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Re: Aquatic Plants Digest V4 #32

Hello Tom,

You are right, time plays a significant role in reaching an equilibrium,
but it has no influence on the position of that equilibrium.

The entire discussion on this list about the Law of Partial Pressures
treats CO2 as if it were an "IDEAL gas". Well, CO2 is NOT an ideal gas. 

When "normal" gases dissolve in water, they more-or-less follow Henry's
Law of Partial Pressures. However, very soluble gases DO NOT follow that
law at all! Thus, in water, CO2 acts both as a gas and as a reactant,
forming the equilibrium

CO2 (gas) <-H2O-> CO2 (diss.) <--> H2CO3 <-M+-> MHCO3 <--> M2CO3

While it will take substantial time to reach this equilibrium (all
things remaining constant), the position of that equilibrium, in "real"
water (as opposed to pure water), will depend also on factors that have
minimal influence on "normal" and no influence on "ideal" gases: Things
like pH, alkalinity, hardness, etc. because they can radically change
the proportions of the several species in the equilibrium. 

There must be rigorous ways to treat such complex equilibria, but I am
not aware of them.



> Well, we've learned a thing or two now(I have) but the poor newbie who
> asked(and started all this) is likely thinking what is all this about!?
> I must add that "time" also plays a large, often ignored, role in adding,
> removing, binding,equalizing of pressures etc any chemical in a tank. It
> will take some time for this "to happen" in the real applied world so when
> adding CO2 on a continuos basis, this doesn't reflect this theory of partial
> pressures well. Dan's comments seem to be right in this regards IMO. Proof
> is in the Pudding. I know the Wet/dry's work very well and have a very
> small, perhaps nonexsistant, effect. It would take time to remove or drive
> off the gas once the system was out of "balance". It does not happen
> instantly. A lot can happen in a few moments.
> Regards,
> Tom Barr      -who still get humbled by this planted tank thing<g>
> >
> >I believe George was referring to the relationship between O2 and CO2 when
> >both are *in solution*. Henry's law pertains to the relationship of a
> >dissolved gas in proportion to its pressure in a gaseous state. In fact,
> >Henry's Law actually proves that George is right, since the addition of CO2
> >to water does not change the concentration of O2 in solution.
> >
> >> At equilibrium, water will contain these gasses in the
> >> following levels:
> >[snip]
> >> Suppose then you add some CO2 to the system so that the
> >> air now contains 77% N2, 18% O2, and 5% CO2. The total pressure remains the
> >> same. The water will now contain the following:
> >>
> >> N2 585.2 mm Hg
> >> O2 136.8 mm Hg
> >> CO2 38.0 mm Hg
> >
> >Since CO2 is being added to the water via a pressurized tank, the partial
> >pressure in the atmosphere has little to do with it. Unless you change the
> >partial pressure of the O2 in the air above the water (which isn't gonna
> >happen in someone's house where their aquarium is), its concentration in the
> >water will not be affected by the addition of CO2 via a pressurized tank.
> >
> >Dan Dixon