  What's left can be released without losing more than a tiny amount
of CO2.

Your DIY setup produces CO2, but it mixes with the air above the liquid in
the bottle so that you don't get 100% CO2 coming out.

When talking about diffusion of gasses into or out of liquids, people use
the concept of partial pressures, which are proportional to concentrations.
The atmospheric pressure at sea level is 760 mm of mercury.  Assuming an
atmosphere of 79% N2 and 21% O2 (ignoring trace gasses), the partial
pressure of nitrogen is 0.79 x 760 and the partial pressure of Oxygen is
0.21 x 760.  In water where the dissolved gasses are in equilibrium with
the atmosphere, the partial pressures of N2 and O2 would be the same.  Now,
lets suppose you have a big bubble of air in the bell that has been sitting
there for a day or so.  The partial pressure of N2 will be 0.79 x 760 = 600
mm mercury and the partial pressure of the O2 will be 0.21 x 760 = 160
(rounding off).  Now lets say you introduce some CO2 so that the content
becomes 20% CO2.  Partial pressures will now be N2 = 478, O2 = 129, and CO2
= 152.  Because of the differences in partial pressures, N2 and O2 will
dissuse from the water into the mix until their partial pressures are again
equal.  The CO2 will diffuse out untill partial presures for CO2 are equal.

Paul Krombholz                  Tougaloo College, Tougaloo, MS  39174
In Jackson, Mississippi, where we had the coolest April on record.  (I
would *really* like to see the coolest July and August.)