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Re: lime, O2 saturation concentration

> From: olga at arts_ubc.ca
>
(snip)
 
> Will dolomite do the
> same thing as Calcium Carbonate? 
>
	Dolomite is a mixed calcium/magnesium carbonate, one 
magnesium ion for every calcium ion, and one carbonate ion for each.
It will dissolve in the aquarium by reacting with CO2 in the water, in
exactly the same way as calcium carbonate, but will give a mixed calcium/
magnesium bicarbonate solution.  Both KH and GH will rise, but you need 
slightly less of the dolomite than you would of calcium carbonate for
the same effect (about 92% as much), because magnesium has a lower atomic
weight.
	I don't know what the snails will make of all that magnesium!

> ------------------------------
> 
> From: George Booth <booth at hpmtlgb1_lvld.hp.com>
> Subject: Re: Oxygen saturation concentration
> 
> > From: psears at NRCan_gc.ca (Paul Sears)
> > Date: Tue, 25 Jun 1996 08:35:04 -0400 (EDT)
> > 
> > 	This oxygen concentration corresponds to the equilibrium
> > between oxygen in solution and the oxygen in the air.  It corresponds
> > to an oxygen partial pressure of about 0.2 atmospheres.  If you want
> > oxygen bubbles to form, you need a local concentration corresponding
> > to 1 atmosphere pressure of oxygen, i.e. thirty-odd mg/litre.
> > 
> > 	The word "saturation" has to be used carefully - it must be
> > clear what the other phase is; in this case a bubble of oxygen gas at
> > 1 atmosphere.
> 
> OK, I'm confused.  I see bubbles forming at around 110% of saturation.
> The dissolved O2 is measured with a Lamotte kit and then compared with
> a chart I have which shows saturation at something like 6.1 mg/l (from
> memory) at my altitude.  Are we talking two different things here?
>
	The oxygen concentration in the bulk of the aquarium water may
only be 6.1 mg/L, but the local concentration where the oxygen gas is
formed must be above 30 mg/L or so, or oxygen gas at 1 atm pressure
could not leave the water.  This may well be taking place _inside_ the
leaf.  If the gas is then leaving the leaf more quickly than it can
dissolve in the bulk water at the exit, then you will see a bubble.

> ------------------------------
> 
> From: eis at alto1_altonet.com (Paul Nicholson)
> 
> George Booth repled to Jim Kostich:
> 
> >> Another possibility along this line of thought is that the water might
> >> well continue dissolving O2, but be dumping some other gas
> >> simultaneously to "make room".
> >
> >No, each gas has its own soluability (partial pressure).  One type
> >will not leave to make room for another.
> >
> >> The above is all based on my assumption that the maximum solubility of
> >> each of the gases involved is limited by the presence of other gases
> >> already dissolved - an assumption which makes sense to me,
> >
> >That is incorrect.  Many fish stores continue to advise against CO2
> >injection "because it will reduce the oxygen available to the fish".
> >It just doesn't work that way.
> 
> I disagree with this analysis. Addition of any gas will lower the partial
> pressures of other gases. A liquid in equlibrium with a mixed gas
> atmosphere can hold exactly 1 atmosphere of mixed gas. 

	Don't forget that each gas component has its own solubility.
You are pointing out an extra constraint - that the total gas pressure
would be 1 atm.

> If we increase the
> amount of one gas by a diffusion scheme like our CO2 "reactors" we increase
> its partial pressure, therefore the total partial pressures of the other
> gasses will be reduced.
>
	We are also getting away from an equilibrium situation.  I don't
think your constraint above applies too well now.
 
> Imagine an aquarium with half of the surface exposed to air and half
> exposed to CO2. In this case the average atmosphere above this tank is 50
> percent CO2, 10 percent O2, and 40 percent N2. Obviously the partial
> pressures of N2 and O2 will be reduced.
>

	This is not a "mixed gas", though.
 
> In this case since a continuous transport of the the gases from one
> atmosphere to the other will be established the partial pressures in the
> liquid will also be affected by how efficiently each gas diffuses
> into/outof the liquid, I don't know if different gases have different
> diffusion rates into a liquid or not, however I suspect so, since this is
> "sorta" how a gas chromatograph works.
>
	The point is that it is a very complicated situation!  

> Anyway not to worry, the partial pressures of CO2 that we are using are so
> low that not much O2 or N2 leaves.
>
	There is another little wrinkle to this.  If you cover your tank
well, you could get CO2 buildup under the cover.  I think it
would be difficult to do this well enough to cut the O2 concentration
at the surface of the water, but it _could_ be done.
	What are plants for anyway?


Paul Sears    Ottawa, Canada