"NetMax" wrote in message
...

For air at atmospheric pressure and water at 20 C, you end up
with 3 ppm of CO2 and 9.1 ppm of O2.

in water

Yes, right -- it would be different for oil ;-)

Thanks, I think I have it now. O2 is 32 times less soluble in water than
CO2, but since the atmospheric O2 concentrations are higher, the
resultant equilibrium has O2 at 3 times higher than CO2. Can I
extrapolate that atmospheric O2 is in concentrations of 96 times higher
than CO2?

Not quite. When I said O2 is 32 times more soluble, I was referring to
volume and the saturation point. When you say that O2 is 3 times higher
than CO2, you are using weight (because ppm is a measure of weight,
not volume), and you are referring to equilibrium, not saturation.

We can work out the ratio by volume:

Using partial pressure, air is 21% O2 and 0.033% CO2, so that's 636
times more O2 than CO2.

In water, at 20 C, we get 9.1 ppm O2 and 3 ppm CO2. As volume, that's around
6.4 ml O2/l and 1.5 ml CO2/l. That's a ratio of 4.26.


So, you could say that O2 concentration in air is 636 times that of CO2,
and that it is 4.26 times that of CO2 in water. (Incidentally, the difference
in O2 concentration between water and air is one reason why animals
originally evolved to leave the water and move onto land. In air, you
have 21% oxygen whereas, in water, you only have around 0.6%
oxygen. The maneuverability of an animal is largely determined by
how much energy it can produce, and energy is produced by
oxidizing nutrients. Ergo, an animal on land can move faster and
for longer periods than an animal in water because air contains so
much more oxygen so a land animal can produce more energy
per unit of time.)

So why is the actual CO2/O2 equlibrium ratio in water not 636?
The ratio at equilibrium isn't just determined by partial pressure, but
also by a constant that is different for each gas and essentially captures
solubility at a given temperature. For CO2, the constant is
3.38 x 10^-2, for O2, it is 1.28 x 10^-3 at a temperature of
25C. Solubility is determined by multiplying
the constant by the partial pressure of the gas.
To work the problem for different temperatures, you also need to
know the enthalpy of dissolution for each gas. For CO2, that's
-5.242 kcal, and for O2, that's -3.5 kcal. An equation called
the Clausius-Clapeyron equation then allows you adjust
solubility for different temperatures. If you do all that, you
end up with the 32 times at 20 C that I mentioned earlier.

Your description explains how they can put a large amount of pure O2 into
fishbags for trans-shipping. The water simply will not absorb more than
about 4 times what it already has. I imagine fishbags don't have the
best O2 barrier anyways.

Right. There is limit to how much O2 can dissolve into the water. However,
as the fish breath and use up O2, more O2 from the gas volume above
the water can dissolve into the water, repleneshing what's used up by the
fish.

Cheers,

Michi.

--
Michi Henning Ph: +61 4 1118-2700
ZeroC, Inc. http://www.zeroc.com