Re: [APD] Re:CO2 not dissolving

["S. Hieber" <shieber at yahoo_com>]

Good stuff, Paul -- the kind of stuff that ends up in
"Stranded onthe List" in _The Aquatic Gardener_  ;-)

I wouldn't disagree with a word of what you say about the
partial pressures. However, I am disinclined to adopt your
conclusion about Jams's situation.  The partial pressures 
doesn't explain what James needs to do to solve his problem
of gas build-up in the reactor.

It's certainly possible to run one of Tom B's reactors on a
30g tank with a powerhead the size James is using and not
have an ever increasing amount of gas in the reactor and
difficulty maintaining CO2 levels in the tank. Given that
gases obey the same rules in my tank as his, I think the
solution has to be one of the things mentioned already.
When the problem is solved, if he reports back, it will be
useful for all of us.

thanks,
Scott H.

--- Paul Krombholz <krombhol at teclink_net> wrote:
> Probably it is dissolving, but that does not mean that
> the bubbles 
> will disappear.  The bubble may start out as 100% CO2,
> but if it 
> stays around in the tank, it winds up as roughly 80% N2
> and 20% O2 
> and just a little bit of CO2.  How does this happen?  It
> has to do 
> with partial pressures of gasses.
> 
> Partial pressures predict whether a gas will dissolve
> from a gas into 
> water or come out of solution from the water back into
> the gas phase. 
> If the atmospheric pressure is 760 mm Hg---one
> atmosphere---then the 
> partial pressure of nitrogen will be 80% of 760 or 608 mm
> Hg, and the 
> partial pressure of oxygen will be 21% of 780, or 164 mm
> Hg.  Since 
> there is only 0.03% CO2 in the atmosphere, the partial
> pressure of 
> CO2 will be almost nothing---around 0.2 mm Hg.
> 
> If the partial pressure of oxygen in the water is less
> than that in 
> the air, then O2 will dissolve into the water from the
> air until the 
> partial pressure in the water is equal to that in the
> air.  The same 
> is true for all other gasses.
> 
> Now, let's take a bubble of 100% CO2, just released into
> your tank. 
> Since CO2 is the only gas in the bubble, and since the
> total pressure 
> of the bubble is, lets say, one atmosphere, then the
> partial pressure 
> of CO2 is 760 mmm Hg.  Obviously, the partial pressure of
> CO2 in the 
> water is much lower, so CO2 will immediately start
> dissolving in the 
> water.
> 
> But, the water has nitrogen and oxygen dissolved in it,
> and in pretty 
> good equilibrium with the atmosphere.  Let's say that the
> partial 
> pressure of N2 is 600 mm Hg, and the partial pressure of
> O2 is 160 mm 
> Hg. in your tank water.
> 
> Therefore, N2 and O2 will diffuse INTO the bubble from
> the tank water 
> as the CO2 is diffusing out.  Remember that, initially,
> the partial 
> pressures of N2 and O2 in the bubble are zero, since the
> bubble 
> starts out as 100% CO2
> 
> While the bubble may get smaller because CO2 diffuses out
> faster than 
> N2 and O2 diffuse in, The bubble is not going to
> disappear.  Instead 
> it comes into equilibrium with the partial pressures of
> O2, N2, and 
> CO2 in your tank water.
> 
> So, therefore, you can not conclude that you must have
> been sold got 
> 'watered down' CO2 or that your reactor is somehow
> sucking in gas 
> from an outside source.  You just have to remember that
> gasses are 
> diffusing into those CO2 bubbles, while the CO2 is
> diffusing out.


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