CO2, calcium sulphate

> From: Doug Valverde <75051.160 at compuserve_com>
> Date: 24 Nov 95 21:47:12 EST
> Subject: Aeration/agitation and H2O/CO2
> Have any of you done experiments with Trickle Filters and alleged CO2 drive
> off?  (George didn't you do something?)
> I'd appreciate some good solid chemistry if any of you have it.  Does CO2 have
> a greater affinity for water then does O2?  If it does then how can surface
> agitation or aeration decrease CO2?  If it does not, then why do we always hear
> TF's drive off CO2, same story for air pumps, surface agitation, waterfalls,
> and so on.
> Can anyone give me good scientific answers as to the relation of CO2 and O2
> content of water as it relates to exposure to the air?
	The solubility of a gas in water depends both on its "affinity" for
water and the amount of it around - its partial pressure.  The partial 
pressure of a gas in air is the fraction of the air pressure provided by
that gas.
	For oxygen, the partial pressure is about 0.2 atmospheres (O2 is
about 20% of the air), while for CO2, the partial pressure is about
0.03 atmospheres.  This is a very large difference, and the result is
that, at equilibrium with atmospheric air, water contains about 9 ppm O2,
but only 0.5 ppm CO2, even though CO2 is intrinsically more soluble
in water.
	If the water is _not_ at equilibrium with the air, then the speed
with which equilibrium is attained depends heavily on the agitation of
the water surface.  If your water contains more than 0.5 ppm of CO2, then
the more you agitate the surface the more quickly that CO2 will 
leave, and the faster you will approach the equilibrium concentration.
If your water contains _less_ than 0.5 ppm, the more surface agitation
you have the faster CO2 will _enter_ the water. 

> From: gomberg at wcf_com (Dave Gomberg)
> Date: Fri, 24 Nov 95 23:13:13 PST
> Subject: Re: Calcium carbonate
> You can buy calcium sulfate at your hardware store as plaster of
> paris.  You can mold it into a small lump that can be put into your
> tank to dissolve slowly as required.  The addition of CO2 will
> promote the formation of CaCO3.  It is not clear that calcium
> carbonate is in and of itself useful, tho.  
	Sorry, but this won't work.  For this to happen, the SO4--
would have to go somewhere.  You can't displace the anion of a strong
acid (sulphuric) with a weak acid (carbonic).

Paul Sears       Ottawa, Canada.