Re: Gas bubbles

A lot of discussion has taken place about the observation of
gas bubbles streaming from plant under illumination, particulary
after a water change.  Three points:
1. The aquarium is not at equilibrium, so the total dissolved gas
is not determined by the atmospheres and plant surfaces in contact
with the water only, but on the rates of transport processes.
2. The solubilities of different gases is different, especially
that of CO2 differs from N2 and O2, so the total gas content
at equilibrium does depend on the atmosphere content, even if its
total pressure is unchanged.
3. The composition of the bubbles should not be assumed to be pure
oxygen, but will be determined by all the dissolved gases in solution.

Therefore, bubbles can be observed even if the Oxygen concentration
is slightly above the equilibrium saturation concentration, as George
has observed, because overall there is more gas in solution than would
be the case at equilibrium. Most of the gas in the bubbles will be
nitrogen, not oxygen.

The aquarium after a water change is clearly not at equilibrium,
especially if little time is allowed for the tap water to equilibrate
to its new (low) pressure.  In the pipe, it is under high pressure
and has dissolved more nitrogen and oxygen gas than is soluble at
atmospheric pressure.  If this water is vigorously aerated before
adding it to the tank, the oxygen and nitrogen can be made to reach
their equilibrium concentrations.  (Of course carbon dioxide would
also be depeted.  I believe, the original poster said that the CO2
concentration was not particularly high in his tap water--that's why
he couldn't explain the observation.)
Addition of very fresh tap water to the aquarium would produce bubbles
of nitrogen and oxygen, on many surfaces.  With illumination, the
most bubbles should come from living plants, as they will further
increase the content of dissolved gas at the surfaces of their leaves.

Curtis Hoganson
Dept. of Chemistry, Michigan State University
East Lansing, MI 48824   517-355-9715 ext 260